bluuebunny/crossref_metadata_2025 huggingface dataset:
Quantum‐Chemical modeling of catecholamine storage including continuum solvent effects
Authors: Góamez‐Jeria J. S. , Parra‐Mouchet J. , Morales‐Lagos D.
Published: January 2005

A model for catecholamine storage in vesicles is analyzed within the SCRF‐CNDO/2 approach including continuum solvent effects. The model considers the approach of cationic norepinephrine (NE) to a positively charged guanidinium moiety. Ion‐pair formation is found for the whole range of dielectric constants. Even though stable states of H‐bonded partners are found for large dielectric constants, this process is ruled out to occur because it involves too high energies. It appears that the medium's polarity is determinant in lowering the energy barrier between the ion‐pair complex and the separated partners. Thus, as the medium dielectric constant increases, the equilibrium between the two states is enhanced.
Normalized irreducible tensorial matrix expansions applied to an effective sp‐type Hamiltonian for the PES of water
Authors: Ellzey M. L.
Published: January 2005

Any matrix can be expanded on a basis of SU(2) normalized irreducible tensorial matrices, NITM, defined in terms of 3‐j symbols or coupling coefficients of SU(2). The NITM transform under rotations according to Wigner's matrices. If one dimension of an NITM is odd and the other even, the tensor has half‐integer rank. A simple NITM basis consists of all NITM having the same numbers of rows and columns as the expanded matrix. A compound NITM basis consists of two or more simple bases, each spanning a corresponding block in the expanded matrix. The choice of NITM basis for expanding an effective Hamiltonian matrix is a crucial step in formulating a model. To illustrate the use of a compound NITM basis, including nonsquare NITM, an effective sp‐type overlap‐free superposition Hamiltonian is constructed and applied to the photoelectron ionization potential spectrum of water.
Second‐order correlation potential in the Kohn–Sham approximation for atoms
Authors: Boada R. L. , Karasiov V. V. , Labzowsky L. N.
Published: January 2005

A new type of correlation functional derived from the second‐order expression for the correlation energy of an atom is proposed. The derived correlation potential contains one free parameter, which is determined by fitting the known pair correlation energy. The calculations with this potential in the Kohn–Sham approximation give rather accurate values for the matrix elements of different operators.
Second‐order perturbational treatment of normal coordinates in the string model for the hydration reaction of formaldehyde
Authors: Tachibana Akitomo , Fueno Hiroyuki , Yamato Masaya , Yamabe Tokio
Published: January 2005

A perturbation theory for normal coordinates of nonadiabatic solvation is presented by means of the “string model” of chemical reactions. The dynamic normal coordinate is introduced for the perturbational treatment of the “intrinsic” normal coordinates that are orthogonal to the reaction path. The reaction is defined as the intrinsic reaction coordinate (IRC) that is treated as a string. The string is thrown in the external force field that acts as a nonadiabatic source of perturbation. As an application of the present treatment, the effect of a water molecule for hydration reaction of formaldehyde is calculated. A second‐order perturbation effect for the enhancement of the reaction rate is found.
A quantum chemical investigation of pyrolysis reactions of glyoxylic acid ethylester
Authors: Bøhm Stanislav , Skancke P. N.
Published: January 2005

Two possible reaction paths for the pyrolysis of the ethylester of glyoxylic acid have been studied by ab initio molecular orbital calculations. The basis sets 3‐21G and 6‐31G* have been used, and electron correlation has been included by Møller–Plesset calculations up to fourth order. Our calculations indicate that the reaction leading to acid and ethylene through a 6‐membered ring transition state is favored relative to a process involving a formyl hydrogen transfer via a 5‐membered ring to the alkyl unit leading to ethane, CO, and CO2. The predicted activation energies for these two reactions obtained at the highest level of calculation, MP4(SDTQ)/6–31G*, are 50.4 and 71.7 kcal/mol, respectively. The transition states have RHF wave functions that are stable relative to UHF solutions using the 3–21G basis. The geometry of the transition states and IRC following indicate that both reactions are strongly asynchronous: The CO bond rupture is virtually completed before hydrogen transfer occurs. For comparative purposes, analogous calculations have been performed for the ethylester of formic acid, where it is confirmed that a 6‐membered ring transition state is preferred relative to a 4‐membered one by around 42 kcal/mol at the highest level of calculation.
Analysis in terms of valence‐bond structures of environmental effects on the electronic structure of molecules
Authors: Solà Miquel , Lledòs Agustí , Duran Miquel , Bertràn Juan
Published: January 2005

Environmental effects on the electronic structure of molecules caused by external perturbations are studied by means of an expansion of molecular orbital wave functions into valence‐bond (VB) wave functions. Applications are carried out to the π system of butadiene, hydroxyethylene, acrolein, and carbonyl oxide and also to the full electronic system of water. The external perturbations considered are uniform electric fields, nonuniform electric fields, and reaction fields created by solvents. A different behavior is found between nonpolar and polar species in the presence of solvents. Some consequences on the behavior of molecules under the influence of the different electric fields are discussed.
A time‐dependent molecular orbital approach to electron transfer in ion–metal surface collisions
Authors: Feng Eric Q. , Micha David A. , Runge Keith
Published: January 2005

