bluuebunny/crossref_metadata_2025_split huggingface dataset:
The externally contracted CI method applied to N2
Authors: Siegbahn Per E. M. 
Published: December 2004

AbstractAn approximate multireference CI method is presented. By grouping together configurations with the same internal parts and freezing their relative weights by the use of perturbation theory, the number of variational parameters is drastically reduced. The loss of correlation energy is shown to be usually less than 2%, and the timing is less than one ordinary CI iteration. Examples from calculations on some states of the nitrogen atom and nitrogen molecule are given. The basis set convergence for the lowest excitation energy in the atom is very slow. Less than 50% of the correlation effect is obtained at the s, p, d limit. After the inclusion of ƒ functions this value is improved to 83%. The dissociation energies of the molecule also show slow basis set convergence with errors of 0.5 eV even after addition of ƒ functions. The bond distances are, howeever, accurately reproduced with errors of less than 0.005 Å for all the states. A qualitative discussion of predissociation in the a 1Πg and B 3Πgstates caused by spin–orbit interaction with the 5Σg+ state, is finally presented. Rapidly oscillating lifetimes between the different vibrational states are predicted.
Hartree–Fock exchange energy of an inhomogeneous electron gas
Authors: Becke A. D. 
Published: December 2004

AbstractWe examine the short‐range behavior of the spherically averaged Hartree–Fock exchange charge density by performing a simple Taylor expansion. On the basis of this expansion, a theoretical model is constructed that generates gradient correction terms to the local density approximation for the exchange energy of an inhomogeneous electron gas. In particular, we derive the Xαβ exchange energy functional and a theoretical value for the parameter β. Our value for β agrees well with previous empirical estimates, and with empirical calculations in the present work.
Avoided crossing and intermediate states in certain photochemical reactions
Authors: Lee Tieh‐Sheng , Chu San‐Yan 
Published: December 2004

AbstractAn attempt to classify various types of avoided surface crossings originally discussed by Salem and co‐workers is presented. From the present scheme, an order‐of‐forbiddenness criterion regarding reactivity is established from a consideration of electron and orbital counting. Since the new classification is qualitatively related to the energy gap created in the avoided crossing, the classification appears to be more systematic and informative. Energy transfer processes involved in electronically excited states may also be rationalized by such an application. Three major mechanistic types are distinguished: concerted two‐electron process, concerted one‐electron process, and stepwise one‐electron process involving an ionic‐pair intermediate.
Quantum model of coherent X‐ray diffraction: Extension to Bloch orbitals
Authors: Goldberg M. J. , Massa L. J. 
Published: December 2004

AbstractThe description of electron density in solids by quantum theory and by x‐ray diffraction experiments should closely coincide. A formalism for describing the electronic structure of single crystals in terms of a density matrix is presented. Iterative equations for calculating the density matrix in a basis of Bloch orbitals using measured crystallographic intensity data are derived. Numerical results are presented. The Bloch orbital formalism explicitly accounts for the mutual interaction of atoms in different unit cells, and is useful for insulators, semiconductors, and metals.
Discussion of several analytical approximate expressions for the eigenvalues of the bounded harmonic oscillator and hydrogen atom
Authors: Arteca Gustavo A. , Maluendes Sergio A. , Fernández Francisco M. , Castro Eduardo A. 
Published: December 2004

AbstractSeveral approximate analytical formulas for the multidimensional isotropic bounded oscillators and the bounded hydrogen atom are compared. Numerical results show that the coth z method is in both cases better than the Padé approximants method. Perturbational polynomials, necessary in order to build the approximate eigenvalues, are obtained through the hypervirial perturbative method.
Violations of the noncrossing rule due to special symmetry and alternance
Authors: Zhixing Chen , Ruiyu Hong , Yala Zhang 
Published: December 2004

