bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonlinear dynamics of a model hydrogen bond
Authors: Besnainou S.
Published: January 2005

The dynamics of a model triatomic hydrogen bond are analyzed through classical mechanics. The approximate separability of the equations of motion induces the existence of an adiabatic invariant for the intramolecular mode motion. This mode is governed by a Mathieu equation, a feature already present for the antisymmetric mode in symmetric triatomics. The corresponding spectrum readily obtained as the Fourier transform of a classical trajectory shows that the fundamental frequency is shifted to lower values owing to the anharmonicity of the potential. We observe also a substructure of combination lines generated by a nonlinear resonance with the intermolecular mode. These properties are consistent with experimental observation. In a four‐atoms model, the lines are split when intramolecular Fermi resonance occurs. When the intermolecular mode becomes chaotic there is no vibrational heating of the intramolecular neighboring mode which tends to behave like a local isolated oscillator.
A computational analysis of ortho‐lithiation in aromatic systems
Authors: Jayasuriya Keerthi , Iyer Sury
Published: January 2005

We have analyzed the ability of a tertiary amide group to activate an ortho position on aromatic system toward an electrophilic attack using an ab initio SCF molecular orbital method. The molecular electrostatic potential was used to analyze the electronic nature of the ortho‐lithiated intermediate complex and possible sites for electrophilic attack.
Interaction energy in small N‐particle systems and the permutation group
Authors: Holas A. , Działak H. , Olszewski S.
Published: January 2005

The distribution of energy levels of an N‐identical‐particle system among the irreducible representations of the permutation group is examined for a “spin‐free” problem. There are calculated the sums of powers of the eigenenergies obtained from the original N!‐fold degenerate energy level when the interaction between the particles is switched on. The first‐order perturbation theory is applied. Some results of the present approach are shown to be equivalent to the results of the Weyl's theory of the permutation group. The distribution of energy levels is found to be symmetric with respect to the average energy level belonging to a self‐conjugate representation. For a special case of the Coulomb interaction the wave function of the highest level is fully symmetric, whereas that of the lowest level is fully antisymmetric with respect to the interchange of the coordinates of any two particles. The average energy and the dispersion of the levels belonging to a given representation are calculated for some N.
Core/valence partition and relativistic effects in effective potentials for transition metals
Authors: Pacios L. Fernandez , Calzada P. Gomez
Published: January 2005

Two different versions, relativistic and nonrelativistic, of the “shape‐consistent” effective potential (EP) formalism are analyzed in ab initio calculations for transition metals. The influence of alternative core/valence partitions on the reliability of the procedure is discussed. The accuracy of EP results for transition metals is shown to depend on adequate choices of the valence subspace and proper inclusion of relativistic effects.
Intershell nl4f electron correlation effects
Authors: Jankowski K. , Malinowski P. , Sokołowski A.
Published: January 2005

The pair correlation energies for some nl4f pairs of the ground state of the Yb atom are calculated for the first time. The partial wave (PW) increments to the second‐order pair energies are generated using numerical first‐order radial pair functions obtained as the solution of two‐dimensional differential equations. The analysis of the PWs contributions shows the dominant role of the df, fg, and gh PWs for the 4d4f pair, of the pf and dg PWs for the 4p4f and 5p4f pairs, and of the sf and pg PWs for the 4s4f, 5s4f, and 6s4f pairs. A discussion of the similarities and differences of the structure of the correlation energy found in this paper with those calculated earlier for smaller atoms is given.
Natural energy orbitals and the one‐particle Green's function
Authors: Holleboom L. J. , Snijders J. G. , Baerends E. J.
Published: January 2005

It is shown how the properties of the one‐particle Green's function lead naturally to the definition of the so‐called natural energy orbitals. These orbitals allow the fully correlated total energy of a system to be written in Hartree–Fock‐like fashion and might therefore provide a bridge between sophisticated correlated wave functions and approximate theories of chemical structure and reactivity based on a Hartree–Fock‐like energy expression. Moreover these orbitals form the basis for a self‐consistent scheme to calculate the one‐particle Green's function. The relation between these natural energy orbitals and the extended Koopmans' theorem is considered. Finally it is shown that the exactness of the lowest extended Koopmans' ionization potential implies the linear independence of the corresponding Dyson orbital from all other Dyson orbitals.
Estimation of minima in 1A1 states of the ozone molecule
Authors: Moscardó Federico , Andarias Ramón , San‐Fabián Emilio
Published: January 2005

Calculations have been made at the minima of the X1A1 ground state of the ozone molecule. The equilibrium geometries have been obtained by means of CID calculations. The criterion adopted for the choice of configurations gives realistic results. CIPSI calculations at the two minima lead to an estimated gap of 0.92 eV between them.

