bluuebunny/crossref_metadata_2025_split huggingface dataset:
MNDO Calculations as a valuable tool for structure evaluation of contact ion pairs
Authors: Havlas Z. , Nick S. , Bock H. 
Published: January 2005

AbstractContact ion pairs, essential intermediates of both chemical and biochemical reactions in solution, can be satisfactorily approximated by MNDO calculations as exemplified by the lithium and potassium salts of 2,5‐trimethylsilyl‐p‐benzosemiquinone radical anion and by sodium tetraphenylallyl anion diethyl ether. Topics discussed are the structural changes of p‐benzoquinone derivatives on single‐electron reduction, the dynamics of the countercations in semiquinone radical contact ion pairs measured by temperature‐dependent ESR spectra, as well as structural details of the sodium tetraphenyl‐allylanion complex isolated such as its intramolecular Na dibenzene sandwich or its diethylether solvation. © 1992 John Wiley & Sons, Inc.
Ab initio study of the internal rotation barrier of formamide and the formamide–H2O complex
Authors: Wang Xiao‐Chuan , Facelli Julio C. , Simons Jack 
Published: January 2005

AbstractThe barriers to internal rotation about the NC bond of formamide and the formamide–H2O complex have been examined by ab initio quantum chemistry methods. Both self‐consistent field and correlated approaches have been utilized to determine the geometries, energies, and local harmonic vibrational frequencies of the minimum‐energy and transition‐state structures of these two systems. We find that formamide's rotation barrier of 14–15 kcal/mol is increased to 16–18 kcal/mol when a single H2O molecule is attached. This result contrasts with the effect of a single H2O solvent molecule on the barrier to tautomerization of formamide (to form formamidic acid HNCHOH) for which a barrier lowering of ca. 20 kcal/mol has been observed. The rotation barriers obtained for formamide and for its H2O complex are compared with barriers obtained experimentally in various solvents. © 1993 John Wiley & Sons, Inc.
The MRSDCI studies of four low‐lying electronic states of the BF2 radical
Authors: Cai Z.‐L. 
Published: January 2005

AbstractThe equilibrium geometries, excitation energies, force constants, and vibrational frequencies for four low‐lying electronic states X 2A1, 2B1, 2B2, and 2A2 of the BF2 radical have been calculated at the MRSDCI level with a double zeta plus polarization basis set. Our calculated excitation energy for X2A1 → 2B1 is in agreement with available experimental data. The electronic transition dipole moments, oscillator strengths for the 2B1 → X2A1 and 2B2 → X2A1 transitions, radiative lifetimes for the 2B1 and 2B2 states, and the spin properties for the X2A1 state are calculated based on the MRSDCI wave functions, predicting results in reasonable agreement with available experimental data. © 1993 John Wiley & Sons, Inc.
Møller–Plesset expansion of the dispersion energy in the ring approximation
Authors: Moszynski Robert , Jeziorski Bogumił , Szalewicz Krzysztof 
Published: January 2005

AbstractCoupled‐cluster equations for the calculation of the nonexpanded (fully damped) dispersion energy are derived. These equations are solved in the ring approximation using the Møller–Plesset expansion in terms of the fluctuation potentials WA and WB for the individual molecules. Numerical results of high‐order perturbative calculations for the He, H2, LiH, H2O, and HF dimers are presented and compared with the converged results computed using the same basis sets. It is found that the convergence of the Møller‐Plesset expansion of the dispersion energy in the ring approximation is very fast. The padé approximants still accelerate this already good convergence. For all complexes studied in this paper, the sum of the corrections through the second‐order in WA + WB reproduces over 99% of the converged value. The sum of third‐ and higher‐order corrections in the ring approximation is found to be one or two orders of magnitude smaller than the sum of second‐order terms not included in the ring approximation and, therefore, may be safely neglected. Thus, it appears that a second‐order calculation, which does not require iterating coupled‐cluster equations or solving random phase approximation equations, offers the best compromise between accuracy and computational requirements. © 1993 John Wiley & Sons, Inc.
General algebraic expressions of totally symmetric potential functions for AXn (n = 3,4) molecules
Authors: Ischtwan Josef , Peyerimhoff Sigrid D. 
Published: January 2005