AbstractThis paper reports a study on which behavior of the Hamiltonian gives rise to violation of the noncrossing rule. In principle, the noncrossing rule may be violated when a special symmetry other than spatial and spin symmetries is present or there exists the so‐called alternance, which corresponds to a Hamiltonian in a real vector space anticommuting with a Hermitian operator. In the HMO models for pericyclic reactions, violations due to special symmetry or alternance have been found. The [m,n] supra–antara cycloadditions have no symmetry in the traditional sense, but have special symmetry leading to the existence of crossings in the correlation diagram. Alternance results in one crossing in the middle of the correlation diagram of a forbidden pericyclic reaction with intermediate states in the form of even alternant hydrocarbon. For the reactions with intermediate states in the form of odd alternant hydrocarbon such as [2,4]‐cycloaddition of an allyl cation or an allyl anion to butadiene, there should be no crossing in the correlation diagrams, and both the supra–supra and the supra–antara processes are predicted to be allowed. Such a prediction is beyond the Woodward–Hoffmann rule.
Analytical MO study of the singlet–triplet coupling in xylylenes
Authors: Karafiloglou Padeleimon 
Published: December 2004

AbstractThe singlet–triplet energy difference in para‐, meta‐, and ortho‐xylylenes is studied as the interaction of two radical centers through the benzene ring. An SCF perturbative procedure adapted to open‐shell systems leads to two benzyl‐like nonbonding molecular orbitals (NBMOS) and to benzene‐like occupied and vacant MOS whatever the xylylene isomer. The superposition of these NBMOS in para‐, meta‐, and ortho‐positions and their interaction with the benzene‐like MOS lead, at the configuration interaction level, to the following results: The exchange energy (which favors the triplet state) and the charge transfer energy (which favors the singlet state) are important only in the meta‐xylylene; the dynamic (or double) spin polarization favors the triplet in meta and the singlet in para and ortho‐isomers; the super‐exchange energy (which favors the singlet) is important only in para‐ and ortho‐isomers. The above results are independent of the chosen geometry.
Gap equations for molecular systems
Authors: Sykja Besnik , Calais Jean‐Louis 
Published: December 2004

AbstractThe gap equations for different Fukutome classes are derived in the formalism of second quantization. Compact orbital forms are given for the gap and related to energy difference of the new LUMO and HOMO, ϵLU–ϵHO, obtained from the old RHF molecular orbitals. Approximate expressions are obtained by maximizing the average electron–electron repulsion energy. The simplifications obtained and possible applications to molecular systems are discussed and compared with the instability analysis of an RHF state.
A theoretical study of medium effects on the transmission mechanisms of the fermi contact term of spin–spin coupling constants in the acetamide molecule
Authors: Facelli J. C. , Giribet C. G. , Contreras R. H. 
Published: December 2004

AbstractSolvent effects on different transmission mechanisms of spin–spin coupling constants are analyzed from a theoretical viewpoint. Medium effects are introduced using the solvaton model, and the decomposition of coupling constants in σ‐ and π‐electron transmitted components is accomplished with the PRMO method. Trends thus obtained are in fairly good agreement with experimental findings reported in the literature. In all types of couplings studied in this work, σ and π components show opposite behavior when increasing the polarity of the solvent.
Molecular theory of critical phenomena in aliphatic carboxylic acid monolayers
Authors: Dunne Lawrence J. , Bell George M. , Combs Leon L. 
Published: December 2004

AbstractA statistical mechanical theory is presented for the behavior of monolayers of aliphatic carboxylic acids at the air/water interface. A lattice fluid model is introduced with monomer states to represent molecules in the all‐trans conformation directed perpendicular to the interface, dimer states to represent rotational isomers, and vacant sites or “holes.” Using a Flory–Huggins‐type approximation, the temperature and chain length dependences of the monolayer behavior are satisfactorily reproduced for a certain range of parameters. These parameters are compared with those intermolecular energies calculated using the methods of computational quantum chemistry. The agreement thus obtained is fairly satisfactory indicating that the higher‐order nature of fatty acid monolayer transitions is due to the onset of the rotational ordering of pairs of chains about their longitudinal axes, while the sensitive temperature dependence is attributed to the flexible nature of fatty acid molecules. Some possible effects due to the presence of the substrate are discussed.
The band structure of porphyrinatonickel(II). A semiempirical crystal orbital study based on the tight‐binding formalism
Authors: Böhm Michael C. 
Published: December 2004