A time‐dependent molecular orbital method has been developed to study charge transfer in collisions of ions with metal surfaces at energies between 1 and 100 au. A set of localized basis functions consisting of generalized Wannier functions for the surface and s‐ and p‐atomic functions for the ion, is used to separate the system into primary and secondary regions. An effective Hamiltonian and time‐dependent equations for the electron density matrix are obtained in the primary region, where most charge transfer occurs. The equations for the electron density matrix are solved with a linearization scheme. The method is suitable to study atomic orbital orientation for collisions of ions and surfaces. A model calculation for Na+ + W(110) collisions with a prescribed trajectory is presented. The interaction potentials between the W(110) surface and Na+ 3s and 3p orbitals are calculated from Na+ pseudopotentials. Results show that the yield of neutralized atoms in 3p states changes as the collision energy is lowered.
On the conditions for physical admissibility of Schrödinger wave functions
Authors: Hui‐Yun Pan , Sen Zhao Zu
Published: January 2005

It is shown that the dispute about the conditions for physical admissibility of Schrödinger wave functions may be settled if the following two questions are clarified: (a) Are these condittions rooted in the foundations of quantum mechanics or invoked only for particular physical models? (b) Do we consider either the position‐space wave function or the momentum‐space wave function? Both the case of closed states and scattering states are considered. Some general remarks about these conditions are made in the Conclusion.
Pauling's criterion of bond strength and the relative bond lengths in molecule MLk
Authors: Zhan Chang‐Guo
Published: January 2005

In this paper, the bond strengths, defined by Pauling, for a series of molecules in the type of MLk have been calculated by using the generalized method obtained from the maximum overlap method in a preceding paper and by using Pauling's pair–defect–sum approximation. A number of geometrical bonding situations are investigated. It is demonstrated why a previous study purporting to use of Pauling's criterion of bond strength to find that the axial bonds in trigonal bipyramidal ML5 are stronger than the equatorial bonds is incorrect. The results obtained from the two methods approach each other and are in good agreement with the experimental bond lengths, which show that Pauling's criterion is viable and that the pair–defect–sum approximation is indeed an excellent one that agrees with the maximum overlap method.
Variational calculation of vibrational energies of triatomic molecules using SCF optimized modes
Authors: Hidalgo Antonio , Zúñiga José , Francés José M. , Bastida Adolfo , Requena Alberto
Published: January 2005

A self‐consistent field optimization of the vibrational coordinates for nonlinear triatomic molecules is presented. The optimal coordinates are obtained by making a three‐dimensional rotational transformation of the normal modes and determining the rotation angles as those for which the SCF energy is stationary. The utility of the optimized coordinates in full variational calculations of vibrational energies is studied for the molecules of H2O, O3, H2D+, H2T+, and D2T+. For H2O and O3, the optimization procedure leads to the local mode representation. It is shown that the use of the optimal coordinates in variational calculations allows a large reduction of the dimension of the Hamiltonian matrix to be diagonalized in order to reach convergence.
Transforming Gaussians into Wannier functions using inversion of cyclic matrix
Authors: Iyakutti K. , Thiagarajan M. , Kumarasamy K.
Published: January 2005

One‐dimensional Gaussians are transformed into orthonormalized atomic orbitals (OAOS) using the inverse of the cyclic overlap matrix. During that process it was found that these OAOS are nothing but the Wannier functions and have properties similar to the Wannier functions obtained by Wannier himself from Gaussians using a different method. The present method gives rise to a better Wannier function with less distortion.
Normalized irreducible tensorial matrices and the Wigner–Eckart theorem for unitary groups: A superposition hamiltonian constructed from octahedral NITM
Authors: Ellzey M. L.
Published: January 2005

The concepts of normalized irreducible tensorial matrices (NITM) are extended to all finite and compact unitary groups by a development that clarifies their relationship to group theory and matrix algebra. NITM for a unitary group G are shown to be elements of a basis obtained by symmetry adapting to G the matrix basis of a matrix space M(α1 × α2). Elements [X] ∈ M(α1 α2) transform under Ga ∈ G according to [Ga] [X][G−1a], where [Ga] and [G−1a] belong to irreducible representations of G. The usual properties of NITM and the Wigner–Eckart theorem follow from these results, which are valid for both finite and compact unitary groups. The NITM span M(α1 × α2) are orthonormal under the trace and transform irreducibly with respect to G. This NITM basis of M(α1 × α2) is said to be simple. A compound NITM basis of a matrix space results when the space is partitioned into two or more subspaces, each spanned by a simple NITM basis. NITM determined from Griffith's V coefficients for the octahedral group are tabulated and used to construct a six‐coordinate superposition Hamiltonian.
Valence and hypervalence in compounds with second‐row elements
Authors: Jug Karl , Fasold Eckhard
Published: January 2005

A recently introduced concept of charge and valence for ab initio wave functions is applied to molecules with second‐row elements. Mulliken and Löwdin charges and their valence counterparts by Mayer and Gopinathan‐Jug are calculated for selected molecules with 4‐31G and 6‐31G* wave functions and compared with results derived from the new valence concept. It is concluded that the term hypervalence can be reasonalby used in phosphorus compounds PF5 and PCl5 and sulfur compounds SO3 and SF6.
Molecular equilibrium geometries and vibrational frequencies by maximum overlap symmetry molecular orbital method
Authors: Zhan Chang‐Guo
Published: January 2005