AbstractThe integrity bases for representations of the nuclear permutation groups Sn (n = 3,4) in the space of internal coordinates describing AXn molecules are determined. This allows optimal expressions of the molecular potential energy functions in terms of internal coordinates that are totally symmetric with respect to permutations of indistinguishable nuclei. © 1993 John Wiley & Sons, Inc.
Some applications of nonlinear convergence accelerators
Authors: Homeier Herbert H. H. 
Published: January 2005

AbstractA number of established nonlinear convergence acceleration methods are presented. It is discussed how these methods, and also some new algorithms, can be applied successfully to Fourier series. Some of the new accelerators are tailored for these‐series. Others are also useful for other types of series and sequences, and not only for convergence acceleration, but also for extrapolation or summation problems. Applications to the numerical inversion of the Laplace transform and to a Fourier series arising in the Matsubara formalism are presented. © 1993 John Wiley & Sons, Inc.
On the solid state of hydrogen fluoride: A self‐consistent crystal field study
Authors: Panas Itai 
Published: January 2005

AbstractA recently developed self‐consistent crystal field (SCCF) method is applied to two suggested crystal structures of solid hydrogen fluoride, polar and nonpolar, respectively. Constrained geometry optimizations are performed and the results are compared with experiment and previous theoretical studies. Comparisons are also made with a previous SCCF study on hydrogen cyanide. The Pauli repulsion, dispersion, and “classical” crystal field contributions to the lattice energy are calculated. © 1993 John Wiley & Sons, Inc.
Electronic states and nature of bonding in the molecule YN by all‐electron ab initio CASSCF calculations
Authors: Shim Irene , Gingerich Karl A. 
Published: January 2005

AbstractIn the present work, we present results of all‐electron ab initio CASSCF calculations of nine electronic states of the molecule YN. Also reported are the spectroscopic constants derived on the basis of the calculated potential energies. The predicted electronic ground state is 1∑+, and this state is found to be separated from the excited states 3∑+, 3Π, and 1Π by 5177, 9290, and 9915 cm−1, respectively. The chemical bond in the YN molecule is polar with charge transfer from Y to N, giving rise to a dipole moment of 8.19 Debye at 3.3 au in the 1∑+ ground state is basically a double bond composed of two π bonds. The dissociation energy of the YN molecule has been derived as 4.59 eV. © 1993 John Wiley & Sons, Inc.
The complex coordinate scattering theroy and its application to the study of the surface asymmetry effect in helium diffraction from copper
Authors: Peskin Uri , Moiseyev Nimrod 
Published: January 2005

AbstractThe Complex Coordinate Scattering Theory is reformulated for the general case of a time‐independent Hamiltonian. It is applied to scattering of He atoms from a Cu(115) crystal surface by constracting the Green operator for the T‐matrix from the eigenvectors of both the complex scaled Hamiltonian and its transposed (“right” and “left” eigenvectors), which are different in this case. The weakly asymmetric corrugation function describing the (115) face of Cu is shown to cause a strong dependence of the calculated diffraction intensities upon the direction of the incident atomic beam. The calculated transition probabilities are in excellent agreement with the experimentally measured ones, previously obtained by Perreau and Lapujoulade. We show that additional information about the gas atom/surface physisorption interaction potential can be obtained if the incident angle of the atomic beam (the angles between the beam and the surface normal) is changed from γ to −γ. © 1993 John Wiley & Sons, Inc.
A configuration‐interaction‐oriented implementation of the complex coordinate method
Authors: Saenz Alejandro , Weyrich Wolf , Froelich Piotr 
Published: January 2005

AbstractAn implementation of the complex coordinate method is demonstrated that exploits a new technique for obtaining the matrix representation of the complex dilated Hamiltonian. The purpose is to make the complex coordinate method applicable together with standard numerical ab initio codes designed for large‐scale calculations on many‐electron atoms and/or molecules. No complex intergrals have to be calculated, and no changes of the standard codes are required even in the common case where the kinetic and potential energy components are not stored separately. Instead, two standard (real) CI calculations are used to generate the dilated (complex) CI matrix representation. The performance of the procedure is demonstrated in the context of the GAMESS program and applied to obtain the resonant structure of the Bethe surface pertinent to the absorption spectrum of the helium atom. © 1993 John Wiley & Sons, Inc.
A multiple‐transition‐point WKB investigation of complex energy resonances
Authors: Andersson Nils 
Published: January 2005

