bluuebunny/crossref_metadata_2025_split huggingface dataset:
Quantum chemical simulations of hole self‐trapping in semi‐ionic crystals
Authors: Kantorovich L. , Stashans A. , Kotomin E. , Jacobs P. W. M. 
Published: January 2005

AbstractA novel formalism is presented for reliable calculations of the energetics of hole self‐trapping in semi‐ionic solids with mixed valence bands. Unlike previous model‐Hamiltonian‐type approaches, it is based on self‐consistent quantum chemical INDO simulations of the atomistic and electronic structure of a self‐trapped hole, making no a priori assumptions about a particular form of its localization (if any). This formalism is applied to the problem of hole self‐trapping in corundum crystals (α–Al2O3). The hole self‐trapping is found to be energetically favorable in the form of a diatomic O2 molecule with strong covalent bonding quite similar to the self‐trapped hole (VK‐center) in alkali halides. The so‐called localization energy (i.e., the energy that is required to localize the Bloch‐like wave packet of the free hole on the molecule, as the first stage of further trapping) is essentially less than one‐half of the upper valence band width, which is the estimate commonly used for ionic solids. © 1994 John Wiley & Sons, Inc.
Semidirect algorithms for third‐order electron propagator calculations
Authors: Zakrzewski V. G. , Ortiz J. V. 
Published: January 2005

AbstractAn efficient procedure for third‐order electron propagator calculations of ionization energies and electron affinities is reported. Diagonal self‐energy expressions that are suitable for large molecules are empolyed. The outer‐valence Green's function method also is implemented. An integral transformation program for direct and semidirect algorithms is modified to store only nonzero integrals according to Abelian point group symmetry. Contributions to self‐energy matrix elements that depend on electron repulsion integrals with four virtual orbital indices are computed in a direct way. Intermediate batches of integrals are created by sort procedures while avoiding storage of transformed integrals in the main memory. This method permits calculation of electron binding energies for C with a 231 atomic orbital basis and for Zn(C5H5)2 with a 220 atomic orbital basis on an IBM RISC/6000 Model 550. During these calculations, the CPU is engaged approximately 90% of the time. © 1995 John Wiley & Sons, Inc.
Derivation and reinterpretation of approximations in Schröudinger and Kohn–;Sham theory via a hierarchy within the work formalism
Authors: Sahni Viraht 
Published: January 2005

AbstractIn this article, we derive and there by reinterpret various approximations in Schrödinger theory and Kohn‐Sham density‐funtional theory via a hierarchy within the work formalism of electronic structure due to Harbola and Sahni. In the work formalism, which is based on Coulomb's law, the local potential representing electron correlations as well as the electron correlation energy both arise from the same quantum mechanical source charge distribution that is the pair‐correlation density. The potential is the work done to move an electron in the force field of the pair‐correlation density, and the energy is the energy of interaction between the electronic and pair‐correlation densities. The differential equation governing the system is a sturm‐Liouville equation so that the system wave function can, in principle, be obtained as an infinite linear combination of Slater determinants of the spin‐orbitals corresponding to the occupied and virtual states. The hierarchy is achieved by improvement of the pair‐correlation density either by systematic improvement of the wave function or, as is the case of Kohn‐Sham theory, by an expansion of the pair‐correlation density in gradients of the density about the uniform electron gas result. The derivations of the approximations of Kohn‐Sham theory via the work formalism, in turn, exhibit the existence of additional correlations that are not evident through the Kohn‐Sham prescription, whereby the potential is obtained by functional differentiation. The approximations considered within Schrödinger theory are the Hartree, Hartree–Fock, and configuration‐interaction approximations. Those within Kohn‐Sham theory are the density functional theory Hartree, local density, and gradient expansion approximations. © 1995 John Wiley & Sons, Inc.
Ab initio study of the ground and first excited state of LiAr
Authors: Sadlej Joanna , Edwards W. Daniel 
Published: January 2005