It is demonstrated that the maximum overlap symmetry molecular orbital method can be used for optimization of molecular geometries and calculation of vibrational frequencies by adding a two‐body repulsive energy term and a modification of the Wolfsberg–Helmholz formula. The obtained equilibrium geometries and vibrational frequencies are on the whole in good accordance with experimental data, which shows that the basic idea using the method to optimize molecular geometries and to calculate vibrational frequencies is reasonable.
Vibronic dressed electronic states and nonadiabatic effects in polyacetylene
Authors: Lü Tian‐xiong , Tachibana Akitomo , Yamabe Tokio
Published: January 2005

The nonadiabatic effects in the dynamic Fock equation (DFE) have been studied for the model system, polyacetylene, within the Hückel framework. The validity of the perturbation treatment of DFE has been investigated. It is followed by a proposed iterative procedure for solving the DFE. With this procedure, the self‐consistent vibronic dressed electronic states (VDS) of polyacetylene have been obtained and described. It has been shown that (1) the pairing theorem for an alternant chain remains in the VDS of polyacetylene; (2) the modification of orbital energy due to the nonadiabatic effects decreases with the decreasing of absolute value of adiabatic orbital energy, implying that the energy gap originating from the nonadiabatic effects is too small to be significant; (3) the nonadiabatic mixing happens only for those adiabatic states having the same symmetry; and (4) a dynamic resonance exists between the starred and unstarred atoms, which contributes a dynamic delocalization to the polyacetylene. The size‐dependence of the nonadiabatic effects in polyacetylene has been shown throughout the discussion.
The evaluation of the hybrid one‐center integrals used in the semiempirical NDDO‐type methods for d‐elements from spectra
Authors: Zilberberg I. L. , Filatov M. J. , Zhidomirov G. M.
Published: January 2005

For semiempirical SCF MO methods based on the NDDO assumption, the so‐called one‐center hybrid integrals of the type (ab/cd) are required. These integrals are expressed in terms of the Slater–Condon parameters (SCP) Rk(abcd) with different radial functions. The SCPs from atomic spectra are evaluated by taking into account configurational interaction (CI). As an example, the R1(sdpp), R2(sdpp), and R2(sddd) SCPs for Ti I, Ti II, and Ti III have been evaluated and the values of these parameters are compared with those calculated with Slater‐type orbitals.
Investigation on the intermolecular electrostatic interactions of some polar molecules with cumulative potential‐derived atomic multipole method
Authors: Kong Jing , Yan Ji‐Min
Published: January 2005

In this paper, we propose a systematic method to include cumulatively atomic dipole moments and quadrupole moments in the fitting procedures of the potential‐derived method to derive a set of atomic parameters to calculate the intermolecular electrostatic energy. This method is examined both qualitatively and quantitatively. The description of the charge distributions in polar molecules HF, H2O, and NH3 is agreeable to intuitions and the inverted lone‐pair sites in H2O are explained naturally. It is also shown that atomic dipole moments and quadrupole moments should be included if one wants to calculate the electrostatic energy between molecules precisely. We also use Buckingham's model to calculate the orientations of some H‐bonded complexes and explain the sources of nonlinear forces in terms of atomic dipole moments and quadrupole moments.
A three‐dimensional finite element approach towards molecular SCF computations
Authors: Murakami H. , Sonnad V. , Clementi E.
Published: January 2005

Fully three‐dimensional, SCF ground‐state computations for the Hartree equation are carried out by a finite element approach that completely avoids forming or storing the Fock matrix. A combination of strategies is used to reduce storage and computational requirements by orders of magnitude over the traditional finite element approach, which makes three‐dimensional molecular orbital computations feasible. Results using the three‐dimensional formulation and computer program are shown for one‐electron systems: He+ and H2+, and for two‐electron systems: He and H2. The best results are within about 30–100 micro‐Hartrees of the exact values of the total energies for the ground states of these systems, indicating that our three‐dimensional approach has been correctly implemented in the computer code.
On the calculation of Rys polynomials and quadratures
Authors: Sagar Robin P. , Smith Vedene H.
Published: January 2005

A discretized Stieltjes procedure is used to generate Rys polynomials from a three‐term recurrence relationship. We show that this process is numerically stable in contrast to the evaluation from the moments of the system and polynomial coefficients, i.e., the Stieltjes procedure, where numerical instabilities arise. Rys quadratures may thus be calculated in an accurate manner from the Rys polynomials generated via this discretized Stieltjes procedure.
Photochromic spironaphthoxazines: A theoretical study
Authors: Malatesta V. , Ranghino G. , Romano U. , Allegrini P.
Published: January 2005

Photochromic spironaphthoxazines and their open merocyanine forms were studied via semiempirical Hartree–Fock SCF quantum chemical calculations. The most stable of the merocyanine isomers were found to have a cis–trans–cis conformation and the zwitterionic form is, generally, more stable than is the quinoidal tautomer. The calculated activation energy for the thermal bleaching reaction is in good agreement with the experimental one.
Electronic structure of poly(pyrrole–thiophene) copolymers: Design of polymeric quasi‐one‐dimensional superlattices
Authors: Bakhshi A. K. , Ladik J.
Published: January 2005

