bluuebunny/crossref_metadata_2025 huggingface dataset:
A mixed basis set of slater and gaussian functions for molecular calculations
Authors: Colle Renato , Fortunelli Alessandro , Salvetti Oriano
Published: December 2004

A procedure for using simultaneously Slater and Gaussian basis functions in molecular calculations is presented here. The analytic expressions of the integral prototypes involving both Slater and Gaussian functions are explicitly derived.
Electron transport in condensed matter. A novel approach using quantum chemical methods
Authors: Larsson Sven
Published: December 2004

The theory of mixed valence hopping conductivity is reviewed and prepared in a way suitable for quantum chemical calculations. Metallic conductivity appears as a limit case, if the thermal barrier for electron transfer is approaching zero and the orbital contact between the electron exchanging subsystems is large enough for adiabatic transfer. The theory is exemplified by calculations on the molecular crystal model and a model for doped polyacetylene. In the latter case it is possible to explain the decreasing thermal barrier for increased concentration of doping molecules as due to increased interaction between these molecules through the polymer chain.
Ab initio theory of hydrogen bonding in vitamin B6
Authors: Greenwood H. H. , Plant J. S.
Published: December 2004

Ab initio MO computations have been carried out for the crystalline form of the B6 vitamin, pyridoxinium chloride. In the crystal, the pyridoxine is protonized to pyridoxinium, with the formation of a hydrogen bond NH+ ⃛ Cl−. For an isolated molecule, the calculations predict a single potential well, with H+ placed close to Cl− at the minimum. When neighboring molecules are included in the calculations, a double well is formed, and the lower minimum occurs for the proton placed near the experimentally observed position, in the vicinity of the N atom.

Mulliken populations, which depend on the proton position, are described.
Geometry of density matrices. VI. Superoperators and unitary invariance
Authors: Casida Mark E. , Harriman John E.
Published: December 2004

We examine and compare ways of dividing into subspaces the space whose elements are density matrices or other operators for the class of model problems defined by a finite one‐particle basis set. One method of decomposition makes the significance of the subspaces apparent. We show that this decomposition is also complete, in the group‐theoretic sense, for the group of unitary transformations of the set of one‐electron basis functions. The irreducible subspaces are labeled by particle number and by an additional integer we call the reduction index. For spaces of particle‐number‐conserving operators, all subspaces with the same reduction index are isomorphic, and an analogous isomorphism exists for non‐particle‐number‐conserving cases. The general linear group also plays a key role, and we introduce the term “canonical superoperators” to characterize those superoperators which commute with this group. When an appropriate basis set is chosen for the matrix spaces, the supermatrices corresponding to these superoperators have a particularly simple form: a block structure with the only nonzero blocks being multiples of unit matrices. The superoperators of interest can be constructed in terms of two operators, , and these two have been expressed simply in terms of creation and annihilation operators. When only real orthogonal transformations of the basis are considered, a further decomposition is possible. We have introduced superoperators associated with this decomposition.
Ru‐Oxo catalyzed epoxidations and the woodward–hoffmann rules
Authors: Cundari Thomas R. , Drago Russell S.
Published: December 2004

An analysis of the pathways and intermediates for the epoxidation of olefins using a Ru(IV)‐oxo model complex is presented. Simple bonding concepts, coupled with INDO/1 semiempirical MO calculations, provide a description of the metal‐oxo/olefin interaction that agrees well with experimental facts. Concerted [1 + 2] and [2 + 2] pathways are investigated and found to lead to unfavorable orbital interactions as with organic analogues. Nonconcerted process (those in which one bond is formed before the second), which connect the two concerted pathways, are preferred. A nonconcerted [1 + 2] pathway is preferred over a nonconcerted [2 + 2] pathway on the basis of less steric repulsions between the olefin and the metal ligands. Also, all open structures (those with one CO bond formed) investigated minimized to bound epoxides. In the most favorable pathway, a nonconcerted [1 + 2] pathway, radical cation character on the terminal carbon increases as the nonconcerted process is induced. Preference for the bound epoxide intermediate over the oxometallocycle is simply explained by partial charge considerations.
Ab initio calculations of many‐body energy expansion in LiF− clusters
Authors: Shalabi A. S. , Kamel M. A. , Eid Kh. M.
Published: December 2004

The effects of the basis‐set size on many‐body energy expansion in LiF− clusters are investigated and correlated with previously reported values on LiCl− analogs. Coulomb and non‐Coulomb energies in LiF− at different configurations are also examined. Although at the minimal STO‐3G basis Vna(3, 4) and Vna(4, 4) nonadditivity terms were the smallest in the D3h configuration, they were the largest at the extended 6‐311 ++G basis. V(m, n) terms where m = n ≥ 3 were found to be playing a small role in the chemistry and physics of LiF− clusters compared with V(3, n) terms in LiCl− clusters.
Evaluation of two‐center, two‐ and three‐electron integrals involving correlation factors over Slater‐type orbitals. I. Basic integrals
Authors: Budziński Jan
Published: December 2004

The modified and extended version of the Neumann expansion of the interrelectronic distance function r for u = −1, 0, 1, 2, using the set of orthogonal polynomials normalized to unity, is presented. This expansion has been utilized to obtain analytical expressions for evaluating two‐center two‐ and three‐electron integrals in the Slater orbital basis occurring if variational correlated functions are used.
On the redistribution of electrons for chemical reaction systems
Authors: Tachibana Akitomo , Parr Robert G.
Published: December 2004

