bluuebunny/crossref_metadata_2025 huggingface dataset:
Ab initio calculations on porphin
Authors: Almlöf Jan
Published: December 2004

Ab initio SCF calculations are reported for the porphin molecule. The positions of the central protons have been optimized, and the equilibrium geometry is found to be a linear NH ⃛ HN arrangement. The NH vibrational frequencies have been computed and are compared to experimentally measured quantities. Several low ionized states have also been studied in separate spin‐restricted SCF calculations. The lowest state is found to have B1u symmetry with an ionization potential of 8.0 eV.
Analysis of the space correlation factors in a MCSCF representation of LiH and Li2 molecules
Authors: Besson M. A. , Suard M.
Published: December 2004

The space correlation factor is studied in LiH and Li2 molecules, using MCSCF wave‐functions. The shape of the Fermi hole is related to the localization of molecular space orbitals, while the Coulomb hole study indicates the importance of symmetry properties of the molecular orbitals involved in excited configurations for the representation of the electronic correlation inside the chemical bond.
Quantum chemical calculations on thioridazine
Authors: Kaufman Joyce J. , Kerman Ellen
Published: December 2004

Quantum chemical CNDO/2 calculations for the conformational preference of the side chain of thioridazine as a function of angle indicated the crystallographically determined structure gave the lowest energy. There is also a small region of conformational flexibility within the first 90° of rotation of the side chain. This is commensurate with the results which we had obtained previously for our similar calculations for promazine and its Cl and CF3 derivatives, perazine and its Cl and CF3 derivatives, and for the hypothetical hitherto unknown N‐piperidinopromazine and its Cl and CF3 derivatives. The conformational profile of thioridazine resembles that of the perazines. The calculated gross atomic populations on the alkyl nitrogen in thioridazine was within the range we had previously found necessary for neuroleptic activity.
The simulated ab initio molecular orbital technique. VI. Open‐shell radicals in the spin restricted formalism
Authors: Collins Michael P. S. , Duke Brian J. , Eilers James E. , O'Leary Brian
Published: December 2004

The simulated ab initio molecular orbital (SAMO) method previously applied to RHF closed‐shell and UHF open‐shell situations has been applied to open‐shell radicals, such as the butyl radical and the pentyl radical, within the RHF open‐shell framework. The open‐shell Hartree–Fock theory is developed such that a rapidly convergent iterative method for evaluating the SAMO wave‐function can be employed. Results closely parallel those for the same systems using the UHF method and are of comparable accuracy to SAMO closed‐shell results.
Hulthén and slater type 2d functions in some excited configurations of sulphur and phosphorus
Authors: Chandler G. S.
Published: December 2004

It is shown that a substantial energy improvement is gained by the variational use of Hulthén orbitals, instead of single Slater orbitals, in the 3d shells of some excited configurations of sulphur and phosphorus. The energies obtained are close to those attained with two‐term Slater functions. In some cases the radial distribution functions from Hulthén orbitals are as good an approximation of SCF radial distributions as those from two‐term Slater orbitals. Single term 2d functions with only one parameter are found to give almost identical energies and radial distribution functions as those obtained from two‐parameter Hulthén orbitals. It is shown that the relationship between one‐term 2d orbitals and Hulthén orbitals gives a method of enforcing nuclear cusp conditions on the former with little effect on the energy.
An application of the Brillouin–Löwdin perturbation expansion: Lower bounds to the energy eigenvalues of the rigid rotator in an electric field
Authors: Grau D.
Published: December 2004

One of the variants of Löwdin's partitioning technique that makes use of a Brillouin‐type perturbation expansion for the study of lower bounds to the eigenvalues of a Hamiltonian has not been applied to any practical quantum mechanical problem so far. To illustrate how powerful this method can be, an application is made to the rigid rotator in an electric field, which has already been studied by Choi and Smith using the bracketing function of an intermediate Hamiltonian. It turns out that for a given order of the basis set of the Bazley space the Brillouin–Löwdin perturbation expansion gives closer bounds than the method of Choi and Smith, except for the case l = m, where the procedures can be shown to be mathematically equivalent. Especially for high l − m the number of basis functions needed to attain the same accuracy is by far larger for the method of Choi and Smith than for the Brillouin–Löwdin method.
Point‐group symmetry and transition metal superconductivity: NbMoTc alloys
Authors: Mueller F. M. , Myron H. W.
Published: December 2004

Using a factorization of the band density of states into point‐group symmetries, it is shown that the variation of the fractional density of states of one type—Γ12—correlates closely (ρ12 = 0.997) with variations in λM〈ω2〉. It is also shown that the Hopfield parameter η does not correlate with Np(0)Nd(0)/N(0), which would be true if superconductivity in d‐band materials was dominated by local atomic processes. It is concluded that bonding plays an important role in superconductivity in Nb‐Mo‐Tc alloys.
Perturbation theory in the path‐integral representation and its application to some simple quantum systems
Authors: Perlin Y. U. E. , Gifeisman S. H. N. , Van Quyet Pham
Published: December 2004