Ab initio band structure results of polypyrrole (PPY) and polythiophene (PTP) obtained with a double‐zeta basis set are reported. The electronic density of states (DOS) of the various quasione‐dimensional copolymers (superlattices) of the type (AmBn)x (A = pyrrole; B = thiophene) have been calculated numerically within the ab initio SCF tight‐binding approximation. These copolymers on the basis of the band positions of PPY and PTP are found to belong to the class of type II (staggered) superlattices. The trends in their electronic properties as a function of (i) composition (m/n), (ii) block sizes m and n, and (iii) arrangement of blocks in the copolymer chain are discussed.
A quantum chemical study of the interaction of carbenes with 2‐alkenylfurans and thiophenes
Authors: Ermolaeva L. V. , Vul'fson S. G. , Shostakovsky V. M. , Nefedov O. M.
Published: January 2005

The study of the interaction mechanism of monoalkenyl‐substituted furans and thiophenes with carbenes was performed by AM1 and MNDO methods. The energy profiles of two pathways were calculated, one leading to ylide formation and the second to cyclopropanation species. The insertion of methyl groups into the vinyl fragment does not change the energetic parameters of path 1. The reactivity of the exocyclic double bond was shown to depend on the degree of substitution. The calculations prove the presence of the intermediate ylide formation, based on experimental data. The stability of the ylides is determined by the heights of the cycloaddition barrier and the one of the reversed reaction 1 as shown by calculations.
Calculation of quadratic hyperpolarizabilities for organic π electron chromophores: Molecular geometry sensitivity of second‐order nonlinear optical response
Authors: Kanis David R. , Marks Tobin J. , Ratner Mark A.
Published: January 2005

This contribution explores the sensitivity of computed quadratic hyperpolarizabilities to the choice of chromophore molecular geometry. The nonlinear optical response of 25 organic π‐electron molecular chromophores is calculated for four different types of input geometries using the ZINDO‐SOS formalism. The calculated nonlinear optical susceptibilites are found to be surprisingly sensitive to certain key alternations in molecular structure; this is understandable in terms of modifications in the conjugation strength through the π system. We also describe an efficient, a priori prescription for constructing chromophore input geometries that yield accurate quadratic hyper‐polarizabilities within the ZINDO‐SOS formalism. The first optical absorption maxima, the dipole moments, and the second‐order nonlinear optical responses computed from these idealized geometries are essentially identical to those derived from MOPAC‐optimized structures and correspond well with available experimental data.
Nonlinear optical properties of conjugated polymers from ab initio finite oligomer calculations
Authors: Kirtman Bernard
Published: January 2005

A review of our own work on ab initio calculation of nonlinear optical properties by the finite oligomer method is presented. Various computational considerations such as geometry determination and choice of basis set are discussed as well as extrapolation to the long‐chain limit. For cases where the extrapolation is problematic, two potential solutions are suggested. We also analyze several important areas ripe for investigation including the frequency‐dependence, the effect of vibrational distortion, and the role of electron correlation.
Quantum molecular study of sulfur mustard
Authors: Broch H. , Viani R. , Vasilescu D.
Published: January 2005

The sulfur mustard, HD, a chemical warfare agent, has been studied by ab initio quantum molecular computations (HF/6‐31G* and 6‐31G) on its various forms (neutral, ethylene sulfonium, and free carbonium ions). The geometries of these molecules have been completely optimized and the minimal energy conformations determined with their associated charge distributions. We discuss these results on electrostatic properties with respect to the mechanism of DNA alkylation by HD and compare them with our previous study of the nitrogen mustard mechlorethamine.
Ab initio MO calculations on the stability of the CF, PF, SF, and MoF ions with the F1s core hole
Authors: Falaleyev A. G. , Andreyev V. A. , Vovna V. I.
Published: January 2005

For the CF, PF, SF, and MoF ions appearing after the F1s photoionization, the possibility of dissociation has been shown by the ab initio MO LCAO method within the Z + 1 core equivalent model. According to the calculations, the decay channel AF → AF + F(1s12p6) is energetically open for the ions. So the interpretation of the gas‐phase emission FKα spectra, in which the bands are assigned to the discrete transition energies, can be unacceptable for these ions. The conditions and signs of such failure are discussed.
Polystyrene models. II. Ab initio study of isobutylbenzene
Authors: Lagowski J. B. , Csizmadia I. G. , Vancso G. J.
Published: January 2005

The structure and stability of the various conformations of isobutylbenzene are studied using ab initio molecular orbital theory. The calculations show that coupling between the structural units is important. The results indicate that complete geometry optimization of the stable and transition structures of isobutylbenzene produce significant changes in geometrical parameters and charge distributions of this molecule when compared with the corresponding results obtained using the rigid‐rotor approximation. These changes are particularly noticeable in one of the gauche conformations and in transition structures of isobutylbenzene generated by the phenyl group rotation. For polystyrene, these results present evidence that there is a strong coupling between the chain‐backbone folding and the rotation of the phenyl group. Multidimensional potential energy surfaces are displayed using a topological representation. © 1992 John Wiley & Sons, Inc.
Intermolecular interactions of methyl acetate, β‐propiolactone, ethyl acetate, and γ‐butyrolactone: An AM1 semiempirical study
Authors: Bodor Nicholas , Huang Ming‐Ju
Published: January 2005