A regional density‐functional theory is formulated and applied to the study of ground‐state electron redistributions during the course of a chemical reaction. If for a given increment of the reaction process, accumulation of electrons occurs in a certain region of space, then it is called the dynamic acceptor region, denoted by P. The complement is called the dynamic donor region, denoted by Q. The regional energy itself is determined as a unique functional of the electron density of the total system. The regional transfer potentials are defined in such a way that they add to give the total chemical potential, and their values along the reaction coordinate are found to be different between P and Q. The difference between the regional transfer potentials is shown to provide the driving force for electron transfer from Q to P. A characteristic coordinate for following electron transfer and an associated excitation potential are introduced. The excitation potential is a measure of regional virtual excitation due to regional interactions. The regional transfer potential gives the local character of electron transferability, while the excitation potential gives the global character. The theory encompasses the concepts of regional hardness and softness and sheds light on the HSAB principle.
A global characterization and similarity analysis of two‐dimensional potential energy surfaces
Authors: Luo Xincai , Mezey Paul G.
Published: December 2004

Potential energy surfaces can be classified into combinatiorial equivalence classes, based on their partitioning into catchment regions. Two classification theorems are proven: one for reaction spheres and another for reaction tori. A method for constructing all possible equivalence classes of reaction spheres and reaction tori is presented. As illustration of the general results, it is shown that not all the two‐dimensional reaction spheres are combinatorially equivalent to polyhedra in the three‐dimensional Euclidean space. As examples, several reaction spheres are calculated by using the RHF method at the 3‐21G* level, describing the interactions between a series of polyatomic ions and H+. The calculations show that the potential energy surface of the CO …︁H+ interaction, combinatorially equivalent to that of the NO−3 …︁H+ interaction, is not combinatorially equivalent to any polyhedron in 3‐space; however, the combinatorially different potential energy surface of the PO …︁ H+ interaction is equivalent to a polyhedron in 3‐space. The topological classification scheme is proposed for the study of similarities between various families of chemical reactions.
Electronic‐phase transition associated with instability of the Hartree–Fock solution of infinite one‐dimensional system
Authors: Tanaka Kazuyoshi , Kobayashi Hiromi , Okada Mayumi , Kobashi Masahiro , Yamabe Tokio
Published: December 2004

Electronic‐phase transition associated with the singlet instability problem of the Hartree‐Fock solution of the extended system is studied employing the metallic trans‐polyacetylene. Concerning the eigenstates of the stability matrix set up for the crystal orbitals at the Fermi level, classification of the electronic phases is attempted and the origin of their emergence in relation to the interelectronic interaction is discussed.
Behavior of coupled cluster energy in the strongly correlated limit of the cyclic polyene model. Comparison with the exact results
Authors: Piecuch Piotr , Čížek Jiří , Paldus Josef
Published: December 2004

Perturbation expansion for the ground‐state energy in the strongly correlated limit of cyclic polyenes, as described by the Pariser–Parr‐Pople (PPP) and Hubbard Hamiltonians, is examined. Focus is placed on the first‐ and second‐order contributions in terms of the resonance integral, and the performance of the approximate coupled pair theory corrected for the connected quadruple excitations (ACPQ), as well as ACPTQ and ACPQ + T(ACPQ) methods, which are two different versions of ACPQ approximately accounting for the triexcited clusters, is studied. The second‐order contribution obtained with the ACPQ, ACPTQ, and ACPQ + T(ACPQ) approaches is then compared with the exact result for the fully correlated limit as provided by the diagonalization of the nearest‐neighbor Heisenberg Hamiltonian. It is shown that all three coupled pair theories yield vanishing first‐order contribution, in agreement with the exact result. The best result for the second‐order contribution for the cyclic polyenes considered in this article is provided by the ACPQ + T(ACPQ) method. In this respect, all three coupled pair theories seem to deteriorate as the polyene size approaches infinity, in spite of yielding excellent energies. Comparison is also made with the results of alternant molecular orbital method.
A theoretical study of atomic oxygen chemisorption on the Ni(100) and Ni(111) surfaces
Authors: Siegbahn Per E. M. , Wahlgren Ulf
Published: December 2004

Atomic oxygen chemisorption has been studied for the fourfold hollow site of the Ni(100) surface and for the threefold hollow site of the Ni(111) surface. To model the Ni(100) surface, 10 different clusters in the range Ni5 to Ni41 were used, and for the Ni(111) surface, 11 different clusters in the range Ni13 to Ni43 were used. A detailed analysis of the orbital occupations of the cluster with and without oxygen for the different clusters shows that there are three different possible bonding mechanisms. In two of these, the basic feature is that a1 electrons of the cluster are kicked out by the oxygen 2pz orbital and moved to holes in the 2px, y orbitals. A picture where the oxygen electrons fit into the electronic structure of the cluster is emphasized. The third mechanism, which is applicable for only one cluster, can be described as the formation of two covalent bonds of E symmetry. The final best estimate of the oxygen chemisorption energy for the Ni(100) surface is about 130 kcal/mol, and for the Ni(111) surface, about 115 kcal/mol. In particular for the Ni(111) surface, an excited oxygen state with radical character is identified, which might be a catalytically important species. The excitation energy to reach this state should be on the order of 10 kcal/mol for the Ni(111) surface.
Polarizable continuum model of solvation for biopolymers
Authors: Frecer Vladimír , Miertuš Stanislav
Published: December 2004