In the framework of the path‐integral formalism of nonrelativistic quantum theory the perturbation method is developed. This is based on the cumulative expansion of the partition function in powers of the difference between the classical actions of the exact and trial system. The trial system is constructed by replacing the Coulomb interactions by elastic ones, with parameters defined by Feynman's variational principle. The second order corrections to the ground‐state energy of the hydrogen and helium atoms, the hydrogen molecule, and molecular ion have been evaluated. The results have been found to coincide with the actual values with an accuracy of 4–6%, whereas the corresponding error of the variational solution is of the order of 15–20%.
Variation–iteration solution of the Hartree–Fock equations for atomic systems
Authors: Blakemore M. , Evans G. A. , Hyslop J.
Published: December 2004

A recently proposed scheme based on the variation–iteration method is applied to the solution of the Hartree–Fock equations for atomic systems. The procedure depends on the repeated application of a Green's integral operator which involves a single numerical quadrature at each stage. The integrations are performed by means of a prescription described in a recent paper by the present authors and it is shown that the parallel philosophies of the self‐consistent field and the variation–iteration approaches combine quite naturally. Representative calculations are carried out on three‐and four‐electron systems and the flexibility of the proposed scheme is demonstrated by considering solutions of various forms of the SCF equations.
On the kashiwagi–sasaki generalization of the löwdin orthogonalization and the inverse vibrational problem
Authors: Stepanov N. F.
Published: December 2004

By use of the pseudo‐inverse matrix technique the generalization of the Löwdin orthogonalization, given by Kashiwagi and Sasaki, is shown to be valid in a case of singular metric matrices for two basis sets of functions. The application of the same idea to the inverse vibrational problem brings about new procedures for solving this problem in an iterative way.
Investigation of the convergence of an expansion of the screening‐charge density for impurity ions in semiconductors
Authors: Csavinszky P.
Published: December 2004

In theories of the potential of an impurity ion in semiconductors, the common starting point is an expansion of the Fermi–Dirac integral ℱ1/2 appearing in the screening‐charge density. In the present paper, a formal test of the convergence of the series for ℱ1/2 is carried out. The results show that, in the physically important limits of complete degeneracy and complete nondegeneracy, respectively, the series for ℱ1/2 do converge. This establishes the validity of nonlinear Poisson equations for the impurity‐ion potential, which can be solved approximately by making use of a variational principle.
Reduced first order density matrix for the Be ground state
Authors: Bunge Carlos F.
Published: December 2004

A reduced first order density matrix for the Be ground state is computed from an extensive configuration interaction (CI) wave function. A sequence of increasingly accurate CI wave functions Φq converging towards the exact Ψ is used to assess the quality of the results which include approximate bounds for the overlaps 〈Φq|Ψ〉, electron–nuclear coalescence cusp data, Weinhold's overlap between density matrices, virial ratios, occupation number spectra, and some expectation values. The nuclear magnetic shielding constant and the molar diamagnetic susceptibility are determined with 2.0 and 1.5% of uncertainty, respectively.
Approximate exchange perturbation study of intermolecular interactions in molecular complexes
Authors: Sokalski W. Andrzej , Chojnacki Henryk
Published: December 2004

In an attempt to rationalize and improve an approximate exchange perturbation scheme related to the model of Murrell et al., more accurate approximations are introduced eliminating the use of empirical parameters. The total interaction energy has been evaluated as the sum of additive electrostatic, exchange, charge transfer, and dispersion contributions. It is proven that the assumption of intramolecular ZDO approximation is equivalent to the use of the Mulliken formula for intermolecular intergrals in the region of small overlap. It is also shown that the use of exact two‐center resonance integrals and atomic multipole expansion leads to a substantial improvement of results. The method has been applied to calculation of interaction energies of some model hydrogen‐bonded complexes of water, hydrogen fluoride, adenine, thymine, guanine, and cytosine using approximate all‐valence wave functions.
Electron correlation theories and their application to the study of simple reaction potential surfaces
Authors: Pople J. A. , Krishnan R. , Schlegel H. B. , Binkley J. S.
Published: December 2004

This paper serves two purposes. The first is to describe an implementation of the coupled cluster theory with double substitutions (CCD) previously developed by Cizek. The second is to apply this method and closely related fourth‐order perturbation methods to some simple molecules and reaction potential surfaces. These studies show that CCD theory gives results close to those of a Møller‐Plesset perturbation treatment to fourth order in the space of double and quadruple substitutions MP4(DQ). Addition of contributions from single substitutions at fourth order makes little change in predicted relative energies. Preliminary results on the potential surfaces for 1,2‐hydrogen shifts in C2H2, HCN, CH2O, and N2H2 are discussed and compared with previous studies.
Generalized‐molecular‐orbital theory: Simple multiconfiguration self‐consistent‐field method
Authors: Hall Michael B.
Published: December 2004