AbstractThe intermolecular potentials for the X 2σ and A2Π states of Li… Ar were studied by a variety of multiconfiguration, single‐configuration, and perturbation methods (CASPT2). The A 2Π excited state was calculated to have a well depth of 811 cm−1 at an internuclear separation of 2.59 Å, in excellent agreement with the 810 cm−1 derived from experimental data. A smaller well of 77 cm−1 was found for the X 2σ ground state at an intermolecular separation of 4.8 Å. These results are in better agreement with experimental results than were the previously reported pseudopotential calculations. The comparison of CI calculation with the CAPST2 results shows that the latter is able to give good results for interacting metal–rare gas systems. © 1995 John Wiley & Sons, Inc.
A semiempirical investigation of the mechanisms for the alkylation of arylamines
Authors: Chaka Anne M. 
Published: January 2005

AbstractThe alkylation of arylamines is an important reaction in many large scale industrial processes. Three possible mechanisms are evaluated at the semiempirical level using the alkylation of aniline with ethylene as a model system. A molecular orbital analysis is performed for each mechanism to determine the appropriate type of wavefunction required to describe the transition state. Successive levels of refinement in search algorithms are used to locate the transition state on a potential surface with up to 60 degrees of freedom. A reasonable mechanism for the AlCl3‐catalyzed Friedel‐Crafts alkylation is shown to be analogous to a Diels‐Alder [4 + 2] concerted cycloaddition after initial β‐elimination of HCl. The Brönsted acid‐catalyzed reaction shown is likely to occur via a mechanism similar to the Ene reaction, rather than initial protonation of the olefin followed by electrophilic addition to a nearby aromatic ring. © 1995 John Wiley & Sons, Inc.
Measures of distance for atomic charge and momentum densities and their relationship to physical properties
Authors: Hô Minhhuy , Sagar Robin P. , Schmider Hartmut , Weaver Donald F. , Smith Vedene H. 
Published: January 2005

AbstractAtomic charge and momentum densities of 91 atoms (HeU) are classified in terms of theirL1,L2, and entropic measures of distance from the densities of the preceding atoms. The relationship between these distances and the first ionization energies is also considered. © 1995 John Wiley & Sons, Inc.
The evaluation of matrix elements in the analysis of anharmonic molecular vibrations: Optimized expansions and quadratures
Authors: Schmidt P. P. 
Published: January 2005

AbstractThis article presents methods for computing matrix elements with Cartesian Gaussian wave functions of potential energy operators that depend on functions of the form (r−r0)n exp[−a(r − r0)] as well as matrix elements of the class of polynomial many‐body potentials developed by Murrell et al. The matrix elements arise in the analyses of anharmonic vibrations in molecules. © 1995 John Wiley & Sons, Inc.
Pairon distributions and the spectra of reduced Hamiltonians
Authors: Coleman A. J. , Yukalova E. P. , Yukalov V. I. 
Published: January 2005

AbstractThe reduction of the N‐particle problem to the 2‐ and 1‐particle problems is discussed, and the relation between the statistics of particles and the properties of the pairon distribution is studied. It turns out that not all levels of the pairon spectra can be occupied. A simple way of defining the pairon distribution is suggested. The spectra of the reduced Hamiltonian with power‐law interaction potentials are analyzed. The bound states are calculated by the method of self‐similar approximation, the accuracy of which is compared with and shown to be better than that of the quasi‐classical approximation. © 1995 John Wiley & Sons, Inc.
An analysis of dynamic linear response properties at RPA level
Authors: Packer Martin J. , Pickup Barry T. 
Published: January 2005

AbstractA separation of the dynamic linear response is developed, which distinguishes between the one‐ and two‐electron contributions to the molecular response, by partitioning the RPA equation. The derivation of the partitioning is given in both an RPA, equation of motion, type approach and using the alternative, but equivalent, density matrix method. Three physically distinct contributions are obtained, called the direct, interaction, and back contributions. The direct term is composed entirely of one‐electron effects, while the interaction and back terms account for the electron‐interaction contributions to the response. Results for the dynamic dipole polarizability suggest that while the one‐electron contribution is dominant in the zero‐frequency limit, the two‐electron contribution becomes increasingly important as the frequency of the perturbation increases. This implies that approximation of the linear response by only one‐electron contributions is acceptable for the static case, but is less relevant for the dynamic case. The ramifications of this observation, for the scaling of sum‐over‐states‐type calculations of large molecular systems, is briefly discussed, as is the application of our partitioning method to the higher polarizabilities. © 1995 John Wiley & Sons, Inc.
Conformation‐transitional rate in protein folding
Authors: Luo Liaofu 
Published: January 2005