The original polarizable continuum model (PCM) of solvation based on the boundary element method for the numerical solution of the Poisson equation for a realistic molecular shape is modified through a classical description of solute charge distribution and solute polarization. In this approach, the solute charge distribution is represented by point charges and polarizabilities and the solute–solvent polarization is evaluated. The solvent–solute back polarization is also considered. The effect of electrolyte ionic strength is included via a dielectric function based on the Debye–Hückel model. A more realistic description of molecular cavity boundaries is introduced with the help of variable van der Waals radii. Approaches of evaluation of dispersion–repulsion and cavitation contributions to solvation Gibbs free energy suitable for large biopolymers are also presented. Test calculations illustrating the performance of the model are given together with examples of its application for larger biopolymer structures.
Ab initio CPHF calculations of the static polarizability and second hyperpolarizability of small molecules: Comparisons between standard and moderately large basis sets augmented with diffuse functions
Authors: Dory M. , Beudels L. , Fripiat J. G. , Delhalle J. , André J. M. , Dupuis M.
Published: December 2004

Static polarizability and second hyperpolarizability have been calculated for a number of small moleculesCO2, OCS, CS2, C2H2, C2H6, C3H8, cyclo‐C3H6, C3H4, C3H6, SiH4, Si2H6in the framework of the coupled‐perturbed Hartree‐Fock (CPHF) theory. The linear and nonlinear coefficients have been calculated with standard Gaussian basis sets and 3‐21G bases moderately enlarged with diffuse functions. It is shown that the parallel component of the polarizability saturates rapidly, which suggests that a 3‐21G basis containing s and p diffuse functions is sufficient to reproduce αzz. For the αxx and αyy components, a 3‐21G basis with s, p, and d diffuse functions is required. In general, the concordance between α computed with this basis set and the experimental static polarizability is at least of the order of 80%. On the contrary, the computation of the second hyperpolarizability with the same basis set for CO2, CS2, and C2H2 gives values that are 30% too low, compared to the experimental value. Better results are observed for ethane, propane, and cyclopropane for which the error is lower than 50%. The better agreement observed for the saturated compounds can probably be explained by their saturated character.
Electronic structure and nonlinear optical properties of push–pull conjugated molecules
Authors: Meyers F. , Brédas J. L.
Published: December 2004

We present a review of ab initio calculations we recently performed on organic molecules presenting promising quadratic nonlinear optical properties. These molecules constitute so‐called push–pull conjugated compounds in which a conjugated segment is capped at one end by an acceptor group and at the other end by a donor group. We foucs on three series of systems: (i) p‐amino‐p′‐nitrodiphenylacetylenes for which “unusual” distorsion patterns have been recently reported; (ii) benzodithiapolyenals, which present among the largest second‐order polarizabilities ever measured; and (iii) 2‐methylene‐2H‐pyrrole derivatives. Our results are found to provide a detailed understanding of the avilable experimental data.
A strategic plan for molecular electronics
Authors: Aviram Ari
Published: December 2004

This paper describes the background of microelectronics science and technology that influenced the development of molecular electronics (ME) and includes a description of some thoughts on the development of molecular circuit elements. Three basic principles are proposed and exemplified by a particular example of a suitable molecule. It is pointed out that as the size of electronic devices approaches molecular dimensions, ME promises to have distinct advantages over other technologies because the component function can be an outcome of the electronic structure of the molecule. A plan that can lead to the development of the ME concept with the scientific and technological knowledge of the present day is proposed.
A calculation of exciton energies in periodic systems with helical symmetry: Application to a hydrogen fluoride chain
Authors: Vracko M. G. , Zaider M.
Published: December 2004

Ab initio Hartree–Fock crystal orbital results were used as input for the calculation of exciton energies in the Tamm–Dancoff and random‐phase approximation for polymers with helical symmetry. The calculations were applied to a hydrogen fluoride chain. We show that the Tamm–Dancoff method is a good approximation to the random‐phase theory. Furthermore, the influence of the band–band interaction in the exciton calculation has been investigated.
Orbitally resolved charge sensitivity analysis: Basic concepts and relations
Authors: Nalewajski Roman F. , Mrozek Janusz
Published: December 2004

Following the recent developments of the charge sensitivity analysis (CSA) in the atoms‐in‐molecules (AIM) resolution, the corresponding CSA quantities in the orbital (or shell) resolution (OR) are defined. The OR electron population variables, in the ordinary closed‐shell SCF problem, are the elements of the bond‐order matrix P, and their conjugates, “chemical potentials,” FT = ∂E/∂P, are the respective Fock matrix elements, appropriate for the representation in question; here E is the SCF energy. The second derivatives ∂2E/∂P∂P define the OR hardness tensor from which all related OR CSs, e.g., the hardness, softnesses, Fukui function (FF) indices, etc., can be determined. The rigid potentials and hardness tensor, corresponding to the “frozen” orbital approximation, are examined in more detail, and the decoupled representation of the normal orbitals (NoO) is introduced, in which the rigid hardness tensor becomes diagonal. Illustrative valence‐shell NoO contours for the water molecule are given and discussed. The new approximation for the OR FF indices, as the orbital occupation probabilities, is proposed on the basis of the density matrix functional development of Donnely and Parr for natural orbitals, and the relevant expressions for the molecular fragment (collection of orbitals) quantities are summarized.
An AM1 study of the coupling reaction of radicals with the acetone enolate ion and some related nucleophiles
Authors: Pierini A. B. , Borosky G. L. , Baumgartner M. T.
Published: December 2004