Our results agree with the analysis of previous theoretical works on the relative stability of open and cyclic zone structures, showing that the D3h minimum is stable relative to the ground state dissociation limit.
Convergence radii of the perturbation expansions for the ground‐state energies of finite Hubbard models
Authors: Hashimoto Katsufumi , Čížek Jiří , Paldus Josef
Published: January 2005

The ground state energies of finite Hubbard molecules are calculated by numerically solving the Lieb–Wu equations for a complex Hubbard repulsion parameter U. From the positions of the singular points located in the complex plane, the radii of convergence of the perturbation expansions for the ground state energies are determined.
Modified all‐valence INDO/spd method for ground and excited state properties of isolated molecules and molecular complexes
Authors: Lipiński Jóazef
Published: January 2005

A new semiempirical all‐valence method, GRINDOL (Ghost and Rydberg INDO), based on the INDO approximation, is described. Linderberg–Seamans relation (extended to the d and Rydberg orbitals) for the resonance integrals and a new semitheoretical expression for the core‐core repulsion term and energy correction including basis‐set superposition error (intermolecular as well as intramolecular) has been applied. The proposed method enables calculation of ground and excited state properties. The ground state results (including intermolecular interactions) as well as the spectral properties are in reasonable agreement with relevant experimental (or ab initio) studies for isolated molecules, molecular complexes, and transition metal compounds. The method contains only one adjustable parameter, all two‐center integrals and terms are only basis‐set dependent. The one‐center integrals are evaluated from the respective atomic terms.
An indexing scheme for spin‐free configurations of electrons
Authors: Dinesha K. V. , Rettrup S. , Sarma C. R.
Published: January 2005

A simple procedure has been created for indexing the spin‐free configurations of an N‐electron system in spin state S. This indexing has been linked to a procedure for generating the matrix representations of the generators of the unitary group U(n) to enable a direct configuration study to be undertaken.
Structure of consistent ground state: Re—derivation of RPA vacuum and inclusion of higher RPA effects
Authors: Toyoda Kaoru
Published: January 2005

Here is presented a method to determine the consistent ground state (CGS) which satisfies the so‐called killer condition for the excitation operator. This method may be called an extended application of the procedure employed by Weiner and Goscinski in deriving the random phase approximation (RPA) vacuum.

The RPA vacuum is derived by solving the recurrence formula of the configuration coefficients of a multiconfigurational state vector. The role of boson approximation to the primitive p‐h excitation operator is also investigated and by using the present formalism the cluster‐expansion‐type CGS is derived as the RPA vacuum under the boson approximation.

Inclusion of the effects of a higher RPA in the CGS leads to the simultaneous equations of the configuration coefficients of the CGS. In including the effect of the second RPA, only the symmetry‐broken CGS can exist. When the third RPA effect is involved instead of the second RPA, there can be a symmetry‐adapted CGS, in which the picture of electron pairs acquired in the standard RPA vacuum is modified. Thus the exact CGS vectors are analytically obtained in the case of simple model systems of two or four electrons.
Correlation between the conformation of nucleoside antibiotics and their biological activity
Authors: Saran Anil
Published: January 2005

Nucleoside antibiotics, which result as a consequence of minor modifications in pyrimidine and purine nucleosides, exhibit a wide variety of antiviral, antibacterial, antitumor, and cancerostatic properties. The conformational properties of a number of these antibiotics have been investigated by using the quantummechanical PCILO method, and the results indicate that the nucleoside antibiotics and their parent nucleosides have very similar conformational preferences. This similarity is strikingly marked in the situations which prevail in an aqueous medium. As a result, these antibiotics easily get incorporated in growing chains of RNA and DNA by mimicking their parent nucleosides and then bring about the inhibition of protein, RNA, or DNA syntheses. The experimental observations corroborate these deductions, and thus a correlation has been obtained between the conformation and the biological activity of nucleoside antibiotics; it is the striking conformational similarity between the nucleoside antibiotics and their parent nucleosides which gives rise to their biological activity. The PCILO investigations carried out on two 3‐deazapurine nucleosides demonstrate that the converse of the above correlation also holds true.
The change in the dressed potential of a polyatomic molecule in intense photon fields: Simple rules based on the nuclear charge‐mass ratio
Authors: Saito Nakaya , Hall George G.
Published: January 2005

An explicit expression for the dressed potential of a polyatomic molecule, in the adiabatic approximation, is derived. This expression clearly shows the importance of the nuclear charge‐mass ratio (NCMR) for the change of potential due to photon fields. It is found from a simple calculation that the 1H atom is the only atom having an abnormal NCMR value; all other atoms have similar, or the same, values. This means that only those molecules containing a 1H atom should be strongly affected by fields. On the basis of this new physical insight, we postulate two rules, which enable us to classify molecules, with respect to their response to intense photon fields, into three classes: high‐sensitive, low‐sensitive, and insensitive molecules. Qualitative verification is also given by using water isotopes.
Theoretical study of some heterocyclic amines with applications to the chemistry of 9‐amino‐1,2,3,4‐tetrahydroacridine
Authors: Pop Emil , Brewster Marcus E. , Kaminski J. J. , Bodor Nicholas
Published: January 2005