AbstractApproximate conditions determining the complex energy resonances of a one‐dimensional model potential, originally discussed by Moiseyev et al., are derived within a multiple‐transition‐point WKB treatment. It is shown that an erroneous energy spectrum is generated if too few transition points are considered. The numerical results of a correct semiclassical treatment of the problem, where several transition points are included, agree well with the, presumably exact, results of Rittby, Elander, and Brändas. © 1993 John Wiley & Sons, Inc.
First‐order corrections to sums of zeroth and first power of absolute values of negative eigenvalues of Schrödinger operators with potentials with Coulomb singularity in the semiclassical limit
Authors: Siedentop Heinz 
Published: January 2005

AbstractFirst‐order corrections jointly obtained with B. Helffer, A. Knauf, and R. Weikard to the sum and the number of negative eigenvalues of Schrödinger operators with potentials that have a negative Coulomb singularity in the quasi‐classical limit are reviewed. The relevance of the result to the ground‐state energy of large atoms is pointed out. © 1993 John Wiley & Sons, Inc.
Protonic delocalization and quantum correlations in the H‐bond dynamics of G–C and κ–π DNA base pairs
Authors: Chatzidimitriou‐Dreismann C. A. 
Published: January 2005

AbstractA new base pair (called κ–π) of Watson–Crick type, with a H‐bond pattern different from that in A–T and G–C base pairs, has been recently synthesized and shown to be stable and incorporable into duplex DNA and RNA by polymerases. This new basepair, which contains three H‐bonds, is compared with G–C, in the framework of modern dynamical theory of quantum nonlocality and quantum correlations. Connection with the traditional treatment of proton transfer in DNA base pairs, which uses the adiabatic approximation, is explicitly made. As a result, the dynamics of the H‐bond pattern of G–C is shown to exhibit a specific quantum mechanical phase stability, which is clearly missing in the case of κ–π. This finding is discussed and illustrated, also in connection with recent quantum chemical calculations of proton transfers in DNA base pairs. Additionally, certain speculations concerning the “evolutionary advantage” of G–C with respect to κ–π are briefly considered. © 1993 John Wiley & Sons, Inc.
On some properties of general linear operators
Authors: Löwdin Per‐olov 
Published: January 2005

AbstractSome elementary properties of a general linear operator, which is defined on a Hilbert space but is not necessarily self‐adjoint, are briefly reviewed: the classical canonical form, the bi‐orthogonality theorem, the bivariational principle, the spectral resolution of the resolvent, and the properties of exponential forms. © 1993 John Wiley & Sons, Inc.
The non‐N‐representability of the Colle–Salvetti second‐order reduced density matrix
Authors: Morrison Robert C. 
Published: January 2005

AbstractThe Colle–Salvetti second‐order reduced density matrix (2‐matrix) is an approximation to the 2‐matrix obtained from a wave function that is a product of a reference wave function containing little or no correlation times a product of correlation factors that are functions of the coordinates of pairs of electrons. A formal proof is given for the non‐N‐representability for the Colle–Salvetti 2‐matrix using the nonnegativity condition of the 2‐matrix. The nonnegativity condition of the particle‐hole overlap matrix (G matrix) is also not satisfied. The proof is valid for Colle–Salvetti 2‐matrices obtained from both the Hartree–Fock and small multiconfigurational‐self‐consistent‐field wave functions. Even though the Colle–Salvetti 2‐matrix is not N‐representable, it does satisfy the Pauli principle component of the G‐matrix condition because it reduces to an N‐representable first‐order reduced density matrix. © 1993 John Wiley & Sons, Inc.
Spin‐unrestricted calculations in quantum chemistry
Authors: Cassam‐Chenaï P. , Chandler G. S. 
Published: January 2005

AbstractThe unrestricted complete active space self‐consistent field (UCASSCF) function is defined, and a proof that a UCASSCF eigenfunction of the spin operator S2 is a CASSCF function is given. The spin‐contamination for an unrestricted Hartree–Fock (UHF) function is evaluated by using Araki angle operators, and the UHF function is then projected on the restricted open‐shell Hartree–Fock (ROHF) space. The present analysis has deep consequences since it implies that the only non‐spin‐contaminated UHF functions are the ROHF functions. This is illustrated in a calculation of the spin density of He. © 1993 John Wiley & Sons, Inc.
A study of the weak interaction in SCO/He and SCO/N2 systems
Authors: Sadlej Joanna , Edwards W. Daniel 
Published: January 2005