Even after completing a multiconfiguration self‐consistent‐field (MCSCF) calculation, one must often include additional configuration interaction (CI) to obtain quantitative or semiquantitative results. There is some question of whether the prior MCSCF calculation is worthwhile, if additional CI is needed later. We have developed a new MCSCF computational method, which, because of our assumptions about the nature of the configurations, yields one Fock‐like operator for all the “filled” orbitals (high occupation numbers) and a second Fock‐like operator for all the “virtual” orbitals (low occupation numbers). Since there are only two matrices to build, our method is considerably faster than other MCSCF approaches. Because of these similarities to standard molecular‐orbital (MO) calculations, we have termed our approach generalized‐molecular‐orbital (GMO) theory. However, the “virtual” orbitals, unlike those of standard MO theory, are optimized to correlate the “filled” ones and can he used in a subsequent CI calculation. Results are presented for the correlation energy of H2O, the spectroscopic constants of N2, the singlet–triplet energy separations in CH2, and the nature of the chromium–chromium quadruple bond. Although these results are at a very low level of CI, the GMO approach appears to correct for the gross deficiencies of the single‐determinant SCF procedure.
Theoretical approaches to intramolecular electron transfer processes
Authors: Ratner Mark A.
Published: December 2004

Theoretical issues that must be considered for calculating rates of intramolecular electron transfer are discussed. The process itself is well defined only if the ground state is well approximated by a localized electronic function (Robin–Day class I or II). Under those conditions, the linear‐response formalism yields the rate as the Fourier transform of the particle–hole propagator. We discuss the validity of electron propagators as approximations to particle‐hole propagators, and find that they are valid, for simple models, within the stability range of the Hartree–Fock procedure. The vibronic nature of the transfer process is stressed, and formal schemes are given for calculating direct and through‐bridge transfer rates. Outstanding difficulties, and applications to biological systems, are very briefly noted.
Geometric approximation to nuclear spin–spin coupling constants in the water molecule
Authors: Lazzeretti P.
Published: December 2004

The geometric aproximation is used within the framework of triple perturbation theory to evaluate the contributions to nuclear spin–spin coupling constants in the water molecule provided by the Fermi contact, the spin–orbit, and the spin–dipolar interactions. The results, obtained with SCF wave functions expanded over Gaussian basis sets of increasing quality, are compared with corresponding coupled Hartree–Fock estimates. The limits of the geometric approximation to coupling constants are discussed.
Are intermediate states responsible for certain specific properties of biological macromolecules?
Authors: Biczó G. , Lukovits I.
Published: December 2004

A new electron structural mechanism is proposed for interpretations in molecular biology, in addition to the already existing theories of Szent‐Györgyi (semiconductivity) and Fröhlich (long‐range coherence). The hypothetical “intermediate‐” or “zigzag states” (ZZS) of solids are investigated by the recursion (transfer matrix) method. The physical reality of the ZZS is discussed up to an SCF DODS‐type theory and the necessity of additional less approximate investigations is emphasized. The possible role of ZZS in the explanation of: (i) translation in protein synthesis, (ii) energy and charge transfer processes, as well as (iii) initiation of protein formation is outlined.
What can quantum chemistry contribute to biology?
Authors: Hall George G.
Published: December 2004

The transition from quantum chemistry to quantum biology is discussed, and the contributions that quantum biology can make to the study of biological structure and process are outlined. The need for extensions to the theory to deal with larger systems, to include solvent effects and to account for specificity, are emphasized.
Electron pair correlation energies for ZN2+
Authors: Jankowski K. , Malinowski P. , Polasik M.
Published: December 2004

Pair energies contributing to the correlation energies of the outer‐shell electrons (n = 3) as well as for the 1s2 and 2s2 pairs are computed for the Zn2+ closed‐shell ion by means of the variational‐perturbation method starting with the sum of one‐electron Hartree–Fock operators as the zeroth‐order Hamiltonian. The results allow an understanding of the electron correlation for pairs of electrons of the p and d type. For 3p3d pairs it has been found that the correlation energy for the singlet pair of 1D symmetry is lower than for the triplet pair 3D. The 3l‐3l′ correlation energies are compared with the MBPT results of Kelly and Ron for Fe. The total correlation energy of the outer shell is −1.032 a. u.
Mechanisms of electron transfer through proteins
Authors: Petrov E. G.
Published: December 2004

The analysis of modern physical mechanisms of electron transfer in proteins is given. The tunnel electron transfer and donor–acceptor electron transfer through conducting states of a protein chain are discussed in detail. The expressions for the values of the electron resonance interaction and the formulas for probabilities of electron transfer between vibronic levels of donor and acceptor states in the presence of “transverse” and “longitudinal” relaxation are given.
Carbon‐13 magnetic resonance study of lanthanide‐substituted muscle calcium binding parvalbumins
Authors: Nelson Donald J. , Theoharides Anthony D. , Nieburgs Andra C. , Murray Richard K. , Gonzalez‐Fernandez Federico , Brenner Daeg S.
Published: December 2004