9‐Amino‐1,2,3,4‐tetrahydroacridine (THA), a potent cholinesterase inhibitor, was recently used in the treatment of Alzheimer's disease. On attempting to prepare a dihydropyridine ⇆ pyridinium salt‐based redox chemical delivery system (CDS) to enhance brain delivery of THA, several of the practical synthetic challenges were examined by using a theoretical MO approach. The structures, reactivities and stability of THA, derivatives of THA and a model compound, 4‐aminopyridine, a simple dibasic heterocyclic amine, were studied in the framework of the AM‐1 approximation. The study included the possible protonated forms of THA and 4‐aminopyridine. The calculated heats of formation showed that ring nitrogen protonated forms are more stable for both THA and 4‐aminopyridine. The calculated heats of formation showed that ring nitrogen protonated forms are more stable for both THA and 4‐aminopyridine, consistent with experimental results. Electron delocalization is responsible for the remarkable stability of these molecules and for the observed lack of reactivity of the amino group, both in the basic and protonated forms. The site of N‐alkylation of the 9‐nicotinamide derivative of THA (an intermediate in the synthesis of THA‐CDS) is controlled by electronic, thermodynamic, and steric factors.
New concepts in bonded functions theory. III. Orthogonalization problem
Authors: Gołȩlbiewski A. , Brocławik E. , Witko M.
Published: January 2005

One of possible approaches to the CI method is based on Boys bonded functions which can be generated in a systematic way forming an independent set of high internal symmetry. The main disadvantage of bonded functions is their nonorthogonality. In this paper a scheme is proposed for passing to orthogonalized set of bonded functions together with the appropriate algorithm for the transformation of the energy matrix H. The orthogonalization matrices are shown to reflect high symmetry of the canonical set of bonded functions, and in what follows they can be defined by short vectors. Moreover, the orthogonalization transformation can be handled in a blockwise manner.
Products of class operators of the symmetric group
Authors: Katriel Jacob
Published: January 2005

A combinatorial derivation of the product of the class of three cycles, [(1)N−3(3)]N with an arbitrary class operator of the symmetric group SN is presented. The form of this result suggests a conjecture concerning the expression of the general class operator product in terms of a relatively small number of reduced class coefficients. The conjecture is applied to the determination of the products of [(1)N−4(4)]N, [(1)N−4(2)2]N, and [(1)N−5(5)]N with arbitrary class operators. General expressions for the reduced class coefficients of the simplest type are obtained.
Fifth‐order constant denominator perturbation theory within a localized bond model: Contributions from triple and quadruple excitations
Authors: Cullen J. M.
Published: January 2005

The contributions of the triple and quadruple excitations to the fifth‐order perturbation energy for the perturbation configuration interaction using localized orbitals (PCILO) method are derived. This completes the development of a fifth‐order constant denominator perturbation theory initiated in a previous paper [5] with the single and double excitations. This theory is tested on molecules containing strained ring geometries, stretched bonds, strongly polarized bonds, and delocalized pi systems: cases where the starting zero order reference wave function poorly describes the system. Although the perturbation expansions turn out to be slowly convergent, the Padé approximant taken from an energy series which itself is constructed from Padé approximants provides results accurate to within a few kilocalories/mole of benchmark calculations. Computational times as in the original PCILO procedure remain proportional to N3, where N is the number of bonds.
Explicit formulas for the Hamiltonian matrix elements
Authors: Lim Tiong‐Koon
Published: January 2005

The explicit formulas for the evaluation of the Hamiltonian matrix elements are presented. The calculation of the integral coefficients is independent of both the nature of the orbitals and th spin coupling schemes. It is fully automatic and only dependent on the number of doubly and singly occupied orbitals. Further‐more, the symmetric group representation matrices are not needed, and the N! problem can be avoided.
Recent progress in the understanding of light particle tunneling in metals
Authors: Karlsson Erik B.
Published: January 2005

Light positive particles can be introduced in metallic lattices where they take up interstitial positions. The motion of particles between these sites is usually described as a phonon‐assisted tunneling process, which at high temperatures approaches a classical over‐barrier jump motion.

The present paper reviews briefly the study of such phenomena at very low temperatures where phonon assistance is no longer effective. Experiments on proton transfer between certain sites in niobium metal and on positive muons which diffuse in copper or aluminium lattices show unusual temperature dependencies, which have been explained quantitatively with recent tunneling theories. These approaches take into account the simultaneous dissipation of energy to the electron bath, a phenomenon which was shown by J. Kondo to be determined by the screening electrons following the particle. The temperature dependence is essentially an effect of the Fermi distribution of the conduction electrons. The interaction of the particle with the itinerant electrons will also determine whether the particle wavefunction will be localized or form an extended (Bloch‐like) state.
Microscopic model for heavy‐ion nonrelativistic collisions
Authors: Köhler H. S.
Published: January 2005

A microscopic model for nuclear dynamics has been formulated and applied to the collisions between heavy ions. It may be viewed as an extension of the collisionless time‐dependent Hartree–Fock (TDHF) method to include nucleon‐nucleon collisions. The collision term is written in the form of a time relaxation. The relaxation time is obtained from separate calculations. Some results are shown for the collisions between two 16O nuclei. The computations require a storage of several million words and require a super‐computer. Calculations have been done on the John von Neumann Center (JVNC) CYBER‐205. Future calculations are planned on the JVNC ETA‐10.
Self‐formation and evolution of singletons
Authors: Wilhelmsson H.
Published: January 2005