AbstractThe band structure of porphyrinatonickel(II) (2) has been studied by means of crystal orbital calculations that are based on the tight‐binding approximation; the computational framework is a recently developed INDO model for transition metal compounds of the 3d series. The porphyrinato polymer has been studied in an eclipsed arrangement (2a) and in a staggered conformation (2b) where neighboring layers are rotated by 41°. The total energy of the metallomacrocycle has been decomposed into one‐ and two‐center contributions; the latter interaction parameters have been fragmented into physically feasible resonance, exchange, and classical electrostatic (electron–electron, electron–core, core–core) interactions. It is shown that individual two‐center potentials between atoms in neighboring layers are prevailingly determined by the electrostatic interaction energy. The NiNi coupling in the chain is highly repulsive; important stabilizing interactions are predicted between the 3d center of one cell and the electronegative N atoms in the neighboring layers. Stabilizing and destabilizing electrostatic interaction potentials largely compensate each other; the net stabilization in the polymer comes from the accumulation of resonance and exchange increments. The unoxidized Ni(II) porphyrinato polymer is an insulator. Several ligand bands (π, σ, and lone‐pair) are predicted on top of bands with significant Ni 3d admixtures; the conduction band of the unoxidized strand is of ligand π* character. The dense manifold of ligand states in the vicinity of the Ni 3d states (3d, 3d, 3dxz/3dyz) prevents the formation of bands in the polymer that are strongly localized at the 3d center. Ni 3d and 3d interact strongly with ligand lone‐pair and σ states. Avoided crossings between ϵ(k) curves in k space lead to compositions in the various bands that differ significantly at the bottom and the top. The INDO crystal orbital formalism predicts a partial oxidation of ligand bands in derivatives of 2 that contain oxidants (e.g., halides). The theoretical findings derived for 2 are compared with available experimental data on highly conducting porphyrinatonicke(II) polymers (tetrabenzo and octamethyltetrabenzo derivatives of 2).
Bond length alternation in cyclic polyenes. V. Local finite‐order perturbation theory approach
Authors: Takahashi M. , Paldus J. 
Published: December 2004

AbstractThe finite‐order many‐body perturbation theory using the localized Wannier orbital basis is applied to the problem of bond length alternation in the Pariser–Parr–Pople model of cyclic polyenes CN HN, N = 4v + 2, which may be regarded as a simplified model of polyacetylene. Both the Møller–Plesset and the Epstein–Nesbet‐type partitionings of the model Hamiltonian are employed. The localized orbital basis enables an efficient truncation of the perturbation theory summations over the intermediate states as well as an elimination of energetically unimportant diagrams, thus enabling one to obtain the fourth‐order Møller–Plesset‐perturbation energies with a relatively small computational effort even for large polyenes. The results obtained with the second‐, third‐, and fourth‐order Møller–Plesset and with the third‐order Epstein–Nesbet perturbation theories yield very similar bond length distortions (about 0.05 Å) and stabilization energies per site (about 0.04 eV) as obtained earlier with the RHF, one‐parameter AMO, and delocalized orbital perturbation theories. The effects of truncation and diagram elimination in the fourth‐order Møller–Plesset perturbation theory and the abnormal behavior of the second‐order Epstein–Nesbet perturbation theory results in the localized Wannier basis near the instability threshold of the RHF solutions are discussed.
Effective basis sets for calculations of exchange‐repulsion energy
Authors: Gutowski Maciej , Chałasiǹski Grzegorz , Van Duijneveldt‐Van De Rijdt Jeanne 
Published: December 2004