Closely related structures, like esters and lactones, have vastly different physical properties. This is apparently due to differences in the intermolecular interactions. The intermolecular interactions of methyl acetate, β‐propiolactone, ethyl acetate, and γ‐butyrolactone have been studied using the AM1 semiempirical method. Some of the “arranged clusters” were also compared to possible covalently bound trimers and tetramers of β‐propiolactone and γ‐butyrolactone. © 1992 John Wiley & Sons, Inc.
Maximum overlap symmetry molecular orbital model
Authors: Zhan Chang–Guo
Published: January 2005

This paper provides a brief and systematic presentation of the basic principle and method of the maximum overlap symmetry molecular orbital (MOSMO) model and its application to simplification of molecular orbital calculation and to calculation of molecular structures and properties, together with some new results about the MOSMO calculation and new insights concerning the further extension of the principle and method. It has been shown that the theoretical method of the MOSMO model is very simple, reliable, and useful and can be employed to study the structure–property relation in even very large molecular systems. The numerical results obtained from the MOSMO calculation on various semiempirical molecular orbital approximation levels show that when the same parametrization, such as one of those employed in EHMO, CNDO/2, and HMO methods, is adopted, the MOSMOS are very close to the canonical molecular orbitals obtained from the customary LCAO method and the MOSMO calculation requires less computing time than does the LCAO method. The MOSMO calculation can be used for rapidly obtaining reasonably good molecular geometries, vibrational frequencies, and other properties of molecules by employing a simple improved semiempirical parametrization. Equilibrium geometries, vibrational frequencies, and other results are in good agreement with the experimental data and the results obtained from ab initio molecular orbital calculation. The basic calculational procedure of the MOSMO model can be extended further and has been employed to give some new results, to propose some new theoretical schemes and principles, and to introduce some new interesting theoretical problems that deserve to be studied further.
Role of axial ligand in the electronic structure of model compound I complexes
Authors: Du Ping , Loew Gilda H.
Published: January 2005

A quantum chemical study of the two low‐lying quartet states of seven model compound I iron–porphyrin complexes with varying axial ligands has been carried out using the INDO method. The varying axial ligands included in this study are five that are models for those in the intact enzymes: imidazole and imidazolate (model peroxidase HRP and CCP), CH3CONH2 (Gln175 mutant of CCP), PhO−1 (catalase), CH3S−1 (P450), and two that have been used in biomimetics of these enzymes: Cl−1 (hemin) and PhS−1 (model P450s). The purpose of these studies was to determine the role of the axial ligands in determining (i) the relative energies of the two nearly degenerate quartet electronic states of compound I, involved either as an a1u or a2u porphyrin π cation radical and (ii) the electron and spin distributions in the a1u and a2u radical cations of compound I. For most of the model complexes, including both HRP‐I and CAT‐I, a moderate effect of the axial ligand on the relative energy of these two states was observed and the a1u radical cation was found to be the ground state. The energy order of these two radical cations, however, was reversed in the P450‐I model complexes, indicating an association of the unique property of the FeO bond breaking with an a2u radical cation. The symmetry‐allowed overlap between the FeO and 3a2u orbitals may lower the activation energy for the FeO bond cleavage in P450‐I. However, the calculated electronic and spin properties, including the unpaired spin and net charge on the oxygen and the FeO bond overlap density, important determinants of the reactivity of this complex in the ligand–FeO region, are very similar for all complexes and in both cation states. © 1992 John Wiley & Sons, Inc.
MCSCF/MCLR Studies of potential energy surfaces, spectra, and properties of the X1A1 and a3B2 states of ozone
Authors: Nordfors David , Årgren Hans , Jensen Hans Jørgen Aa.
Published: January 2005

No Absract.
Peptide conformational analysis using the TRIPOS force field
Authors: Wall Craig G. , Healy Eamonn F. , Fox Marye Anne
Published: January 2005

To explain observed electron transfer rates between the terminal aromatic moieties in the arylsubstituted alanine dimer 1, a conformational analysis of the dipeptide was performed using a standard molecular mechanics force field. The resulting low‐energy conformers could conveniently be grouped into two families with an average energy difference of ca. 2 kcal mol−1 and populations of 64% and 30%, respectively. These results correlate well with the 78:22 ratio of the two experimentally distinguishable decay processes for the radical anion of 1. © 1992 John Wiley & Sons, Inc.
NDDO/MC: A new semiempirical SCFMO method for transition metal complexes
Authors: Filatov Michael J. , Zilberberg Igor L. , Zhidomirov George M.
Published: January 2005

A new semiempirical SCF MO procedure available for prediction of the transition‐metal complexes' binding energy and molecular geometry is developed. The features of the method are (i) an explicit account of the orthogonality of the basis set; (ii) use of a new formula for the resonance integral; and (iii) an effective account of the Coulomb correlation of electrons in the calculation of the two‐electron integrals based on approach of a model Coulomb hole function. The parameterization for H, C, N, O, Co, and Ni atoms is presented. The results of NDDO/MC (NDDO for Metal Compounds) calculations of molecular geometries and binding energies for a number of organic compounds and more than 30 cobalt and nickel complexes are compared with the available experimental and ab initio data. The average absolute errors for the binding energies of organic molecules and metal complexes are 8.8 and 5.0 kcal/mol, respectively. © 1992 John Wiley & Sons, Inc.
Prototropic changes in cationic base‐pair adducts. I. Guanine protonation
Authors: Ford George P. , Wang Bingze
Published: January 2005