New solutions for reaction‐diffusion equations, called singletons and polytons, are found to be of fundamental significance. They exhibit properties of self‐formation. In the presence of nonlinear saturation effects their evolutions in time have pronounced maxima. Information about details of an initial spatial distribution is found to be carried along in time with the dynamic process of forming a growing singleton and to be restored at a later stage.
On the validity of the mass–velocity operator in quantum chemistry
Authors: Hui‐Yun Pan , Sen Zhao Zu
Published: January 2005

The problem of the validity of the mass–velocity operator in computational quantum chemistry is discussed. The opinion that the mass–velocity operator is incorrect is shown not to be well founded.
Spin adaptation of antisymmetrized geminal product wave functions
Authors: Sarma C. R. , Paldus J. , Öhrn Y.
Published: January 2005

A spin‐free polynomial representation of antisymmetrized geminal products is presented for several cases. In particular, products of identical geminals, which possess different spin multiplicity, are considered. The cases of singlet geminals, singlet geminals with one or two triplet geminals coupled to the lowest possible spin multiplet, and triplet geminals coupled to an arbitrary multiplet are considered in detail, and explicit polynomial representation is given.
On the Ricatti equation for eigenvalue problems
Authors: Fernández Francisco M. , Castro Eduardo A. , Ogilvie John F.
Published: January 2005

The Riccati equation is shown to be suitable for obtaining implicit approximate analytic expressions for the eigenvalues of quantum‐mechanical systems. The Hamiltonian operator H = (1/2)p2 – (Z/r) + λr2 is used as a test example, and the resulting formulae are modified to deal with the Zeeman effect in hydrogen.
Improved intermolecular SCF theory and the BSSE problem
Authors: Mayer I. , Surján P. R.
Published: January 2005

Analytical and numerical studies are performed concerning the exclusion of the basis set superposition error (BSSE) from the SCF calculations of intermolecular interactions. Based on these studies a new procedure is proposed, which consists of the following steps: (1) determine the orbitals by the SCF scheme based on the recent “chemical Hamiltonian approach” (CHA‐SCF method), i.e., excluding the delocalization effects caused by BSSE, and then (2) calculate the usual energy expectation value. (This gives results superior to those obtained by the previous nonsymmetric CHA energy formula.) The actual numerical calculations performed for different simple systems (He2, water dimer) by using various basis sets indicate that the CHA/CE (CHA with “conventional energy” formula) potential curves are well‐balanced and are close to those obtained by the Boys–Bernardi (BB) method and usually (but not necessarily) go slightly beyond the latter. So our method gives results better than (or close to) those given by the BB method by performing only a single ∼N4 calculation at each geometrical arrangement of the system.
The stochastic theory and its relations to the general reaction rate theory
Authors: Christov S. G.
Published: January 2005

A short review of the recent development of the stochastic approach to chemical reactions in condensed media is made. The relations of this approach to a general formulation of chemical kinetics are discussed on the basis of a many‐frequency oscillator model. It is shown that the classical transition‐state theory is not a particular case of the stochastic theory neither in the case of small viscosity nor in the case of large viscosity. The recent quantum generalizations of the stochastic theory are identical to earlier results of a quantum transition‐state theory that correspond to the case of large viscosity. Such generalizations based on previous results of a collision theory are made for the case of small viscosity. It is shown that these quantum generalizations of the stochastic theory are possible only in the temperature range of moderate tunneling defined in terms of a characteristic temperature.
Self‐interaction‐corrected electronegativities and hardness for atoms
Authors: Goycoolea Candelaria , Barrera Mauricio , Zuloaga Fernando
Published: January 2005

Electronegativity χ and hardness η for 54 atoms and their positive and negative ions are calculated by means of self‐interaction‐corrected DFT including correlation terms. The exchange potential energy is treated by local spin density approximation corrected to account for self‐interaction effects as suggested by Rae. The highest occupied orbital eigenvalues for ions are identified to the chemical potential μ± for positive and negative charged atoms depending upon the developing charge process. Values of χ±δ and η± for the different ionic species are given for several values of δ. Average values for 〈χ〉 and 〈η〉 in the sense of Mulliken finite formula for neutral atoms are also tabulated and compared with Mulliken values from experimental data. The agreement among them is almost quantitative.
Are oxygen rings (On) and their negative ions (O) unstable?
Authors: Sinanoğlu Oktay
Published: January 2005

Oxygen clusters of the equilateral ring types On and their negative ions O are studied by the recently developed pictorial‐topological quantum chemistry method, VIF (valency interaction formulas), of this author. The species are found to be of high energy relative to separated oxygen atoms, the cause being evident from the VIF pictures. The odd n rings should be at local minima but are not likely to have observable negative ions. The even n rings have distortional instabilities, yet their negative ions are more likely to be observed as transients in beam experiments.
Application of box quantization to the determination of the dissociation rates of a system of two coupled morse oscillators
Authors: Lefebvre R. , Atabek O. , Garcia‐Sucre M.
Published: January 2005

A well‐known model of two coupled Morse oscillators is studied within the context of box quantization. Two variants are considered: analysis of stabilization graphs or addition of an optical potential. These methods are shown to give efficient alternatives for the determination of unimolecular dissociation rates.
Symmetric and symmetry‐broken Hartree–Fock solutions of one‐dimensional models
Authors: Hu Chengzheng , Monkhorst Hendrik J.
Published: January 2005