AbstractUsefulness of different Gaussian basis sets for reproducing the “tail” region of the SCF wavefunctions employed in calculations of the exchange‐repulsion effect is investigated for the model He‐He interaction. It has been shown that extension of the monomer‐centered basis set in the scheme of regularized even‐tempered basis sets [M. W. Schmidt and K. Ruedenberg, J. Chem. Phys. 71, 3951 (1979)] can be more efficient than augmentation of the fully energy‐optimized basis set with diffuse basis functions. It has been also found that Landshoff term vanishes and the “tail” region is well reproduced if monomer wavefunctions are calculated with the basis set of the dimer.
Many‐electron theory of autoionizing states using complex coordinates: The position and the partial and total widths of the Ne+ 1s hole state
Authors: Nicolaides C. A. , Mercouris Th. , Komninos Y. 
Published: December 2004

AbstractThis article presents the first results of the application of quantum mechanics with complex coordinates to the calculation of partial widths for the radiationless decay of an inner‐hole excited autoionizing state, the Ne+1s2s22p6 2S. This is succeeded by the reduction of the multi‐electron, multichannel problem in the complex energy plane to five, symmetry adapted, two‐electron problems, in accordance with a published theory of many‐electron resonances. These two‐electron problems are solved independently by using rotated analytic Hartree‐Fock orbitals (expressed in terms of Slater orbitals) for the localized components, and Slater plus Gamow orbitals for the rotated, asymptotic square‐integrable functions carrying the width information. A recently proposed variational principle is employed for the optimization of nonlinear parameters. Within this independent asymptotic pair approximation (IAPA), our results for the partial widths to the five Ne2+ channels are (in 10−2 a.u.): 1s‐2p2 1D: 0.560, 1S: 0.048; 1s‐2s2p, 3P0: 0.029, 1P0: 0.154; 1s‐2s2, 1S: 0.044. The total width is 0.835. These numbers agree reasonably well with those obtained by Kelly [Phys. Rev. A 11, 556 (1975)] from a many‐body perturbation theory (MBPT) calculation, and by Howat et al. [J. Phys. B 11, 1575 (1978)] from a configuration‐interaction in the continuum calculation. The most recent experimental results yield 0.604, 0.089, 0.063, 0.174, and 0.060, respectively, with a total width of 0.99. Previous real‐coordinate many‐electron calculations by Beck and Nicolaides–including relativistic and radiative effects–have predicted the position of the Ne+ 1s hole state at E0 = 870.4 eV above the Ne ground state. It has already been shown that the real energy corresponding to the localized component of the autoionizing state is stable under rotations of the function space describing it. Therefore, the earlier E0 can be incorporated into the present calculation in the complex plane. The shift due to the additive contribution of the IAPA is found to be − 0.09 eV. When this is added to E0, the final E = 870.3 eV is in excellent agreement with experiment [870.3 eV; T. D. Thomas and R. W. Shaw, Jr., J. Electron. Spectrosc. Relat. Phenom. 8, 45 (1976)].
Electronic structure of the peptide linkage. II. A molecular orbital treatment of the electronic spectra of benzohydroxamic acids
Authors: Abu‐Eittah Rafie , Hilal Rifaat , Moustafa Hussein 
Published: December 2004

AbstractThe electronic absorption spectra of some benzohydroxamic acids have been analyzed and interpreted using the results of molecular‐orbital calculations. Charge density distributions, dipole moments, and the extent of delocalization of the MOS were used to interpret the observed solvent effects. Comparison between the spectra of benzohydroxamic acids and that of simple amides led to conclusions regarding the structural differences between the two classes of compounds.
What do local energy maps tell about local error?
Authors: Casida Mark 
Published: December 2004

AbstractThe usefulness of the notion that a one‐particle reduced local energy map can measure local errors in a trial wave function is investigated. Examination of the literature suggests that this notion rests on the assumption that a map of the relative error in the one‐particle density behaves like the one‐particle reduced local energy map. This is tested for several spinless one‐dimensional one‐particle systems. The assumption is discovered to be ill founded when applied to a certain contrived situation, and not to be quantitative when applied to familiar situations. The possibility remains, however, that the assumption may sometimes be useful for obtaining the correct ordering of trial wave functions with respect to the accuracy of certain types of expectation values.
A note on the convergence of multiconfigurational many‐body perturbation theory
Authors: Iijima Naoto , Saika A. 
Published: December 2004