High resolution natural abundance carbon‐13 NMR has been used previously to examine molecular motions and conformational transitions in the muscle calcium binding parvalbumins isolated from mirror carp. [S. J. Opella, D. J. Nelson, and O. Jardetzky, J. Chem. Phys. 64, 2533 (1976) and D. J. Nelson, S. J. Opella, and O. Jardetzky, Biochemistry 15, 5552 (1976).] The carbon‐13 NMR spectrum of parvalbumin typically reveals the presence of a number of well‐resolved resonances from single‐carbon sites in the protein. Since accurate assignment of these resonances to specific carbon atoms in the protein is essential before these resonances can be employed as probes of local conformational events, an investigation of the origin of a number of single‐carbon resonances has been performed. Carbon‐13 NMR on Tb(III)‐ and Yb(III)‐substituted parvalbumin is the principal spectroscopic technique employed; however, results from terbium fluorescence spectroscopy, γ‐ray scintillation spectroscopy, and x‐ray difference Fourier analysis all contribute to single‐carbon site assignment. The principal conclusions drawn from the combined spectroscopic results are that: (1) the downfield carboxyl resonance at 184.6 ppm, previously attributed to Glu‐81, whose carboxyl group is involved in an internal ionic bond, originates rather from a carboxyl function coordinating the solvent exposed metal ion, (2) the carbonyl resonance at 168.9 ppm arises from Lys‐96, as was previously suggested; and (3) the 11.2 ppm resonance derives from the δ‐methyl carbon of Ile‐97, a resonance assignment that could not be made previously.
Quantum chemical and physicochemical influences on structure–activity relations and drug design
Authors: Kaufman Joyce J.
Published: December 2004

Quantum chemical results will be presented on drugs, carcinogens, teratogens, and endogenous biomolecules using our new nonempirical ab initio MODPOT/VRDDO method, which incorporates as options to our ab initio LCAO‐MO‐SCF/CI programs ab initio effective core model potentials (MODPOT) permitting one to calculate only the valence electrons explicitly yet accurately and an integral prescreening technique (VRDDO, variable retention of diatomic differential overlap) especially effective for spatially extended molecules. For molecules of the size of those of interest the MODPOT/VRDDO calculations run an order‐of‐magnitude faster than with our own fast ab initio programs and still retain accuracy to the third decimal place for the valence electron properties. We have also just implemented a new efficient MERGE technique which allows us to reuse integrals from a common skeletal fragment and only to have to recalculate those for a new atom or group or a change in its position. Examples will be presented of the use of this technique on a carcinogenic polycyclic aromatic hydrocarbon and its metabolites. The pKa's, oil‐water partition, and drug distribution coefficients as a sensitive function of pH have been measured for a number of drugs as well as for relevant endogenous biomolecules. The pH dependence of the lipophilicities of such molecules has profound implication on appropriate use of such data in QSAR studies.
Role of entropy in formation of van der Waals complexes
Authors: Hobza P. , Čársky P. , Zahradník R.
Published: December 2004

The TΔS0 term has been estimated for various types of van der Waals molecules and its importance for the stability predictions has been demonstrated. With hydrogen‐bonded complexes the absolute values of ΔH0 and TΔS0 are comparable at 289°K. With true van der Waals molecules the TΔS0 term even represents a dominant contribution to ΔG0.
Charge transfer interactions between nucleic acid bases and strong acceptors
Authors: Sheina G. G. , Radchenko E. D. , Egupov S. A. , Blagoy Yu. P. , Orlov V. M.
Published: December 2004

The regularities of the donor‐acceptor interaction of nucleic acid bases and some of their derivatives with the strong acceptor 7,7,8,8‐tetracyanoquinodimethane (TCNQ) in thin amorphous films at 77 K were studied using UV and visible spectroscopy. Purine and pyrimidine bases are shown to form charge transfer TCNQ complexes whose band energies hνct = Ip − EA − (H* − H0) are linearly dependent of the donor ionization potential Ip. Calculated differences between the interaction energies of uncharged molecules and ion radicals, ΔE ≈ 3.0 eV, are in good agreement with experimental variations of the complex dissociation energies in the ground and excited states, (H* − H0) = 2.73 eV. Annealing of the films of complexes containing cytosine, 1‐methylcytosine, uracil, and caffeine leads to ordering of their structure and the formation of ion radical salts.
Some peculiarities of the electronic transfer in redox enzymes
Authors: Likhtenstein G. I. , Kotelnikov A. I. , Kulikov A. W. , Syrtsova L. A. , Bogatyrenko V. R. , Melnikov A. I. , Frolov E. N. , Berg A. I.
Published: December 2004

The new properties of clusters—the polynuclear Fe, Cu, Mo—containing metal‐protein complexes have been discussed. The properties arose as a result of strong electron interaction and of the multiorbitals system existence in which the orbitals energetically fall close together. The clusters are characterized by (i) a high electronic capacity, (ii) an ability to multielectronic transfer without essential rearrangement of nuclei configuration, (iii) a high degree of donor‐acceptor energetical levels fitting at tunnel transfer, (iv) high possibilities for avoiding of reaction pathways being quantum mechanically forbidded, and (v) an ability to provide smooth reaction energetic relief in coordinated sphere. The analysis of data on spin exchange between paramagnetic centers (binuclear transition‐metal complexes, nitroxile biradicals, triplet exited chromophores) showed that in the range of spinexchanged constants Kex = 1014−1 sec −1 of the distances between the centers r = 3−17 Å the approximate relation Kex =1017 exp (−2.3r)sec−1 takes place. This relation may be considered as a criterion of nonspecific electron density transfer through nonconducting medium. The quantitation of exchange triplet‐triplet energy migration permits one to estimate the degree of quantum‐mechanical electron density penetration through biological matrix. By means of measurement of spin‐lattice relaxation rate for oxidized primary donor in bacterial photosynthetic system—bacteriochlorophyll cation (BChl+ )—it is shown that the distance between BChl+ and primary acceptor (complex FeQ) is about 34 Å. The proposed two‐step photoelectron transfer model explains the effective charge separation by relatively slow tunnel recombination of the charges BChl+ FeQ−. As spin and Mössbauer labeling experiments showed the conformation mobility of surrounding protein and membrane matrix with frequency more than 107 sec−1 is required for photoelectron output from primary photosynthetic cell in chromotophores and reaction center to secondary acceptor.
Possible connection between lipophilicity and steric substituent constants
Authors: Tichy̌ Miloň
Published: December 2004