We here report an AM1 study for the reaction of different types of radicals with the acetone enolate ion 1 and other ambident nucleophiles. The theoretical reactivity order determined is phenyl > methyl > tert‐butyl ≅ bicyclo‐[2.2.1]hept‐1‐yl. The activation energy for the coupling of aliphatic radicals is ascribed mainly to nonbonded interactions. Considering possible solvent effects, we propose the same order of reactivity in solution. The coupling reaction of phenyl radical with 1 is an exothermic process with nonactivation energy in the gas phase. Any energy barrier in solution is here ascribed to desolvation of the anion. The regiochemistry of the coupling of radicals with ambident nucleophiles such as 1, phenoxide, or phenylamide anions is also explained on a theoretical basis. © 1992 John Wiley & Sons, Inc.
Semiempirical calculations of molecular polarizabilities and hyperpolarizabilities of polycyclic aromatic compounds
Authors: Lu Yuh‐Jy , Lee Shyi‐Long
Published: December 2004

The correlation of 2D conjugation and the nonlinear properties of a series of polycyclic aromatic compounds were studied by performing semiempirical calculations using the MOPAC package. For linear polyacenes, both linear and nonlinear properties are found to be increased as the number of six‐membered rings increases. In general, the polarizabilities, 〈α〉, and second hyperpolarizabilities, 〈γ〉, are found to correlate very well with total resonance energies. In the case of first hyperpolarizabilities, 〈β〉, symmetry is the dominating factor. © 1992 John Wiley & Sons, Inc.
Extension of MNDO to d orbitals: Parameters and results for the halogens
Authors: Thiel Walter , Voityuk Alexander A.
Published: December 2004

A recently proposed extension of the MNDO formalism to d orbitals has been parameterized for the halogens CI, Br, and I. Extensive test calculations indicate slight consistent improvements for normalvalent molecules and dramatic improvements for hypervalent molecules, in comparison with established MNDO‐type methods without d orbitals. The mean absolute errors in calculated heats of formation are 3.9 kcal/mol for 155 normalvalent compounds and 2.8 kcal/mol for 23 hypervalent compounds. The predicted structures of the hypervalent molecules are qualitatively correct, with a mean absolute error of 2° in 19 bond angles.
Calculation of the long‐range coupling between electrons in lone pairs and double bonds using semiempirical and ab initio methods
Authors: Braga Manuel , Larsson Sven
Published: December 2004

The electronic coupling between symmetrically placed lone pairs or π electrons are calculated using semiempirical and ab initio methods and the results compared to experimental numbers. The agreement is satisfactory in cases when there is a small tendency for cancellation between contributions of different signs. We have found nonexponential behavior when the number of intervening cyclohexane units is increased in the case of phenyl endgroups. In this case as well as in the cases with exponential decrease, the CNDO/S method shows a similar behavior as that of the ab initio Hartree–Fock method, which suggests that the former method may be used to calculate the electronic factor in electron transfer systems. © 1992 John Wiley & Sons, Inc.
On the agostic interaction in lanthanide phenylene complexes: An INDO study of [(CpM)2C6H4] (M = Sc, Lu) employing an energy‐partitioning analysis
Authors: Jahns V. , Köstlmeier S. , Kotzian M. , Röusch N. , Watson P. L.
Published: December 2004

INDO ground‐state calculations have been performed to elucidate the electronic structure of the two para‐dimetallated σ‐phenylene complexes [(CpM)2C6H4], M = Sc, Lu. The lutetium complex exhibits an extraordinarily small MCC angle of about 88°. This unusual geometry is attributed to an agostic interaction between the metal center and the (CH)β group of the σ‐bonded aryl ring. For the analogous scandium compound, the corresponding angle is 118°, rather close to the “normal” value of 120°. Both experimental findings are very well reproduced in the computational models. An energy‐partitioning analysis that exploits the features of the INDO formalism is applied in order to rationalize this geometry difference. The bending of the aryl ligand out of the plane defined by the metal coordination is favored by covalent ligand–metal interactions, but is counteracted by electrostatic repulsion effects. For the lutetium complex, the covalent interactions dominate due to the extended nature of the Lu 5d orbitals and an agostic distortion of the geometry results.
A critical examination of H‐bonding interactions calculated using the AM1 molecular orbital method
Authors: Dannenberg J. J. , Evleth E. M.
Published: December 2004

AM1 calculations for various intermolecular and internally hydrogen‐bonding species are compared with experiment. The calculations generally provide a useful, but not perfect, means for modeling H‐bonds. The average stabilization energy for 33 gas‐phase hydrations of ions was 1.4, whereas that for amination of 10 ions was 4.2 kcal/mol, too weak when compared to experimental results. H‐bonding distances have a tendency to be long. The tendency of AM1 to predict multicentered H‐bonds is discussed in the context of recent surveys of crystallographic data banks. Caution is recommended in comparing AM1 results with inappropriate experiments and inadequate ab initio calculations. © 1992 John Wiley & Sons, Inc.
Valence‐bond calculations on ZNO and HGO using integrals computed through the semiempirical AM1 method
Authors: Sola Miquel , Balcells Merce , Duran Miquel , Lledos Agusti , Bertran Juan
Published: December 2004