AbstractThe convergence of multiconfigurational many‐body perturbation theory (MC MBPT) is discussed in connection with the intruder state. Its convergence properties are first examined with a fictitious three‐level system employing a Hermitian version of MC MBPT, which permits a general model space. It is then applied to the H2—H2 and N2 systems. The results suggest that a more extensive model space is likely to embrace new intruder states and the space extension be executed with due caution.
Controlled orthogonalization of localized orbitals
Authors: Pipek J. 
Published: December 2004

AbstractThe electron correlation theories for extended systems require an orthogonal and at the same time well‐localized virtual orbital system. An iterative method is suggested, by which extremely nonorthogonal basis sets can be orthogonalized without destroying the localization, in contrast to other well‐known procedures. With the help of a general formulation of the problem not only the localization but other properties can be achieved as well. The method is compared to Löwdin's orthogonalization. Calculations for model and real systems were carried out and the convergence properties and the stability of the fixed points of the iterative procedure were investigated.
Local exchange‐correlation approximations and first‐row molecular dissociation energies
Authors: Becke A. D. 
Published: December 2004

AbstractRecent Xα calculations of bond energies and other related properties of first‐row diatomic molecules show very encouraging agreement with experiment. In the worst cases, however, the Xα dissociation energies overestimate the experimental values by almost 2 eV. Therefore, we have examined several refinements of the Xα theory and their effects on molecular bond lengths, bond energies, and vibrational frequencies. Among them, gradient corrections to the Xα exchange energy and also some variations of the local spin‐density correlation energy approximation are considered. We find that a local exchange‐correlation functional with gradient corrections gives dissociation energies in significantly better agreement with experiment than the Xα approximation.
An extension of the pairing theorem
Authors: Karadakov Peter 
Published: December 2004

AbstractAn extended pairing scheme is presented which ensures the fulfillment of pairing conditions not only between the sets of occupied orbitals for spin α and for spin β, but also between their orthogonal complements, i.e., the sets of virtual orbitals for spin α and spin β, as well as between occupied orbitals for spin α and virtual orbitals for spin β and between virtual orbitals for spin α and occupied orbitals for spin β. It is shown that the extended pairing properties are suggested by some aspects of the construction of alternant molecular orbitals. The algorithm for singular value decomposition of rectangular matrices is proposed for use in practical implementations of the (extended) pairing scheme.
Molecular properties from perturbation theory: A Unified treatment of energy derivatives
Authors: Almlöf Jan , Taylor Peter R. 
Published: December 2004

AbstractThe definition of a molecular property as a derivative of the electronic energy with respect to one or more applied perturbations is reviewed. The explicit enumeration of terms entering the derivative formulas is performed by considering in turn the various parameter spaces on which the energy and wave function depend. After deriving general expressions for first, second, and third derivatives for different types of perturbation, the parameter spaces involved in MCSCF and CI cases are identified and used to obtain expressions for the first and second derivatives. An example of an MCSCF third derivative is also given. In addition, the various equation systems defining the perturbed wave functions in each order are derived. Some attention is given to the efficient computer implementation of derivative calculations, and the present work is compared with that of other authors.
Molecular orbital study of the ring nitrogen basicity of various dihydrofolate reductase inhibitors
Authors: Ghose Arup K. , Crippen Gordon M. 
Published: December 2004

AbstractWe present molecular orbital (CNDO/2) calculations on the key fragments of different dihydrofolate reductase inhibitors. Distance geometry analysis, physicochemical parameter dependent QSAR, and molecular shape analysis raised some questions regarding the basicity of the ring nitrogen (N1) in these inhibitors and the effect of the various substituents on the basicity. We show that the ring nitrogen N1 of methotrexate has a considerably higher tendency to be protonated compared to that of folic acid. However, not all 2,4‐diamino inhibitors are equally basic. Even 2‐amino‐4‐hydroxyquinazoline is sufficiently basic to be protonated, but not the 2,4‐diamino‐5‐sulfonyl derivatives. The pyrimidinium ion seems to be highly solvated, since in spite of its high protonation energy it is strongly basic. Triazines were found to be the most basic of all the classes studied.