AM1 calculations have been used to study the effects of protonation on the structures, energies, and, in some cases, proton transfer reactions of guanine cytosine base pairs. Protonation at the guanine O6‐position, or at various ring sites, leads to a relatively facile conversion to a surprisingly stable complementary base pairs following proton transfer to the cytosine 3‐position. In the case of O6‐protonation, this constitutes a direct route to guanine enolization. It is suggested that the spontaneous formation of apyrimidinic sites in nucleic acids take place via prior protonation of guanine moieties in the opposite strand. © 1992 John Wiley & Sons, Inc.
Static polarizabilities for the ne‐isoelectronic series using Harbola–Sahni potential
Authors: Samuel J. , Sen K. D.
Published: January 2005

The recently proposed Harbola–Sahni local potential in its exchange‐only version [Phys. Rev. Lett. 62, 689 (1989)] is found to give the static dipole and quadrupole polarizabilities for the Neisoelectronic series in excellent agreement with the corresponding Hartree–Fock estimates. © 1992 John Wiley & Sons, Inc.
Atomic theorems
Authors: Bader R. F. W. , Popelier P. L. A.
Published: January 2005

The atomic statement of the principle of stationary action determines the theorems that establish the mechanics of an atom in a molecule. The atomic torque, power, and current theorems are derived. They, along with the atomic force, virial, and continuity theorems, provide a complete description of the behavior of matter at the atomic level, including the effects of electric and magnetic fields. The atomic power theorem, for example, enables one to determine the atomic or molecular source of the creation and dissipation of power in combustion or in an explosion. An atom in a molecule is an open system and an atomic equation of motion for an observable contains a component corresponding to the flux in its current density through the atomic surface. The differential form of each theorem yields an equation of motion for the corresponding property density in which the same surface flux appears as a divergence of the current density. The differential and integrated atomic theorems complement those for the total system, enabling one to relate the property to its atomic components and ultimately to a local contribution from each element in real space. © 1993 John Wiley & Sons, Inc.
Classical and quantum study of a generalized Kepler–Coulomb system
Authors: Kibler Maurice , Campigotto Corrado
Published: January 2005

The nonrelativistic potential V = α/r + β cos θ/(r sin θ)2 + γ/(r sin θ)2, which constitutes an extension of the Kepler‐Coulomb potential and of the Hartmann potential, is investigated from both a classical and a quantum mechanical viewpoint. The corresponding Schrödinger equation is solved in parabolic coordinates by using the Kustaanheimo‐Stiefel transformation. The accidental degeneracy for the discrete spectrum is explained through the introduction of an su(2) dynamical invariance algebra. The Hamilton–Jacobi equation is solved in parabolic coordinates. The planarity and the periodicity of the trajectories are investigated. All finite trajectories are found to be quasi‐periodic (rather than periodic as in the case of the Kepler–Coulomb system). © 1993 John Wiley & Sons, Inc.
Dipole, quadrupole, octupole, and dipole–octupole polarizabilities at real and imaginary frequencies for H, HE, and H2 and the dispersion‐energy coefficients for interactions between them
Authors: Bishop David M. , Pipin Janusz
Published: January 2005

We have calculated certain dynamic polarizabilities (for both real and imaginary frequencies) for H, He, and H2 and the dispersion‐energy coefficients for long‐range interactions between them. We have done so in a sum‐over‐states formalism with explicitly electron‐correlated wave functions to describe the states. To be precise, we have determined the dipole (α1), quadrupole (α2), and octupole (α3) polarizabilities of H and He for real frequencies (ω) in a range between zero and the first electronic‐transition frequency and for imaginary frequencies (iω) on a 32‐point Gauss‐Legendre grid running from zero to ħω = 20 Eh, and for H2, we have found the dipole (α), quadrupole (C), and dipole–octupole (E) polarizability tensors for the same real and imaginary frequencies. The dispersion‐energy coefficients, obtained by combining the sum‐over‐states for‐malism for the polarizabilities with analytic integration over ω, gave values of C6, C8, and C10 for the atom–atom systems; C, C, C, C, and C for the atom–diatom systems; and C6, C and C for the H2H2 system. Nearly all the results are considered to be more reliable than those hitherto published and some have been obtained for the first time, e.g., C(iω), E(ω), and E(iω) for H2 and C, C, and C for the HH2 system. © 1993 John Wiley & Sons, Inc.
Solution of atomic and molecular Schrödinger equation described by hyperspherical coordinates
Authors: Deng Conghao , Zhang Ruiqin , Feng Dacheng
Published: January 2005

The Schrödinger equation for an atom or molecule is expressed in terms of hyperspherical coordinates. The eigenfunction is expanded in a series of orthonormal complete sets: Yλ,μ(Ω), eigenfunctions of generalized angular momentum scalar operator, and L, generalized Laguerre polynomials. The recurrence relation of the expansion coefficients are derived and the eigenvalues can be obtained from the secular equation. © 1993 John Wiley & Sons, Inc.
A theoretical study of the effect of basis sets on stationary structures for the addition of carbon dioxide to methylamine: A relation among geometries, energy status, and electronic structures
Authors: Andréas J. , Moliner V. , Krechl J. , Silla E.
Published: January 2005