Valence‐bond calculations have been carried out on ZnO and HgO using a basis set of Slatertype atomic orbitals and the one‐ and two‐electron integrals as computed in the semiempirical AM1 molecular orbital method. The zero differential overlap approximation has been used to calculate integrals between atomic orbital Slater determinants using the rules for matrix elements between determinants formed by orthogonal orbitals. Diabatic and adiabatic curves have been analyzed for the two systems, and results compared with molecular orbital AM1 results. © 1992 John Wiley & Sons, Inc.
Ab initio study of low‐lying electronic states of the FNO2 molecule
Authors: Cai Z.‐L. , Wang Y.‐F. , Xiao H.‐M.
Published: December 2004

Ab initio electronic structure calculations are reported for low‐lying electronic states, 1A1, 1A2, 3A2, 1B1, 3B1, 1B2, and 3B2 of the FNO2 molecule. Geometric parameters for the ground state 1A1 are predicted by MRSDCI calculations with a double‐zeta plus polarization basis set. The vertical excitation energies for these electronic states are determined using MRSDCI/DZ+P calculations at the ground‐state equilibrium conformation. The oscillator strengths and radiative lifetimes for some electronic states are calculated based on the MRSDCI wave functions. © 1993 John Wiley & Sons, Inc.
Hylleraas–CI with linked correlation terms
Authors: Kleindienst H. , Lüchow A.
Published: December 2004

Hylleraas–CI calculations with linked correlation terms of the form rr are discussed. Formulas for the integration of the angular part are deduced and a method for the reduction of the radial part to auxiliary integrals is given. In the case of the Li atom, it is shown that for the calculation of the ground‐state energy an ansatz for the wave function with at most two factors rr is sufficient to achieve spectroscopic accuracy. © 1993 John Wiley & Sons, Inc.
Direct solution of the many‐body schröudinger equation in the hyperspherical formalism: Application of the HH–GLF method to the positronium ion e+e−e+
Authors: Bian Wensheng , Deng Conghao
Published: December 2004

We apply the HH–GLF method, a new simple hyperspherical harmonic method proposed recently by one of us, to directly solve the three‐body Schrödinger equation for e+e−e+. Uniformally convergent energy eigenvalues are obtained with only several GLF and the obtained ground‐state energy with 200 HH and 6 GLF is −0.26124 au, which is very near the exact value of −0.26200 au. Energy results for maximum global momentum Km ≤ 20 are compared with those from some other hyperspherical techniques carefully, and we find that, in the example of e+e−e+, the HH–GLF method can yield results as accurate as the best available other HH method, but is conceptually simpler and more convenient for practical calculations with a large number of hyperspherical harmonics. © 1994 John Wiley & Sons, Inc.
Vibrational Stark effect and vibrational static electric properties of N2O
Authors: Andrés José Luis , Bertrán Juan , Duran Miquel , Marti Josep
Published: December 2004

The vibrational Stark effect together with nuclear relaxation and vibrational contributions to the static molecular electric properties of the N2O molecule are computed using ab initio molecular orbital thoery. Contributions to the molecular properties are computed by finite‐difference techniques involving the energy vs. the uniform electric‐field strength. © 1994 John Wiley & Sons, Inc.
Ab initio calculations to the reactions of HF and HCl with Si(OH)4 and (HO)3SiOSi(OH)3: Modeling of SiO2 etching reactions
Authors: Jenichen A.
Published: December 2004

The local mechanisms of dissociative chemisorption and reactive etching are investigated by molecular models modeling SiOH groups and SiOSi bridges at SiO2 surfaces. The elementary reaction paths for the reactions of HF and HCl with SiO bonds of the models, Si(OH)4 and (HO)3SiOSi(OH)3, are calculated by the Hartree–Fock method. The local transition‐state structures are nearly plane quadrangles with neighboring pentacoordinated silicon and tricoordinated oxygen atoms. For the reaction and activation energies, the electron correlation is considered by the Møller–Plesset perturbation theory. The results agree with experimental findings. © 1994 John Wiley & Sons, Inc.
Iso‐energy cutoff for the calculation of interionic potential of mean force in water
Authors: Mezei Mihaly
Published: December 2004

An iso‐energy cutoff scheme is introduced for the calculation of the potential of mean force between two ions in water. The cutoff criterion is based on the optimal interaction of the water dipole with the ion pair, for which analytical expressions are derived. Formulas are also derived to characterize the solvent reorganization contribution to the potential of mean force. Treatment of the contributions from waters outside the cutoff is also discussed. © 1994 John Wiley & Sons, Inc.
Nuclear spin–spin coupling and nuclear motion
Authors: Raynes W. T. , Geertsen J. , Oddershede J.
Published: December 2004