The steric constants Es and ν have the greatest chance of being independent of lipophilic substituent constants. There is a high probability among hydrocarbon substituents that the steric constants correlate with the lipophilic ones, while in the case of polar substituents the probability is low.
Theory of the excitation spectrum of nondiagonal disordered systems
Authors: Onipko A. I.
Published: December 2004

The spectrum of a two‐component solid solution with a nondiagonal disorder is studied in the framework of the average T matrix method. For a one‐dimensional system in the nearest‐neighbor approximation the criteria for the system parameters are given such that at an in‐band resonance, one or two “impurity bands” may be realized, and the corresponding model calculation is performed. In the single‐site approximation an expression of the self‐energy part of a nondiagonal disordered system Green's function is found taking into account multiple occupancy corrections. The possibility of using it to describe a disordered system excitation spectrum and the calculation of state density moments are discussed.
Tautomerism of pyrimidine bases—uracil, cytosine, isocytosine: Theoretical study with complete optimization of geometry
Authors: Czermiński R. , Lesyng B. , Pohorille A.
Published: December 2004

The semiempirical MINDO/3 method with complete optimization of geometry is employed to calculate the electronic ground‐state properties (dipole moments and ionization potentials) and the energies of various tautomeric forms of uracil, uracil monoanion, cytosine, and isocytosine. The results are shown to be consistent with most of the experimental data. Accuracy of various quantum–mechanical methods is discussed. Particular attention is paid to the influence of the geometry optimization on energy differences between various tautomers. Some qualitative conclusions of biological importance are drawn from these calculations.
Quantum‐mechanical study of the interaction of glyoxal with arginine
Authors: Demoulin D. , Armbruster A‐M. , Pullman B.
Published: December 2004

Ab initio SCF computations indicate that the formation of an adduct of glyoxal to the guanidinium ion occurs in two steps. The first addition should occur on an NH2 group, rather than on NHCH3, and the formation of an unsymmetrical adduct IV is competitive and may be even favored over that of the symmetrical adduct III suggested by Takahashi. The barrier for that reaction is higher than for a similar reaction with guanine. The formation of a Schiff base between glyoxal and the guanidinium ion is disfavored because of the large endothermicity calculated for the process, much larger than that predicted for Schiff base formation with simple neutral or protonated amines.
Recent developments on the mechanism of chemical carcinogenesis by aromatic hydrocarbons
Authors: Pullman B.
Published: December 2004

This paper presents a critical discussion of recent discoveries on the metabolic transformations of polycyclic aromatic hydrocarbons and their interactions with nucleic acids in the light of quantum‐mechanical theories of chemical carcinogenesis.
Theory of electron transfer in donor–acceptor pairs
Authors: Zasukha V. A. , Volkov S. V.
Published: December 2004

The wave functions of donor–acceptor pairs before and after electron transfer are written as a product of the electron‐vibrational wave functions of the donor and acceptor with allowance for the change in the number of electrons on these particles by one after transition. In this approximation, the energy of the initial state is represented as a sum of the electron‐vibrational levels of the donor and acceptor and that of the final state as a sum of donor cation and acceptor anion levels. Formulas for the electron transfer probability of symmetrical and nonsymmetrical donor–acceptor pairs have been derived that express the dependence of this process on the ionization potential difference of the donor and the electron affinity of the acceptor, on the vibrational frequencies of these particles, and on temperature.
Electron‐conformational interactions and functioning of enzyme molecules
Authors: Volkenstein M. V. , Golovanov I. B. , Sobolev V. M.
Published: December 2004

Electron‐configuration interaction is one of the most general physicochemical approaches used to explain an enzymes functioning. Studying enzymatic reactions requires investigation of the motions of nuclei on a certain PES. In order to obtain the shape of the PES numerous calculations of the total energy of the enzyme–substrate complex were calculated, and are presented in this paper.
Interfragment interactions in peptide molecules
Authors: Volkenstein M. V. , Golovanov I. B. , Sobolev V. M.
Published: December 2004

This paper calculates the interfragment interactions in peptide molecules by taking into account the barriers to internal rotation.
Estimation of some parameters of the exchange mechanism of luminescence decay
Authors: Bazhin N. M. , Yakobson B. I. , Burshtein A. I.
Published: December 2004

The rates of variations of the relative yield (αη) and decay time (ατ) with the acceptor concentration (CA) have been calculated in the vicinity of CA = 0. On the basis of the equations obtained, the ratio αη/ατ has been found and a simple scheme of experimental data analysis proposed.
Transferable integrals in a deformation‐density approach to crystal orbital calculations. II
Authors: Avery John , Berg Erik
Published: December 2004