The carbon dioxide (CO2) to methylamine (CH3NH2) addition reaction is investigated using an ab initio MO method with the aim of obtaining an insight into the relation between transition‐state structure and reactivity. The complete potential energy hypersurface was explored and the stationary points representing reactants, transition state, and product were localized. To establish the relevance of different basis sets, calculations were carried out at different levels of theory: STO‐3G, 3‐21G, 4‐21G, 4‐31G, 6‐31G, 6‐311G, 6‐31G*, and MP2/6‐31G*//HF/6‐31G*. A reaction analysis done by correlation of bond‐order indices using the More O'Ferrall‐Jencks diagram reveals the reaction mechanism as an asynchronous process. The transition state can be described as a four‐membered ring. The lone nitrogen electron pair of the amine appears as the driving force of the reaction, where a positive hydrogen rather than a radical is more likely to be transferred. © 1993 John Wiley & Sons, Inc.
Electron attachment to homonuclear diatomic molecules
Authors: Mosley D. H. , Pickup B. T.
Published: January 2005

The electron propagator method is applied to the calculation to the electron affinities of some first‐ and second‐row homonuclear diatomic molecules Li2, Be2, C2, F2, Na2, Si2, and Cl2. Perturbation theory is applied through second order to analyze the results in order to determine the relative importance of correlation and relaxation effects in the binding of the additional electron. © 1993 John Wiley & Sons, Inc.
Polysila analogs of aromatic hydrocarbon ions: Structures and energies of Si3H3+, Si4H, and Si5H5−
Authors: Korkin Anatoli , Glukhovtsev Mikhail , von Ragué Schleyer Paul
Published: January 2005

Silicon analogs of aromatic monocyclic ions, (SiH) (4), (SiH) (5), and (SiH) (6) have been studied ab initio at MP2(full)/6‐31G*. The D3h structure of Si3H3+ is the global minimum, whereas other two ions are nonplanar. The D2d structure of (SiH) is less folded than the carbon analog and possesses a higher stabilization energy. Stabilization energies for the monocharged ions are diminished with respect to the corresponding carbons © 1993 John Wiley & Sons, Inc.
Ab initio investigation of phloroglucinol
Authors: Mandix Kim , Colding Arne , Elming Knud , Sunesen Leif , Shim Irene
Published: January 2005

All‐electron ab initio Hartree–Fock (RHF) calculations have been carried out to investigate the keto/enol equilibrium of phloroglucinol. The calculations predict that the enol form of phloroglucinol, 1,3,5‐benzenetriol, is by far the most stable of the two. This is confirmed by NMR spectra taken on phloroglucinol. A comparison of the keto enol form transformation of phloroglucinol with that of the phenol system shows that the keto form of phloroglucinol, 1,3,5‐cyclohexanetrion, is more abundant in the phloroglucinol system, and the keto form of phenol, 2,4‐cyclohexadien‐1‐on, in the phenol system. © 1993 John Wiley & Sons, Inc.
Resonances and irreversibility for Schrödinger evolution
Authors: Kurasov P. , Elander N. , Pavlov B.
Published: January 2005

The relations between the Schrödinger and wave evolutions are investigated. A generalization of the Lax‐Phillips scattering theory for the case of the Schrödinger equation is constructed. An analog of the wave decay operator is obtained. The general ideas are illustrated by an explicitly solvable model of multichannel scattering on the halfaxis. A Lax‐Phillips scattering theory is developed for the model operator obtained by a self‐adjoint perturbation of the background operator. © 1993 John Wiley & Sons, Int.
Some comments on H‐bond dynamics in DNA base pairs
Authors: Brändas Erkki J.
Published: January 2005

It is advocated that the H‐bond patterns of the standard C–G and A–T base pairs have an evolutionary advantage over any other H‐bond scheme accommodated within the Watson–Crick‐type geometry. A suggested proof of the statement is given by the Longuet–Higgins' sign‐reversing loop argument. The present analysis indicates a close relation with self‐organizational principles. © 1993 John Wiley & Sons, Inc.
Self‐returning walks and fractional electronic charges of atoms in molecules
Authors: Bonchev D. , Kier L. B. , Mekenyan O.
Published: January 2005

Three hierarchically ordered topological factors, i.e., atom connectivity, centrality, and cyclicity, were found to control the number of self‐returning walks (SRWS) associated with every atom in the molecule. The reversal of their order was observed in a few cases where the central location of atoms had a stronger influence than did their connectivity on the number of SRWS. Three atomic topological indices, i.e., the Morgan extended connectivity, the Balaban, Mekenyan, and Bonchev hierarchical extended connectivity, the Randić atomic path code, were found to closely match the ordering of atoms in molecules determined by their number of SRWS. New atomic graph invariants fi = lim SRW/SRWn and ti = fi · SRW2 were specified and may find application in QSAR and QSPR. The ti indices are nonintegers close to atomic valence. The fi indices represent the limit of the number of SRWS of length n for the atom i, SRW, normalized by dividing it by the total number of SRWS for the molecule. In the case of Hückel MO considerations, these invariants were shown to be numerically equal to the partial electronic charges of the lowest occupied molecular orbital (LOMO). A new class of isocodal atoms (atoms having the same number of SRWns) was observed, i.e., atoms that become isocodal only at n ⩾ 1. A number of open questions following from these findings were formulated, including the possibility for a topological modeling of electron correlation. © 1993 John Wiley & Sons, Inc.
Computational studies of peripheral ring twisting in meso‐N‐methyl pyridyl‐substituted porphyrins
Authors: Monaco Regina R. , Zhao Ming
Published: January 2005