AbstractThe structure and energetics of the complexes formed between SCO/He and SCO/N2 were investigated using ab initio wave functions at both the SCF and correlated levels of theory with a medium‐sized polarized basis set. The energy of the T‐structure for SCO ⃛He was found to be lower than the energy of the corresponding linear complexes with He bonded to the oxygen or sulfur atoms. Two linear structures for SCO ⃛ He were found to be nearly isoenergetic. There is only a small difference in energy between the T‐structure for SCO ⃛N2 and the OCS ⃛NN colinear structure, with the T‐structure being the lowest. The electron correlation contributions to the interaction energy were calculated using Möller–Plesset perturbation theory at the MP2, MP3 and SDTQ–MP4 levels. Analysis shows the importance of the triple excitations in these complexes. © 1993 John Wiley & Sons, Inc.
Theoretical study of ionization potentials in monosubstituted benzenes
Authors: Melo André , Gomes J. A. N. F. 
Published: January 2005

AbstractMonosubstituted benzenes, in which the substituents participate in the π‐electron system, are studied following a classification in two classes according to the π‐electronic structure of the substituent. For this type of molecule, a relation is established between the nature of the substituent and, on the one hand, the energies of the two highest occupied molecular orbitals and, on the other hand, their respective differences. The two orbitals referred to above have π‐character and belong to the a2 and b1 species if a C2v point group is assumed. Simple symmetry arguments lead to the conclusion that the a2 orbitals have, essentially, an intraring character, whereas the π‐orbitals of the substituents do give an important contribution to the b1 orbitals. Therefore, an a2 electron must have a larger interaction with the benzene ring and a smaller kinetic energy, whereas a b1 electron must have a larger interaction with the substituent and a larger kinetic energy. It is also expected that the changes in the π‐electronic structure of the substituent must much more influence the variations on the b1 energies and on the components of orbital energies associated with the substituent than the variations on the a2 energies and on the intraring components of the orbital energies. A modified version of the MOPAC program was prepared to perform the decomposition of the orbital energies in their kinetic and potential energy components and these in their monocentric and bicentric terms. MNDO calculations on nine monosubstituted benzenes, using the modified MOPAC program, give good confirmation of the symmetry predictions and prove the consistency of the classification of the substituents that is introduced. © 1993 John Wiley & Sons, Inc.
A study of excited states in trans‐polyacetylene in the Hartree–Fock, Tamm–Dancoff, and random‐phase approximation
Authors: Vracko M. G. , Zaider M. 
Published: January 2005

AbstractWe report calculations of the excitonic spectra for trans‐polyacetylene obtained in the Hartree–Fock, Tamm–Dancoff, and random‐phase approximations. In the first case, in terms of two‐particle propagator theory, the interaction between the excited electron and the hole is neglected. In the latter two cases, this interaction is considered in the first order. In this framework, the interaction between excitations of different bands and k‐vectors has been included. We discuss the bandwidths and density of states for π–π*, σ–π*, π–σ*, and σ–σ* excitons. © 1993 John Wiley & Sons, Inc.
A mutually consistent procedure for excitation energies and transition densities based on the extended Brillouin's theorem
Authors: Dmitriev Yuri Yu. , Mitrushenkov Alexander O. , Fülscher Markus P. , Roos Björn O. 
Published: January 2005

AbstractA mutually consistent method to calculate excitation energies and corresponding transition densities is proposed. The method is based on the extended Brillouin's theorem that is derived from the nonstationary variation principle. Within the proposed procedure, the Brillouin's conditions, which appear in this extension, are used as a set of nonlinear equations for molecular orbitals and configuration interaction coefficients of the trial ground‐ and excited‐state functions. The excitation energy is an eigenvalue of the set. To some extent, this procedure is related to the variational treatment of the conventional random‐phase approximation within the equation‐of‐motion method. The basic features of the proposed procedure are discussed and it is illustrated by numerical examples. © 1993 John Wiley & Sons, Inc.
Examination of the Hund rule in closed‐shell systems: Investigation of spin correlation effects
Authors: Parrondo Ramón Martín , Karafiloglou Padeleimon , Marcos Enrique Śnchez 
Published: January 2005