A new method for calculating crystal orbitals in the Hartree‐Fock‐Slater approximation is proposed. The method makes use of x‐ray crystallographic measurements of the deformation density, and uses transferable integrals to treat the neutral–atom potentials. Methods for evaluating matrix elements of neutral‐atom potentials are discussed in detail, and in this connection, expansions of displaced Slater‐type orbitals in terms of modified spherical Bessel functions and Legendre polynomials are presented. Tables of transferable integrals (moments of the neutral‐atom potentials) are given for all the elements up to Z = 36, and tables of Fourier transforms of the neutral‐atom potentials are also presented.
Composition of the partial wave expansion as a parameter in the MSXα scheme
Authors: Rosenkrantz Marcy E. , Konowalow Daniel D.
Published: December 2004

We explore how the composition of the partial wave expansion in MSXα calculations affects the binding energies and orbital energies for N2, O2, and F2.
Semigroups and the density matrix formulation of quantum mechanics
Authors: Green H. S.
Published: December 2004

A formulation of quantum mechanics is presented based on the theory of semigroups and the associated enveloping algebras of functions defined on countable subsemigroups. The existence of a unique *‐involution is not assumed. The fundamental elements of a semigroup are identified with experimental precedures for the separation of subensembles from a given ensemble of experimental systems. Observables are represented as elements of enveloping algebras, and ensembles as density matrices within an enveloping algebra. The statistical properties of ensembles are expressed in terms of traces defined on the semigroup and its enveloping algebras. The elements and generators of the Poincaré group can be defined and interpreted in the usual way. A variety of applications is described, in which the theory of the density matrix plays an essential or effective role. Advantages associated with the resulting freedom from the limitations of Hilbert space are illustrated.
Nonadditivity of interaction in water trimers
Authors: Clementi E. , Kołos W. , Lie G. C. , Ranghino G.
Published: December 2004

The energy of (H2O)3 has been calculated for 29 geometrical configurations of the trimer using the SCF LCAO MO method and extended as well as minimal basis sets of Gaussian functions. For two configurations two intermediate basis sets have also been tested. The results show the nonadditive component of the interaction energy to be small. They also indicate that fairly reliable results for the trimer can be obtained using minimal basis sets and the counterpoise method to eliminate the basis set superposition error. The nonadditive contribution to the interaction energy is shown to be mainly due to the long‐range induction interaction.
Exact‐exchange Hartree–Fock calculations for periodic systems. II. Results for graphite and hexagonal boron nitride
Authors: Dovesi R. , Pisani C. , Roetti C.
Published: December 2004

The ab‐initio LCAO‐HF method that was presented in a previous work is applied here to the study of graphite and hexagonal boron nitride monolayers. The dependence of total energy, band structure and density matrix on the computational parameters that control truncations in infinite sums over the translation vectors g of the direct lattice is first considered. For these systems it comes out that very good results can be obtained by neglecting all but the first few terms in the sums. For instance, for exchange contributions it is sufficient to consider integrals associated with the first 13 g vectors, suitably grouped, to obtain an error in total energy below 0.0001 a.u. The calculations were performed using an STO‐3G basis set; results concerning conformational minimum, bond energy, symmetric force constant, band structure, density of states, and population analysis are presented and discussed.
Resonance energy of very large benzenoid hydrocarbons
Authors: Randić Milan
Published: December 2004

The resonance energy of conjugated benzenoid systems is expressed as contributions arising from independent conjugated circuits. The scheme has been applied to numerous very large conjugated systems. In many cases, it was possible to find regularities in the increments for the resonance energy within a family of benzenoid systems as the number of benzene rings is increased.
Quantitative structure–activity study of some enzyme‐inhibitory quinazolines
Authors: Abdul‐Ahad P. G. , Blair T. , Webb G. A.
Published: December 2004

Relationships are formulated to explain the variation in dihydrofolate reductase inhibitory potency for a series of 25 substituted quinazolines, with an antineoplastic potential. Highly significant correlations are obtained using CNDO/2–3R calculated indices and/or the empirically estimated molecular polarizability as independent variables. The MO calculated indices employed are the atomic polarizability, as defined herein, and bond energy. The molecular polarizability is represented by a simple sum of environment independent partial atomic polarizabilities. The partial polarizabilities, reported here for H, C, N, O, F, S, Cl, and Br, are obtained from a multiple regression forced through the origin.
Biological effects of magnetic fields and cooperative phenomena
Authors: Ducla‐Soares Eduardo
Published: December 2004

Cooperative phenomena may be involved in the biological effects of magnetic fields (H), since its effects on isolated atoms or ions, at room temperature, are very weak. It is suggested that interactions at the social level can considerably increase the efficiency of H in orienting the displacements of animals. This effect is discussed with a simple mathematical model inspired on superparamagnetism. In case of very weak interactions of H with isolated animals, it suggests that orientation could only be made visible through cooperativity at the social level. These results are generalized to other physical stimuli and effects, leading to the concept of cooperative sensory response, i.e., a group response to an external physical agent even when its effect on isolated members of the group is undetectable.
Pair density analysis of semiempirical wave functions: Study of water molecule including environmental effects
Authors: Constanciel R. , Gritli H.
Published: December 2004