Pariser–Parr–Pople Hartree–Fock crystal orbital calculations (both restricted and unrestricted versions) are performed for one‐dimensional models of π‐electron polymers. The π‐electron band structures corresponding to symmetric and nonsymmetric solutions have been calculated. To investigate the influence of the form of the interelectronic interaction, the calculations were performed with the Mataga–Nishimoto (MN) formula and a modified Mataga–Nishimoto (MMN) formula for two‐electron integrals. We have found that curves of the points of the minimum total energy per unit cell for the alternating models are very similar, but band structures and energy gaps are quite different when the MN formula was substituted with the MMN formula for two‐electron integrals.
The origin of energy functional in Roothaan open shell SCF theory
Authors: Klimko G. T. , Mestechkin M. M. , Plakhutin B. N. , Zhidomirov G. M. , Evarestov R. A.
Published: January 2005

Different methods of averaging of energy over the states of electronic configurations γN (nγ = 1, 2, 3 and N = 1, 2, …, 2nγ − 1) leading to Roothaan' energy expression are considered. The consequent values of vector coupling coefficients (VCC) in energy functionals for various states as well as for average values of energy are presented.

It is shown also that in molecular systems of cubic and tetragonal symmetry having electronic configurations tN (N = 2–4) and e2 there exist states for which VCC are dependent on the choice of basis set of degenerate open‐shell molecular orbitals. The origin of such “non‐Roothaan” terms and peculiarities of its calculation by the restricted Hartree–Fock method are discussed.
Calculation of integrals with slater basis from the one‐range expansion of the 0s function
Authors: Rico J. Fernández , López R. , Ramírez G.
Published: January 2005

A one‐range expansion of the 0s‐STO—previously reported by us—is employed to derive explicit expressions for the elements of auxiliary matrices—appearing in the calculation of one‐ and two‐electron multicenter integrals with STOS—in the form of infinite series. Then, we are able to perform the formal summation of the series giving both closed expressions for these elements and several relations between them. A computational algorithm based on these formal developments is proposed and its numerical behavior is analyzed.
Graphical and color‐pairing symmetries
Authors: Klein D. J. , Živković T. P.
Published: January 2005

Symmetries are considered for a class of Hamiltonian models with one (spin‐free) orbital per site. The models include common types of Parisier‐Parr‐Pople and valence‐bond Hamiltonians, defined over a continuous range of parametrizations. The symmetries investigated are linear canonical transformations and include the common point‐group and alternancy symmetries. We find “graphical” symmetries extending the usual point‐group symmetries and novel “color‐pairing” symmetries which involve hybrids of point‐group–like and alternancy symmetries of relevance for certain heteroatomic species. The occurence of recolorpairing transformations relating the eigensolutions of models for different molecules is also noted.
On the difference in bond orders between HMO and PPP methods
Authors: Randić Milan , Plavšića Dejan , Trinajstić Nenad
Published: January 2005

We have examined variations in bond orders in conjugated hydrocarbons that follow improvement in MO calculations on going from topological Hückel MO to self‐consistent Pariser‐Parr‐Pople–type calculations. Differences in individual bond orders can be interpreted as pointing to increased weights of selected Kekulé valence structures. Significantly, so indicated valence structures are those with the largest number of formal Kekulé benzene rings or the largest contribution to molecular resonance energy in terms of conjugated circuits.
PPP (Peel) calculations on iron porphyrins and iron phthalocyanine. Charge distribution and charge transfer
Authors: Henriksson‐Enflo A.
Published: January 2005

Semiempirical molecular orbital calculations within a modified PPP method, the Peel method, have been performed in order to study the electronic structure and properties of porphyrins and phthalocyanines. It is found that the calculations can reproduce the results from more sophisticated ab initio calculations. It is found that the electronic reactions may take place at the metal atom and in the axial direction. The role of the porphine plane seems to be that of an electron buffer, keeping the electronic population in the porphyrin plane constant. A mechanism is suggested where electron transfer reactions can take place by change in the molecular geometry, as for example by vibrations.
Quantum‐chemical investigation of Buckminsterfullerene and related carbon clusters (I): The electronic structure and UV spectra of Buckminsterfullerene, and other C60 cages
Authors: Feng Jikang , Li Jun , Wang Zhizhong , Zerner Michael C.
Published: January 2005

The electronic structure of C60 cages are examined by use of the INDO and INDO/CI methods. The calculated spectrum of Buckminsterfullerene has one allowed low‐lying band at 27,300 cm−1, to be compared with an experimental value of 25,900 cm−1. These calculations suggest that other cage structures are not stable, or they are not ground state singlets.
Alternative implementation of the unitary group approach to the atomic shell theory
Authors: Zhenyi Wen , Yubin Wang , Zhiyong Zhang
Published: January 2005

Subduction coefficients adapted to the group chain, which appeared in Racah's treatment of fn configurations, are defined and calculated in the unitary group approach. The coefficients are then utilized to construct successively adapted term functions and evaluate other interesting coefficients. In addition the simplified expressions for the Coulomb and spin‐orbit operators are obtained in terms of generators.
The use of localized molecular orbitals and the polarization propagator to identify transmission mechanisms in nuclear spin‐spin couplings
Authors: Diz A. C. , De Azua M. C. Ruiz , Giribet C. G. , Contreras R. H.
Published: January 2005