Semiempirical molecular orbital calculations for the porphyrins tetrakis(4‐N‐methyl pyridyl)porphine (H2TMpyP‐4) and tetrakis(2‐N‐methyl pyridyl)porphine using the MNDO and AM‐1 Hamiltonians suggest that twisting one or more of the pyridinium rings results, at considerable energy expense, in highly nonplanar macrocycle configurations as the exocyclic ring(s) approach coplanarity. The results imply that the mechanism of intercalation of H2TMpyP‐4 into DNA cannot require twisting the exocyclic rings anywhere close to coplanarity with the central porphine core, but involves, instead, the inherent flexibility of DNA itself. © 1993 John Wiley & Sons, Inc.
Perturbation–variational methods revisited
Authors: Chacon Monique Revoredo , Zerner Michael C.
Published: January 2005

We describe a simple expansion of the Krylov type for the estimation of the electronic correlation energy of the ground state of closed‐shell systems and show its relationship with Brillouin–Wigner perturbation theory. We demonstrate the Brillouin–Wigner perturbation theory can be quickly solved in a noniterative fashion. Examples are presented using the intermediate neglect of the differential overlap model Hamiltonian (INDO/1), which indicates that this scheme captures well over 95% of the configuration interaction over double excitations method (CID) in a fraction of the computational effort. © 1993 John Wiley & Sons, Inc.
Decomposition and interpretation of the SCF interaction and deformation energies by the modified Pauli Blockade method
Authors: Pecul Krzysztof
Published: January 2005

The SCF interaction energy in He … Li+, (H2O) … HOH, and OC … BH3 complexes is decomposed, using the original and modified‐by‐the‐author Pauli Blockade (PB) method of Gutowski and Piela. The changes of the dipole moment of the deformed constituents are similarly decomposed and discussed as some indices of the deformation. The deformation is due mainly to the electrostatic interaction, but in some cases, the valence repulsion deformation effects are dominant. The comparison of the PB technique with some other decompositions is discussed. The PB methods seem to be more widely applicable, e.g., they are applicable for small intermolecular distances and large basis sets. The modified PB method is less‐time consuming than is the original one. © 1993 John Wiley & Sons, Inc.
A scheme for representation matrices of a permutation group using spin‐paired functions
Authors: Kadolkar C. , Sarma C. R. , Ghosh Dipan K.
Published: January 2005

In the present note, a linked form of spin‐paired functions for an N‐electron system in spin state S is suggested. This is found to lead to a simple scheme for generating the representation matrices of the elements of permutation group without searching for linkages in the superposition diagrams. The program based on this is found to generate the representation matrices more efficiently than do currently available procedures. © 1993 John Wiley & Sons, Inc.
Three stages of optimization and simple correlated wave functions
Authors: Hoor Marten J. Ten
Published: January 2005

It is advocated to carry out an optimization procedure, which is based upon the variational method, in such a way that the optimum values of the variational parameters are expressed as functions of physical constants, such as the atomic number, Z. The three stages involved in this treatment are illustrated by the optimization of nine correlated wave functions, which describe the ground states of atomic two‐electron systems. An analysis of the Z‐expansions of the total energies associated with these functions leads to the concept of a class of variational functions. The performances of functions belonging to the same class differ only marginally, especially at larger values of Z. Consequently, the concept of class may be used to bring some order in the plethora of variational functions. © 1994 John Wiley & Sons, Inc.
Substituent effects on chemical hardness
Authors: Kneisler John , Zhou Zhongxiang
Published: January 2005

It is shown that for a number of molecules there exists a Hammett‐like equation for the chemical hardness or the HOMO‐LUMO energy gap. Substituent effects can be factored out of the molecular hardness. Preliminary results show that a hardness substituent constant, similar to the Hammett σ‐constant, can be defined and determined for each substituent. © 1994 John Wiley & Sons, Inc.
A PM3 study of the critical distance factor in the activation/cyclization of selected Neocarzinostatin analogs
Authors: Mitra Abhijit , Capitani J. F.
Published: January 2005

Semiempirical MOPAC RHF/PM3 and BIRADICAL/PM3 calculations are performed on several selected analogs of the Neocarzinostatin chromophore. The critical distance parameter, heat of formation, and pertinent bond orders are reported for each model compound. Heats of reaction are also reported for the cyclization of several model compounds to their corresponding diradical products. It is suggested that 3.49 ÅR represents an upper bound to the critical distance in the cyclization of compound related to Neocarzinostatin chromophore. © 1994 John Wiley & Sons, Inc.
Molecular electrostatic charge models: A topographical approach
Authors: Shrivastava Indira H. , Gadre Shridhar R.
Published: January 2005

Electrostatic charge models for molecules have been developed by employing the critical topographical features of the molecular electrostatic potential (MESP) as the “fitting” criterion. These models include one or more spherical Gaussians for incorporating the continuous electron‐charge distribution in addition to the positive valued point charges representing the nuclei. The model parameters (point charges, the orbital exponents, and Gaussian centers) are optimized so as to mimic the extremal characteristics of the corresponding quantum chemicalMESP. The test cases reported here include methane, ethylene, and methanol molecules. The charge models developed using the present method are seen to satisfactorily reproduce the ab initioMESPand its extremal features. © 1994 John Wiley & Sons, Inc.