AbstractBased on the conformation dynamics of macromolecules, the rate of conformational transition is deduced from the nonadiabaticity operator method which can be used to explain the time scale of milliseconds for protein folding. It is proved that (1) the dependence of the transition rate on inertial moment I of the atomic group obeys the I−2.5 law; (2) its dependence on numbers n of torsional angles participating in the transition obeys the n1.5 law; and (3) the temperature dependence of the transitional rate shows an abnormal character in the high‐ temperature region. © 1995 John Wiley & Sons, Inc.
Band structure studies on crystalline C60, Ca3C60, and Ca5C60
Authors: Cao Yang , Chen Liangjing , Chen Bo , Fen Jianwen , Chen Wenjian , Pan Yuh‐Kang 
Published: January 2005

AbstractThe three‐dimensional EHMO crystal orbital calculations for crystalline C60, Ca3C60, and Ca5C60 are reported. The ground states of both undoped solid C60 and partially doped Ca3C60 are found to be insulating with an indirect energy gap of 1.2 and 0.5 eV, respectively. In contrast, Ca5C60 forms a metallic conducting phase with a set of three half‐filled bands crossing the Fermi level, which is found to be located close to a peak of the density of state. The character of crystal orbitals near the Fermi level for both Ca3C60 and Ca5C60 is completely carbonlike. In both cases, the Ca atoms are almost fully ionized and C60 molecules form a stable negative charge state with six to 10 additional electrons. © 1995 John Wiley & Sons, Inc.
Spin–orbit coupling in the intersystem crossing of the ring‐opened oxirane biradical
Authors: Knuts Sören , Minaev Boris F. , Vahtras Olav , Ågren Hans 
Published: January 2005

AbstractThe intersystem crossing (ISC) between the lowest triplet and singlet states occurring in the reaction of atomic oxygen with ethylene was studied. The importance of spin–orbit coupling (SOC) in oxirane biradicals (ĊR′R″—CRR*—Ȯ) is stressed through calculations where the spin–orbit matrix elements over the full Breit–Pauli SOC operator has been obtained in the singlet–triplet crossing region. The calculations are performed with a multiconfigurational linear response approach, in which the spin–orbit couplings are obtained from triplet response functions using differently correlated singlet‐reference‐state wave functions. Computational results confirm earlier semiempirical predictions of the spin–orbit coupling as an important mechanism behind the ring opening of oxiranes and addition of oxygen O(3P) atoms to alkenes. © 1995 John Wiley & Sons, Inc.
PHGLF calculations of heliumlike three‐body systems
Authors: Wang Yixuan , Deng Conghao 
Published: January 2005

AbstractBy expanding the atomic wave function in potential harmonics (PH) and generalized Laguerre functions (GLF), we derived the recurrence relation of the expansion coefficients and then performed calculations for four He‐like three‐body systems. In comparison with the complete set calculation (HHGLF), we find that the errors in eigenenergies introduced by the PHGLF scheme are approximately 0.00033, 0.00025, 0.00022, and 0.00021 au for the ground‐state H−, He, Li+, and Be2+ systems, respectively, and that for excited states the error is slightly higher and decreases with the grand angular momentum. The present results indicate that the PHGLF scheme is such an efficient means to reduce the degeneracy of the hyperspherical harmonics (HH) for three‐body problems that it can give more satisfactory results than can the HHGLF method with much smaller eigenmatrices. © 1995 John Wiley & Sons, Inc.
Hybridization displacement charge in molecules and its effects on electrostatic potentials and fields
Authors: Kumar Anil , Mohan C. G. , Mishra P. C. 
Published: January 2005

AbstractThe hybridization displacement charges in molecules which preserve individual atomic contributions to the hybridization dipole moment, besides preserving the total dipole moment, have been defined. These charges have been used in combination with those obtained by Mulliken and Löwdin population analyses to compute electrostatic potentials and fields in several molecules. Correlations of these potentials and fields with experimentally observed hydrogen‐bonding parameters (α and β) and with ab initio electrostatic potentials of the molecules have been studied. It is found that inclusion of hybridization effects through hybridization displacement charges improves the description of electrostatic potentials and fields appreciably. However, comparison of electrostatic potentials and fields at a specific site in a given class of molecules can also usually be done satisfactorily using Mulliken or Löwdin charges. © 1995 John Wiley & Sons, Inc.
Conformational structure of the amphipathic peptide insecticide L‐KALA
Authors: Nandel Fateh S. , Ahluwalia Anupam , Kaur Amandeep 
Published: January 2005