An extension of the IPPP (inner projections of the polarization propagator) method to theoretically analyze transmission mechanisms of indirect nuclear spin‐spin couplings is presented. The localization technique used is modified so that all the canonical molecular orbitals of a compound may be localized to represent chemical bonds, lone pairs, and the corresponding antibonding molecular orbitals. These localized molecular orbitals, together with the polarization propagator, are used to obtain an intuitive picture of how a coupling is generated as a sum of terms, each one consisting of two particle‐hole single excitations. This picture can be used to identify underlying transmission mechanisms and quantitatively evaluate their importance toward the total coupling. The paramagnetic spin‐orbit and the spin‐dipole interactions are studied in detail.
Computational models for proton transfer in biological systems
Authors: Pardo Leonardo , Mazurek Aleksander P. , Osman Roman
Published: January 2005

A computational scheme based on a “mixed basis set” approach is applied to the study of the structure and the energetics in proton transfer systems. Five hydrogen‐bonded systems of the type (CH3HnA ‥ H ‥ BHmCH3)+, where A and B can be N, O, or S, have been investigated with various minimal and extended basis sets. Calculations with the extended basis set yield double‐well potential energy curves, which the minimal basis set is unable to reproduce. Calculations with the mixed basis set, constructed from an extended basis set on the atoms engaged in the hydrogen transfer part and a minimal basis set on the rest of the molecule, give predictions of geometries, potential energy curves, and relative energies similar to the results from the extended basis set. Inclusion of polarization functions in the mixed basis set becomes essential in systems that contain third row atoms. This scheme should become useful in studies of large molecules in which different parts can be represented at different levels of computational complexity.
Vibronic spectral behavior of molecules. XIII. Theoretical contribution to the vibronic coupling and the dushinsky effect on the S1 ← S0 absorption and the S1 → S0, S2 → S1, and S2 → S0 fluorescences of azulene
Authors: Gustav Klaus , Storch Michael
Published: January 2005

Based on the completely optimized S0, S1, and S2 molecular geometries of azulene, the vibronic structure of the S1 ← S0 absorption as well as of the S1 → S0, S2 → S1, and S2 → S0 fluorescences is investigated theoretically within the adiabatic approximation. By means of theory‐experiment comparisons, the influence of non‐Condon terms and of the Dushinsky effect on the vibronic structure of azulene spectral behavior is discussed. Typical for the S1 ← S0 absorption and the S1 → S0 fluorescence are vibronic transition moment contributions of Condon type, whereas the interpretation of azulene S2 → S1 and S2 → S0 fluorescences is successful only within the scope of the Herzberg–Teller approach by taking into account vibronic coupling terms and, additionally, the Dushinsky effect in the latter case. An analysis of the relevant vibrational modes is given.
Computational chemistry on a PC
Authors: Náray‐Szabó Gábor , Ángyán János G. , Surján Péter R. , Szalóczy Zsolt , Ösapay Klára , Kövesdi István , Kolossváry István
Published: January 2005

We describe a package of some IBM PC programs that may find application in computer‐aided molecular design. PCGEOM constructs and visualizes molecular models from bond lengths, bond angles, and dihedral angles, from Cartesian coordinates, or from stored fragments. It may prepare output files to be used as input for other programs, like CNDOB (conventional CNDO/2) or PCMEP using the bond increment (BI) method for the calculation of molecular electrostatic potentials. PCPROT is in preparation and will use Protein Data Bank coordinates to visualize and manipulate protein molecular models. Starting from these, it will calculate electrostatic potentials using the BI method and/or monopoles adjusted to reproduce ab initio values for amino acid residues. FSCF is based on a CNDO‐type approximation and uses strictly localized molecular orbitals in order to partition large molecules into a central fragment, a polarizable region, and a fully transferable environment. The partition allows one to handle relatively large systems with up to 200 atoms. To illustrate applications, we present estimation of relative inhibitory potencies of a series of substituted triazines on chicken liver dihydrofolate reductase.
Localization of virtual orbitals
Authors: Bartha Ferenc , Bogár Ferenc , Kapuy Ede
Published: January 2005

The application of the MBPT in the localized representation requires that both the occupied and the virtual orbitals obtained by the canonical HF equation should be localized. The localization of the occupied orbitals is straightforward in general by any localization method. It is shown that by using Boys' method the localized virtual orbitals are spatially well separated and transferable not only in minimal basis sets.
Double bonds in the second and third periods. Ab initio study of the conjugation in HnX  YHmZH2 (X, Y = C, N, SI, and P; Z = B and N; n = 1, 2; m = 0, 1)
Authors: Korkin A. A.
Published: January 2005

Ab initio SCF calculations were performed to study the conjugation of C, N, Si, and P double bonds with BH2 (π‐acceptor) and NH2 (π‐donor). The variations of the energy, geometry, and electronic distribution on rotation ZH2 groups connected to the double bonds depend greatly on the polarities and polarizabilities of the molecules under study. The repulsive (attractive) interactions of the lone pairs lying in the plane of the double bond with donor (acceptor) orbital can modify strongly the relative stabilities of the conformations and the parameters of the molecule and electronic structures.
Nonstandard turn of MDGI protein binding site of fatty acids?
Authors: Suessmilch R. , Jaeger J. , Behlke J.
Published: January 2005