The vibration and rotation of molecules affects nuclear spin–spin coupling constants. This manifests itself as a temperature dependence of the coupling and also as an isotope effect (after allowing, where necessary, for differing magnetogyric ratios of the two nuclei involved in the isotopic substitution). Within the Born–Oppenheimer approximation, a nuclear spin–spin coupling surface can be defined for each pair of coupled nuclei. This surface is sampled by the nuclei as they undergo the excursions about equilibrium geometry that are governed by the force field. An accurate ab initio carbon–proton spin–spin coupling surface for the methane molecule has been calculated. This was obtained by summing the surfaces for each of the four contributions—Fermi contact, spin–dipolar, orbital paramagnetic, and orbital diamagnetic—expressed as power series in terms of symmetry coordinates. Preliminary calculations for 13CH4 and 13CD4 give a difference of only 6% between the calculated and observed nuclear motion contributions. The observed temperature dependence is also accounted for by the calculations. For these isotopomers, bond stretching plays the dominant role. © 1994 John Wiley & Sons, Inc.
Theoretical study of the structure and vibrational spectra of the (H2O)2…HF and H2O…(HF)2 molecular complexes
Authors: Rovira Carme , Constans Pere , Whangbo M.‐H. , Novoa Juan J.
Published: December 2004

Using the Hartree–Fock and MP2 methods with bases of up to 6‐31++G(2d, 2p) quality, the optimum geometry of the 1:2 and 2:1 (H2O)n… (HF)m complexes of water and hydrogen fluoride is searched in a systematic way. Two minimum‐energy conformations are found for the 1:2 complex connected through a low‐energy transition state. For the 2:1 complex, only one minimum‐energy structure is obtained. The analysis of the geometries of the minima and their vibrational frequencies shows that none of them can be used to explain the existence of the H …FH reverse complex detected experimentally. © 1994 John Wiley & Sons, Inc.
Structure and stability of BN microclusters: Ab initio calculations for (BN)n (n = 2–4)
Authors: Sutjianto Amin , Pandey Ravindra , Manuel Recio J.
Published: December 2004

Unrestricted Hartree–Fock calcuations coupled with second‐order Møller–Plesset correlation correction were performed to study the structures and energetics of microlusters. For (BN)2, linear and rhombus forms are almost isoenergetic, whereas cyclic forms are preferable for (BN)3 and (BN)4 clusters. As a general trend, linear isomers prefer the triplet spin state, whereas cyclic isomers prefer the singlet spin state. Total charge density plots show a strong dominace of the BN bond, indicating that the extent of its polar character becomes stronger with the increase in the cluster size. The loss of a BN monomer is shown to be the most likely fragmentation channel for both neutral and single‐ionized clusters. We find that neutral (BN)n clusters have the same structural configurations as those of their corresponding C2n counterparts. This similarity follows the isoelectronic principle and is of importance due to recent interest in the investigations of BN fullerene analogs. © 1994 John Wiley & Sons, Inc.
Internal dynamics of simple floppy molecules
Authors: Zülicke L. , Zuhrt Ch. , Chapuisat X. , Saint‐Espès C.
Published: December 2004

The dynamical behavior of simple nonrigid molecules still attracts much interest both from experimentalists and theoreticians. On the one hand, modern laser‐spectroscopic techniques like SEP allow for the detection of highly excited vibrational–rotational states of a molecule, and advanced theoretical methods, on the other hand, are more and more able to calculate accurate potential energy surfaces and to simulate the intramolecular dynamics. The aim of the present article was to contribute to the understanding of the dynamical properties of simple floppy molecules by means of a comparative study of the two triatomics HCN and HO2. Continuing our earlier work, we start from an analysis of the potential energy surface topography, then we investigate the classical dynamical behavior and the localization of the quantum states. Several conclusions of probably more general validity are drawn. © 1994 John Wiley & Sons, Inc.
The role of the bond midpoint electron density in homonuclear molecular binding
Authors: March N. H.
Published: December 2004

Electron density theory is first employed to express the potential energy curve of the H molecular ion in terms of bond midpoint properties. For heavy homonuclear diatomics, low‐ordr density gradient theory is used, but now for the chemical potential, with a similar conclusion to that for H. Homonuclear clusters of alkali atoms are then treated in some detail. Finally the dissociation of doubly charged clusters is considered, by a study of supermolecular treated in some detail. Finally the dissociation of doubly charged clusters is considered, by a study of supermolecular ions (Na)2 and (K)2, again using a density gradient expansion in low order. The deviation between the barrier for fission and the Coulomb barrier is linear in the bond midpoint density over a substantial range of fragment separation. © 1994 John Wiley & Sons, Inc.
Lower and upper bounds of the energy spectrum for potentials with multiminima
Authors: Žitňan Peter
Published: December 2004

An extended Rayleigh–Ritz method for computing two‐sided eigenvalue bounds of the one‐dimensional Schrödinger equation without using a complementary method is presented. The method is based on the B‐spline approximation over the truncated domain, which results in a generalized banded matrix eigenvalue problem. Numerical results for two multiminima potentials of different natures are presented. © 1994 John Wiley & Sons, Inc.
Spectroscopic properties and potential energy curves of some low‐lying electronic states of AlO, AlO+, LaO, and LaO+: An ab initio CASSCF study
Authors: Márquez Antonio , Capitán Maria José , Odriozola José Antonio , Sanz Javier Fernández
Published: December 2004