The pair‐density‐analysis method is applied to the study of CNDO/2 wave functions of the water molecule in various configurations. Particular attention is devoted to the modifications introduced by the consideration of environmental effects. The validity of the separated pair approximation is discussed and a detailed analysis of group functions is given.
Nature of DNA interactions with cations: UV Spectroscopic investigations and Marcus theory
Authors: Bregadze V. G.
Published: December 2004

This paper is devoted to the interpretation of DNA UDS caused by complexing DNA with Me ions using the correlation between the values of the spectral changes of the DNA absorption bands and the constants of reaction velocities of hydrated electrons with Me2+ and H3O+ ions.
Self‐consistent calculation of 1s2s1S state wave function of helium
Authors: Datta Dilip Kumar , Sengupta Sankar
Published: December 2004

A simple self consistent variation perturbation method in the coupled Hartree–Fock scheme has been proposed to calculate 1s2s 1S state of the He atom. The present paper deals with an 1s2s 1S wave function in which all the relevant orthogonality conditions are imposed in successive stages. The resulting wave functions together with some interesting features are discussed.
Simplification of an earlier MCSCF procedure for CI expansions involving orbital excitations
Authors: Mukherjee Nirad Gopal , Ray Pratip
Published: December 2004

An earlier version of the MCSCF procedure has been simplified for CI expansions involving orbital excitations only without decoupling. The computational time has been thereby cut down to a quarter of that taken by the earlier version.
Applicability of coupled‐pair theories to quasidegenerate electronic states: A model study
Authors: Jankowski K. , Paldus J.
Published: December 2004

The coupled‐pair many‐electron theory, its linear approximation, and various other approximate coupled‐pair approaches based on the orthogonally spin‐adapted coupled‐pair theory are applied to simple model systems in which the degree of quasidegeneracy can be continuously varied over a wide range. The four‐electron models studied are three ab initio minimum basis set models involving four hydrogen atoms in various spatial arrangements as well as the semiempirical Pariser–Parr–Pople π‐electron model of cis‐butadiene. The results are compared with the exact full configuration interaction results as well as with those obtained with other approximate limited configuration interaction approaches and Davidson's formula estimate, and the effect of quasidegeneracy on various approaches is discussed. We also examine the difference between the correlation energies obtained with the coupled‐pair many‐electron theory and its linear version and discuss the source of these differences and of the breakdown of the latter approximation in the case of very strong quasidegeneracies.
Definition of invariant molecular tensors in an origin change. III. Comparison of the methods suitable for the obtainment of invariant tensors for refraction and the Faraday effect
Authors: Duprez‐Quesnoit C.
Published: December 2004

In a previous paper, denoted I [C. Duprez and J. Tillieu, Int. J. Quantum Chem. 17, 155 (1980)], a global tensor, invariant in an origin change, has been introduced for all optical phenomena. The results obtained have been applied to two particular cases: refraction and the Faraday effect, in Paper II [C. Duprez, Int. J. Quantum Chem. 17, 169 (1980)]. In this paper, we study the various methods suitable for the calculation of these tensors. Then, we compare the results obtained in II with those previously obtained by other authors who, when studying the two particular cases mentioned above, omitted some terms necessary to obtain the invariance.
Self‐consistent calculation of 1s2s3S state of helium and heliumlike ions
Authors: Dutta Dilip K. , Sengupta Sankar
Published: December 2004

In this paper we present a simple method within the coupled Hartree–Fock framework to calculate the 1s2s 3S state of helium and heliumlike ions. The results are in very good agreement with those obtained by the use of multiterm correlated wave functions. Some interesting observations associated with the wave function are presented.
Molecular orbital treatment of phenylfurans and bifurans
Authors: Abu‐Eittah Rafie , Hilal Rifaat , Hamed Maher M.
Published: December 2004

The molecule‐in‐molecule method of computation has been applied in a SCF study of the electronic absorption spectra of 2‐ and 3‐phenylfurans and 2,2′‐ and 3,3′‐bifurans. The computed values for the transition energies and band intensities differ for the different isomers and help one to assign the preferred conformer of the studied molecules. Computations were performed on planar as well as nonplanar conformers. The calculated parameters as well as the interaction energy suggest all‐planar configuration for the studied molecules. A model potential for the sigma framework for the ortho‐hydrogens is considered. The calculated steric repulsion energy is negligible compared to the π delocalization. The height of the rotational barrier was larger for 2‐phenylfuran than for 3‐phenylfuran, and larger for 2,2′‐bifuran than for 3,3′‐bifuran. This result is in accord with predictions based on the extent of “π” conjugation in these molecules.
Improved lower bound on the indirect Coulomb energy
Authors: Lieb Elliott H. , Oxford Stephen
Published: December 2004

For a Coulomb system of particles of charge e, it has previously been shown that the indirect part of the repulsive Coulomb energy (exchange plus correlation energy) has a lower bound of the form –Ce2/3∫ρ(x)4/3 dx, where ρ is the single particle charge density. Here we lower the constant C from the 8.52 previously given to 1.68. We also show that the best possible C is greater than 1.23.
Addendum to “variational principle for obtaining approximate analytical solutions of the temperature‐perturbed Thomas–Fermi equation for compressed atoms”
Authors: Csavinszky P. , Tarr C. E.
Published: December 2004