The growth‐regulating factor MDGI belongs to the fatty acid binding proteins of which the amino acid sequence is known. Starting from hydrodynamic and dichroic measurements which yield estimations about mass, diameter, thickness, and content of α and β structure of the MDGI protein, we used theoretical methods to calculate the structures and electrostatic properties of selected β‐chain sequences. The hydrophobic character of possible binding sites for fatty acids is demonstrated by the molecular electrostatic potential and field of this structures. A nonstandard turn element, formed by an H‐bridge between polar side chains, Lys‐Leu‐Gly‐Val‐Glu, which connects two extended β chains, was found. Bound oleic acid could be able to stabilize such turn structure, which seems to appear also at the entrance of the cavity of two other proteins. The optimization of an oleic acid‐turn van der Waals complex results in an interaction energy of about –50 kJ/mol originating mainly from the dispersion term and shows the participation of the four hydrophobic side chains Phe 64, Leu 66, Val 68, and Phe 70. Of these, at least Phe 64 and Phe 70 are known as possible binding sites of fatty acids.
1S core‐level shifts of Al and Ar atoms in aluminum clusters
Authors: Varga Zs. , Gyémánt I. K.
Published: January 2005

Theoretical 1s core‐electron binding energies are presented for Al and Ar atoms in free space and in AlAl12, AlAl12Al6, and ArAl12 clusters. The binding energies have been calculated by the self‐consistent field Xα scattered‐wave (SCF Xα SW) method using various exchange parameters and different atomic‐sphere overlaps. The atom/cluster binding‐energy shifts have been obtained both from the Slater's transition‐state energies and from the total‐energy differences; these values are in better agreement with each other if calculated with proper overlapping than if with nonoverlapping spheres. A comparison with available experimental and theoretical data is given as well.
Optimization of molecular electronic structure calculations. 2. Calculation of symmetry‐reduced matrix elements
Authors: Kovrikov A. B.
Published: January 2005

A suggested formalism of the local symmetricized orbitals in conjunction with the selection technique for independent blocks of integrals in an original basis is used for a construction of multielectron Hamiltonian matrix elements in the symmetry orbital basis. The optimal molecular electronic structure calculation algorithm with the Hartree–Fock–Roothaan method in the symmetricized basis was obtained as a result. The minimal number of fundamentally distinguished (symmetry attributed) elements both in original and in symmetricized basis is used in the calculations.
Electron correlation described by extended geminal models: The EXGEM7 and EXRHF3 models
Authors: Røeggen I.
Published: January 2005

Within the framework of the general extended geminal model, two new approximate models, EXGEM7 and EXRHF3, are introduced. Compared with previous models, these new models imply a more sophisticated approximation of the four‐electron terms {ϵKL}. The models are tested by calculations on the beryllium atom and the HF molecule.
Methods for electron correlation on large molecules
Authors: Sæbø Svein
Published: January 2005

High‐quality ab initio calculations, i.e., calculations including electron correlations with large atomic basis sets, have been restricted to relatively small systems due to their excessive demand on computer facilities. To extend these methods to large systems, one is faced with two problems: the steep (6 power) increase of the computational task with the molecular size, and the large number of electron repulsion integrals that must be stored and retrieved. We have recently introduced new computational methods where both these problems have been addressed. The local correlation method reduces the steep dependency of computational effort with molecular size without significant loss of accuracy by taking advantage of the localizability of the electronic structure. For electron correlation calculations integrals in the MO basis are needed in addition to the two electron integrals in the AO basis, and the only way electron correlation can be applicable to large systems is by a direct approach in the spirit of Almlof's direct SCF. We have recently extended the direct approach to post Hartree–Fock methods. Description of both these methods including their supercomputer implementations are discussed.
Localized orbitals for the description of molecular interaction
Authors: Kozmutza C. , Kapuy E.
Published: January 2005

The intermolecular interaction between the molecules CH2O and NH3 was investigated by the supermolecule method. The interaction energies were first calculated at the ab initio SCF level, and the electron correlation was included via second‐order Møller‐Plesset perturbation theory (MP2). The basis set superposition error (BSSE) was taken into account by the counter‐poise (CP) method. The occupied and the virtual canonical molecular orbitals (CMOS) of the supermolecule were separately localized by the Boys' procedure. The correlation correction was calculated by the many‐body perturbation theory (MBPT) in the localized representation. Contributions of the third‐ and fourth‐order localized diagrams were added to those of the second‐order canonical diagram. This procedure gives a correction nearly equivalent to that of MP2. The possibility to separate LMO contributions responsible for the dispersion interaction was investigated.
Topological analysis of eigenvalues of particle in one‐ and two‐dimensional simple quantal systems: Net sign approach
Authors: Lee Shyi‐Long , Li Feng‐Yin , Lin Friday
Published: January 2005