AbstractThe validity of the Hund rule in atomic orbitals (AOs) of the carbon atoms inside closed‐shell molecules, such as acetylene, ethylene, and ethane, is examined. Electron‐pair populations and contributions of the two‐electron covalent structures with parallel (⇈) and antiparallel (↑↓) spins are calculated by multielectron population analysis of MO + CI wave functions. Such an analysis, which allows the visualization of various cooperative electronic effects in some target AOs, is extended on the basis of (strictly orthogonal) hybrid orbitals. Although the HF level shows, incorrectly, that the Hund rule is not satisfied, the CI clearly shows a preference for (⇈) spins to those of (↑↓): This holds for both the electron‐pair populations [those of (↑↓) spins diminish with the CI more drastically than those of (⇈) spins], as well as for contributions of the two‐electron covalent structures [those of (⇈) spins increase with the CI more drastically than do those of (⇈) spins]. The calculation of the correlation functions (or dependent functions) in AO space allows the comparison of repulsive or “attractive” behaviour of (⇈) and (↑↓) spins in various AO couples. Mutual dependence of the two electrons inside the sigma system increases in the series ethane < ethylene < acetylene. Also found is that parallel spins in (pure) AOs of sigma systems are preferred to the antiparallel spins when going from ethane to acetylene. The preference parallel–antiparallel spins in AOs belonging to two different atoms, including hydrogens, is also examined. © 1993 John Wiley & Sons, Inc.
Selective dynamic energy transfers in proton collisions with hydrogen fluoride
Authors: Gianturco F. A. , Buonomo E. , Serna S. 
Published: January 2005

AbstractEarlier experimental data by Friedrich et al. found out, from time‐of‐flight experiments of collisions between H+ and HF molecules, that most of the available relative energy becomes transferred into internal rotations of the target and that vibrational excitation only begins to be noticeable around and above 30 eV of relative collision energy. The present theoretical and computational model starts from the potential energy function presented in the preceding paper and carries out quantum calculations that explicitly include the coupling of vibrational target states. The results indeed show very few vibrational excitations and rather large rotational energy transfers as the collision energy is increased up to 50 eV. Microscopic features of both the interaction and the dynamics are found to be responsible for such behavior, which brings the present calculations in agreement with experimental findings. © 1993 John Wiley & Sons, Inc.
Topological analysis of the charge density of solids:bccsodium and lithium
Authors: Mei Changjiang , Edgecombe Kenneth E. , Smith Vedene H. , Heilingbrunner A. 
Published: January 2005

AbstractStudies of electronic structure of solids have concentrated mainly on Hilbert space properties such as energy‐band structures and density of states, as well as some properties that can be reduced from them. Here, we report application of the topological charge density analysis to crystals. It is found that the pseudoatoms (or nonnuclear attractors, which are charge accumulations in nonnuclear regions), first found in Li2, exist in solid lithium and sodium as well. In the former, the pseudoatoms form aconnectednetwork; in the latter, they are separated from one another. Although such networks obey the space group symmetry of the corresponding crystal, their existence is an intrinsic property of a specific crystal. © 1993 John Wiley & Sons, Inc.
Fluorination effect on the structural properties in benzocyclobutenes and benzocyclobutadienes
Authors: Koch Wolfram , Eckert‐Maksić Mirjana , Maksić Zvonimir B. 
Published: January 2005

AbstractStructural features of benzocyclobutenes, benzocyclobutadienes, and some of their fluoroderivatives are studied at the HF/SCF niveau employing a 6–31G* basis set. A Mills–Nixon‐type of distortion (lengthening of the annelated bonds with concomitant shortening of the adjacent bonds in the benzene moiety) is found in all molecules, with the exception of polyannelated benzocyclobutenes with perflouro‐substituted methylene groups. The latter compounds exhibit a weak anti‐Mills–Nixon effect. Peripheral double bonds in benzocyclobutadienes enhance the double‐bond localization in the benzene ring, but the main effect originates from angular deformations of the σ‐skeleton. Salient structural features are rationalized by rehybridization at the carbon junction atoms and variation of Coulson's π‐bond orders. Finally, several scales for quantifying the Mills–Nixon effect are briefly discussed. © 1993 John Wiley & Sons, Inc.