In the present work the total energy of a Ne atom at T = 0 K is calculated as a function of a spherical container radius. The calculation is based on the Thomas–Fermi (TF) equation, which is solved approximately by an equivalent variational principle. The effect of an approximate exchange correction on the variational TF energy values is investigated.
Internal dynamics of nonrigid molecules. I. Application to acetone
Authors: Smeyers Yves G. , Bellido M. Nieves
Published: December 2004

The group theory for nonrigid molecules is used for studying the internal dynamics of the two equivalent C3v rotor “bent” molecules. Special emphasis is given to the deduction of the symmetry basis vectors which represent in box form the Hamiltonian operator. It is shown that these basis vectors may be advantageously employed in order to simplify the resolution of the two‐rotor equation. The procedure is applied to the acetone molecule. It is found that the lowest solutions are clustered into groups of four. The four lowest levels are related to vibrational states, the upper 64 to vibro–rotational states, in which the rotors are rotating in a restricted manner. Only few states show some cogwheel effect. Internal rotation contributions to the principal thermodynamic parameters of acetone are also computed.
Comments on the status of the chemical bond in methods of quantum chemistry
Authors: Berthier G.
Published: December 2004

Chemists do not take the view that chemistry can be derived from quantum mechanics by computations too literally; the difficulties are illustrated by the double bond concept. Quantum mechanics is very useful, but recent calculations have been mainly concerned with physical properties rather than chemical processes. In general, the chemical formula plays a central role in all interpretation of quantum‐chemical results, thus suggesting that the chemical bond is a preexistent notion. Moreover, the Born–Oppenheimer approximation seems to be necessary for deriving the existence of bonds between atoms previously assigned to suitable positions. Recent analyses by Claverie and Diner, Woolley, Primas, and others are briefly recalled, concluding with an expression of hope that use of more general Hamiltonians can lead to progress in obtaining a fully independent quantum theory of chemistry.
Physical approach to chemisorption through Green's function method
Authors: Iadonisi G. , Ramaglia V. Marigliano , Preziosi B.
Published: December 2004

For the purpose of illustrating methods for treating the chemisorption problem, which take into account bulk properties of the crystal, it is shown how the single‐particle Green's function may be used to solve the self‐consistent Schrödinger equations for an adatom–substrate system. The absorption problem is then reduced to the solution of a set of integral equations in terms of the infinite crystal Green's function. The method is illustrated using the interaction between an impurity level and surface states in a semiconductor gap and using sp hybridization of an alkali atom on a metal.
Conformation of nucleoside antibiotics
Authors: Saran Anil
Published: December 2004

Minor modifications or substitutions in the sugar or in the base part of pyrimidine and purine nucleosides have a profound effect on their biological activity. These modified nucleosides usually become antiviral, antibacterial, or cancerostatic agents and they are collectively called nucleoside antibiotics. The conformational properties of some of these nucleoside antibiotics have been studied by the PCILO method. The results obtained from such study indicate that the conformational preferences of these nucleoside antibiotics are very similar to those of their parent nucleosides and especially so in the situations that occur in aqueous solutions. The important biological significance of these results is that these nucleoside antibiotics can easily get incorporated into growing chains of DNA and RNA by mimicking their parent nucleosides and can interfere with the protein synthesis of RNA or DNA synthesis.
Superoxide and lens of the eye: A new theory of cataractogenesis
Authors: Varma Shambhu D.
Published: December 2004

Based on in vitro culture studies, it has been proposed that the photochemical generation of superoxide in the intraocular chambers provides a significant cataractous insult. Rat lens when maintained in medium generating excessive O is damaged as indexed by deterioration of the cation pump. If O levels are low (ambient levels), peroxidative degradation of lipids has been observed. These reactions are intuitively deleterious to tissue physiology and may contribute to the manner through which light may be toxic to lens in the long run, giving rise to senile cataracts. It was interesting to note that these deleterious reactions could be attenuated greatly by physiological levels of ascorbic acid. The significance of these in vitro studies to the in vitro situatiion as offered by the diurnal habitat of mankind remains yet to be known, but studies simulating such situation in experimental animal models are in progress.
Application of the laser microprobe mass analyzer (LAMMA) to qualitative and quantitative single cell analysis
Authors: Seydel U. , Lindner B.
Published: December 2004

The laser microprobe mass analyzer (LAMMA) is a new type of analytical instrument. It is a combination of a laser microscope with a time‐of‐flight (TOF) mass spectrometer. By focusing an UV pulse laser beam onto the specimen positive and negative atomic and molecular ions are formed which are detected by the TOF mass analyzer. The detection limits reach 10–20 g for some elements in an excited sample volume of 10–13 cm3. With the aim of finding out whether the LAMMA instrument is capable of quantitative measurements on very small biological samples the 23Na+ and 39K+ contents of single mycobacterial cells (M. tuberculosis H 37 Ra) were measured and the results were compared with those obtained from “integral” methods (AAS, NAA, tracer). Furthermore, the influence of different temperatures applied during the washing procedure of the bacteria on the distribution of the Na+/K+ ratio within the cell population was investigated. First attempts to trace isonicotinic acid hydrazide (INH) in bacteria were made by comparison of the LAMMA spectra of INH treated and untreated cells.