Results of CASSCF state‐averaged calculations on the lowest electronic states of LaO and LaO+ are reported in this work. For comparison, some low‐lying electronic states of AlO and AlO+ are also reported. The ground state of LaO was found to be the X2Σ+ (Re = 1.987 Å, ωe = 794 cm−1) with a low‐lying A2Δ excited state. Five more excited states below 26000 cm−1 were found. The first ionization potential (IP) is found at 5.16 eV, resulting in an X1Σ+ ground state for the LaO+ diatom, in opposition to AlO+ for which an X3 Π ground state has been found. Analysis of the wave functions, dipole moments, and Mulliken populations reveal that the bonding is quite ionic in both systems. © 1994 John Wiley & Sons, Inc.
Ab initio calculations of relativistic and electron correlation effects in polyatomics using the universal Gaussian basis set: XeF2
Authors: Malli Gulzari L. , Styszynski Jacek , Da Silva Alberico B. F.
Published: December 2004

Ab initio accurate all‐electron relativistic molecular orbital Dirac–Fock self‐consistent field calculations are reported for the linear symmetric XeF2 molecule at various internuclear distances with our recently developed relativistic universal Gaussian basis set. The nonrelativistic limit Hartree–Fock calculations were also performed for XeF2 at various internuclear distances. The relativistic correction to the electronic energy of XeF2 was calculated as ∼ −215 hartrees (−5850 eV) by using the Dirac–Fock method. The dominant magnetic part of the Breit interaction correction to the nonrelativistic interelectron Coulomb repulsion was included in our calculations by both the Dirac–Fock–Breit self‐consistent field and perturbation methods. The calculated Breit correction is ∼6.5 hartrees (177 eV) for XeF2. The relativistic Dirac–Fock as well as the nonrelativistic HF wave functions predict XeF2 to be unbound, due to neglect of electron correlation effects. These effects were incorporated for XeF2 by using various ab initio post Hartree–Fock methods. The calculated dissociation energy obtained using the MP2(full) method with our extensive basis set of 313 primitive Gaussians that included d and f polarization functions on Xe and F is 2.77 eV, whereas the experimental dissociation energy is 2.78 eV. The calculated correlation energy is ∼ −2 hartrees (−54 eV) at the predicted internuclear distance of 1.986 Å, which is in excellent agreement with the experimental Xe—F distance of 1.979 Å in XeF2. In summary, electron correlation effects must be included in accurate ab initio calculations since it has been shown here that their inclusion is crucial for obtaining theoretical dissociation energy (De) close to experimental value for XeF2. Furthermore, relativistic effects have been shown to make an extremely significant contribution to the total energy and orbital binding energies of XeF2. © 1995 John Wiley & Sons, Inc.
Electron correlation and relativistic effects in xenon tetrafluoride
Authors: Styszynski Jacek , Malli Gulzari L.
Published: December 2004

Ab initio all‐electron fully relativistic Dirac–Fock self‐consistent field and Dirac–Fock–Breit calculations are reported for the XeF4 molecule at various internuclear distances assuming the experimental D4h geometry with our recently developed relativistic universal Gaussian basis set. The nonrelativistic limit Hartree–Fock calculations were also performed for XeF4 at various internuclear distances. The calculated relativistic correction to the total energy of molecule at the Dirac–Fock level is ∼ −5856 eV, whereas the magnetic part of the Breit correction to the electron‐electron interaction is calculated as ∼ 177 eV. The electron correlation effects were included in the nonrelativistic Hartree–Fock calculations using the second‐order Møller‐Plesset (MP2) theory, and the calculated correlation energy for XeF4 is −71 eV. The basis‐set superposition error (BSSE) was estimated by using the counterpoise method for Xe and F. The inclusion of both the relativistic and electron correlation effects in the calculated total energies of F, Xe, and XeF4 predicts the Xe—F bond length and dissociation energy of XeF4 as 1.952 Å and 5.59 eV, respectively, which are in excellent agreement with the experimental values of 1.953 Å and 5.69 eV, respectively, for XeF4. The contribution of the electron correlation and relativistic effects to the dissociation energy of XeF4 is 8.11 and 0.05 eV, respectively. The Breit interaction, however, contributes only 0.02 eV to the dissociation energy of XeF4. Electron correlation is most significant for the prediction of an accurate value of dissociation energy, whereas relativistic effects are very important for the prediction of spin‐orbital splitting as well as the energies of the orbitals, especially the inner orbitals of XeF4. © 1995 John Wiley & Sons, Inc.
Effctive nonadiabatic calculations on the ground state of the HD+ molecule
Authors: Kozlowski Pawel M. , Adamowicz Ludwik
Published: December 2004

The nonadiabatic methodology, which is based on an effective elimination of the center‐of‐mass motion rather than explicit separation achieved by a coordinate transformation, is applied to the ground state of the HD+ molecule. The many‐body nonadiabatic wave function is generated in terms of explicitly correlated Gaussian functions. The analytical first and second derivatives of the variational functional with respect to the Gaussian exponents are applied in conjunction with the Newton–Raphson optimization method to find the nonadiabatic energy and the ground–wave function. The numerical results are compared with conventional nonadiabatic calculations. © 1995 John Wiley & Sons, Inc.
Ab initio calculation of stationary points for the ground and the first excited state of HCO
Authors: Bittererová Martina , Lischka Hans , Biskupič Stanislav
Published: December 2004