Simple net sign analysis has been carried out for particle in one‐ and two‐dimensional boxes and harmonic oscillators. The conclusion that no simple rule for the ordering of eigenvalues of two‐dimensional systems can be drawn is conjectured. Cyclobutadiene, naphthalene, benz‐cyclobutadiene, biphenylene, biphenyl, and p‐terphenyl are found to behave topologically as particle in either type of potential. Benzene, anthracene, naphthacene, and pentacene are found to behave topologically as particle in a two‐dimensional harmonic oscillator. This analysis provides the physical basis for the net sign approach in the signed graph theory.
Transition properties from multiple scattering X‐Alpha Calculations: A study of NO2
Authors: Howard S. T. , Huke J. P. , Parsons I. W. , Till S. J.
Published: January 2005

The usefulness of the MS‐Xα method for calculating molecular transition properties is investigated in a study of NO2. Two‐dimensional potential surfaces are obtained in C2v symmetry for the X̃2A1/Ã2B2 system. Potential curves that approximately follow the asymmetric stretch coordinate are also presented: On investigation, no symmetry‐broken solutions were found. At all nuclear geometries investigated, the muffin‐tin sphere radii were chosen by an energy‐minimization technique. The inclusion of corrections to the muffin‐tin form of the Xα total energy is shown to be essential, if the computed surfaces are to be qualitatively correct. The transition dipole moment connecting the two states is evaluated as a function of nuclear coordinates, using the transition‐state approximation. Finally, the radiative decay of a single vibronic level in NO2 is modeled and the relevance to experiment is discussed.
The system of harmonically interacting particles: An exact solution of the quantum‐mechanical problem
Authors: Osadchii M. S. , Murakhtanov V. V.
Published: January 2005

An original procedure has been developed to determine the eigenvalues and the eigenfunctions of quantum‐mechanical many‐body systems with a harmonic oscillator interaction. Exact solutions in the explicit form have been obtained for the case of systems containing three groups of identical particles. For the system containing two groups of identical particles, a solution in the adiabatic approximation has been obtained and compared with the exact one.
A general treatment of vibration‐rotation coordinates for triatomic molecules
Authors: Sutcliffe Brian T. , Tennyson Jonathan
Published: January 2005

An exact, within the Born–Oppenheimer approximation, body‐fixed Hamiltonian for the nuclear motions of a triatomic system is presented. This Hamiltonian is expressed in terms of two arbitrarily defined internal distances and the angle between them. The body‐fixed axis system is related to these coordinates in a general fashion. Problems with singularities and the domain of the Hamiltonian are discussed using specific examples of axis embedding. A number of commonly used coordinate systems including Jacobi, bond‐length‐bond‐angle, and Radau coordinates are special cases of this Hamiltonian. Sample calculations on the H2S molecule are presented using all these and other coordinate systems. The possibility of using this Hamiltonian for reactive scattering calculations is also discussed.
A correction to effective core potentials for transition metals
Authors: Pacios L. Fernandez , Olcina V. Botella
Published: January 2005

The correct description of energetic separations between atomic states arising from s2dn, s1dn+1, and dn+2 electronic configurations is known to be essential to treat transition metals. Effective core potentials calculations for first‐row transition elements with an Ar core show important errors in these energies when compared with all‐electron values. A procedure to eliminate errors while maintaining the formalism unaltered and retaining the ar core is proposed.
Charge‐transfer intensity in complexes with symmetrically equivalent acceptors
Authors: Parker W. L. , Crosby G. A.
Published: January 2005

Mulliken's model for the intensity of charge‐transfer absorption bands in donor/acceptor complexes is extended to transition‐metal compounds. Attention is focused on the analysis of systems containing a central metal ion and symmetrically equivalent π‐acceptor ligands. The theory is illustrated for a one‐donor/two‐acceptor complex in D2d point symmetry. Results are stated for symmetries of commonly occurring transition‐metal compounds that display well‐documented metal‐to‐ligand charge‐transfer transitions.
Model study of the abrupt excess‐energy dependence of radiationless decay in benzene and azabenzenes
Authors: Sobolewski A. L. , Lim E. C. , Siebrand W.
Published: January 2005

Model studies are reported aimed at accounting for the abrupt dependence of radiationless decay‐rate constants on excess energy (known as channel‐three decay in the case of S1 benzene) in singlet and triplet manifolds of benzene and azabenzenes. The favored model involves La(ππ*) state, strongly distorted along an out‐of‐plane coordinate, crossing the ground‐state potential at an energy close to the minimum of the lowest excited state. The results are compared with experimental observations on benzene and three azabenzenes. Implications for photochemical reactions are also discussed.
Approximate separation of the hyperradius in the many‐particle Schrödinger equation
Authors: Avery J. , Goodson D. Z. , Herschbach D. R.
Published: January 2005

For large values of d = 3N, the radial distribution function of an N‐particle system is sharply peaked near the hyperradius rm = (d − 2)/2k0, where k0≡(2/E/)1/2. This fact allows an approximate separation of the hyperradius, leading to many‐dimensional hydrogenlike radial solutions. Kindred applications to dimensional scaling are also discussed, where d = DN, with D the spatial dimension. For the large D regime, illustrative analytic formulas are obtained giving the energy and effective nuclear charge for the lowest few S states of the helium atom.