SCF and single‐reference ACPF calculations were performed in order to determine the structure, stability, and harmonic vibrational frequencies of stationary points for the HCO radical in the ground (2A′) and first excited (2A″) states. Very large and flexible basis sets including two f functions on the heavy atoms and two d functions on hydrogen were used. The calculated geometries and vibrational frequencies are in good agreement with available experimental data. The relative stabilities are now also much better balanced compared to previous theoretical results. © 1995 John Wiley & Sons, Inc.
Complete active‐space configuration interaction with optimized orbitals: Application to Li2
Authors: Evangelisti Stefano , Bendazzoli Gian Luigi , Gagliardi Laura
Published: December 2004

We performed CAS–CI calculations on Li2 using a set of molecular orbitals (MO) optimized with a procedure that, in the case of highly symmetric molecules, permits extraction of a small set of MO out of a large set of atomic orbitals (AO). The dimension of the CAS–CI space was of about 12 million symmetry‐adapted determinants. We determined some spectroscopic constants of Li2 with three different atomic basis sets of increasing quality. The values obtained with the largest atomic basis set are very close to the experimental results. © 1995 John Wiley & Sons, Inc.
The QDPT CI approach for excitation and ionization spectra: A test on the CO molecule
Authors: Lisini A. , Decleva P.
Published: December 2004

A detailed analysis of the performance of the QDPT CI method, which is based on the application of quasi‐degenerate perturbation theory (QDPT) within the configuration interaction (CI) approach, is presented by comparing the QDPT CI results and those obtained from the diagonalization of the perturbative space. Calculations of core and valence excitation and ionization spectra of the CO molecule, which is a strongly correlated system, are carried out using a DZ basis set and the accuracy of the eigenvalues and eigenfunctions is checked through the comparison of excitation and ionization energies and intensities. Excellent agreement is obtained for all values, indicating the computational advantages of QDPT CI which is able to explore wide energy ranges without requiring the diagonalization of large spaces. © 1995 John Wiley & Sons, Inc.
On wave‐function correction to Hellmann–Feynman force: Hartree–fock method
Authors: Kryachko Eugene S.
Published: December 2004

The formula for the wave‐function correction to the Hellmann–Feynman force provided by truncation of a basis set of orbitals centered at nuclei is reexamined. Correct application of a projector technique results in the validity of the Hellmann–Feynman theorem for finite basis sets. © 1995 John Wiley & Sons, Inc.
Why N2O2 is cis while (CHO)2 is trans: MO and VBstudies
Authors: Mo Yirong , Zhang Qianer
Published: December 2004

The nonempirical valence‐bond method as well as the molecular orbital method are applied to investigate the structures of N2O2 and (CHO)2. Results show that in either molecule the σ‐component tends toward a trans conformation, whereas, the π‐component tends toward a cis conformation. Moreover, even if there are some long‐range bonding overlaps in the cis structure, the bonding energy cannot counteract the increased Coulombic repulsive energy compared with the trans structure. In other words, for a cis conformation, there is a competition between the Coulombic destabilization and the π electronic stabilization. The latter is more preferable to a cis conformation of N2O2, while the Coulombic repulsive interaction should be responsible for the trans conformation of (CHO)2. The nonempirical vB calculations show that the π delocalization energy in N2O2 is negligible, while the value in (CHO)2 is about 4.5 kcal/mol. © 1995 John Wiley & Sons, Inc.
Theoretical study on the photodecarboxylation reaction of methacrylic acid in the gas phase
Authors: Fang Wei‐Hai , You Xiao‐Zeng
Published: December 2004

The photochemical decarboxylation mechanism of methacrylic acid near 249 nm was investigated with the unrestricted Hartree–Fock method. Our investigation shows that the methacrylic acid molecules obtained by photoexcitation are likely to undergo unimolecular decarboxylation either from its lowest 1nπ* state or, following internal conversion, from the ground state. These results support the photodecarboxylation mechanism proposed by Rosenfeld and Weiner. © 1995 John Wiley & Sons, Inc.
Spectral density distribution of an N‐electron Hamiltonian in a finite‐dimensional and spin‐adapted model space
Authors: Rajadell F. , Planelles J. , Karwowski J. , Mas V.
Published: December 2004

Expressions for moments of spectral density distributions of a many‐electron Hamiltonian defined in a finite‐dimensional, antisymmetric, and spin‐adapted model space (as, e.g., a full configuration interaction space) are derived. The moments are expressed in terms of combinations of two‐electron integrals corresponding to a symmetric (a two‐electron singlet) and antisymmetric (two‐electron triplet) two‐electron wave functions. A diagrammatic approach based on Hugenholtz‐type diagrams and leading to a simple and universal classification scheme of the terms appearing in the expression for a specific moment is proposed. © 1995 John Wiley & Sons, Inc.
Near‐Dirac–Hartree–Fock results for first‐row atoms calculated with GTO basis sets
Authors: Datta Sambhu N.
Published: December 2004

Relativistic basis sets for first‐row atoms have been constructed by using the near‐Hartree–Fock (nonrelativistic) eigenvectors calculated by Partridge. These bases generate results of near‐Dirac–Hartree–Fock quality. Relativistic total and orbital energies, relativistic corrections to the total energy, and magnetic interaction energies for the first‐row atoms have been presented. The smallest Gaussian expansions (13s8 p expansions) yield Dirac–Hartree–Fock total energies accurate through six significant digits, while the largest expansions (18s13p expansions) give these energies accurate through seven significant digits. These results are more accurate than some of the results reported earlier, particularly for the open‐shell atoms, indicating that the basis employed is reasonably economical for relativistic calculations. © 1995 John Wiley & Sons, Inc.