AbstractThe conformation structure of the tetrapeptide Ac–Lys–Ala–Leu–Ala–OMe, the repeat unit of a 30 residue peptide YAA(KALA)6LAA, and also its analog containing an unusual amino acid, (Aib): Ac–Lys–Aib–Leu–Aib–OMe, has been investigated by the PCILO method. The global/low‐energy minima for all the residues lies at ϕ = −30° and ψ = 120°. In this conformation, the potential hydrogen‐bonding sites are free, i.e., capable of intermolecular hydrogen bonding. As the angles lie closer to the collagen helical region and potential hydrogen‐bonding sites are free, this structure is named the collagen‐type helix. In aqueous solution, this structure is stabilized by solvation of the peptide bond. The local‐energy minimum in all maps correspond to the right‐handed helical region. The helix thus formed generates a pore of 3 Å along the helix axis, with one lateral hydrophobic side and the other hydrophilic side. The pore thus formed is cation‐selective and accounts for the leakage of contents from vesicles. On the basis of the length of the helix and placement of the side chains of lysine and leucine, a model has been proposed for the aggregation and fusion of the vesicles. The amphipathic basic residues interact with the head groups of the acidic liposomes by extending toward the polar face of the helix to insert their charged moieties by overcoming the electrostatic repulsive forces between the opposing vesicles. © 1995 John Wiley & Sons, Inc.
A comparative study of crystal packing vs. conformational energy of N‐acetyl‐2,3‐didehydroproline
Authors: Roychoudhury Mihir , Kumar Devesh 
Published: January 2005

AbstractConformational studies on dehydro‐Pro‐OH and suitable derivatives are of interest in relation to s‐cis‐s‐trans isomeric behavior in solution. The conformation of N‐acetyl‐2,3‐didehydro‐Pro‐OH as described by Ajo et al. [Acta Crystallogr. C 40, 324 (1984)] is as follows: ω = −172.5°, ϕ = −26.3°, and ψ = −58.3°. In the present article, we examined through quantum mechanical calculations the preferred conformation of this molecule using the PCILO (perturbative configuration interaction using localized orbitals) method. The theoretical preference was obtained at ω = 78°, ϕ = −26.3°, and ϕ = 90°. The stacking energy for both conformations, i.e., crystallographic and theoretically preferred, has been evaluated using the Rayleigh‐Schrödinger perturbation method modified by multicentered multipole expansion for the electrostatic components and the Kitaigorodskii method for dispersion and repulsion components. It was observed that crystallographic conformation exhibits a strong preference for stacking as compared to the theoretical conformation. On the basis of these results, one may therefore conclude that the observed crystallographic structure is due to its preference for crystal packing and the conformation in solution may be entirely different. © 1995 John Wiley & Sons, Inc.
Resonance energies of fullerenes with 4‐membered rings
Authors: Babić D. , Trinajstić N. 
Published: January 2005

AbstractWe report the resonance energies (REs) of several fullerenes with 4‐membered rings and their isomers with only 5‐ and 6‐membered rings computed using the conjugated‐circuit model [RE(CC)] and the topological resonance energy (TRE) model. Both aromaticity indices were normalized by dividing by the size of the considered fullerene [RE(CC)/e and TRE/e]. The results parallel the predictions by Gao and Herndon using the much more advanced SCF–UHF π‐electron approach. A good linear correlation is found between the topologically defined indices [RE(CC)/e and TRE/e] and normalized SCF–UHF π‐electron energy. © 1995 John Wiley & Sons, Inc.
Total energy calculations in the DFT on binary compounds
Authors: Van Camp P. E. , Van Doren V. E. 
Published: January 2005

AbstractCalculations are carried out using first‐principles self‐consistent local‐density and nonlocal density theory of the electronic structure, the total energy, and the charge density of a variety of semiconducting and insulating compounds under hydrostatic and uniaxial pressure. For several cases, the transition pressure from one structure to another is determined as well as the pressure coefficients of the main band gaps. It is shown that several properties are calculated with adequate accuracy to be compared with experiment, so that values which have not yet been measured are trustworthy predictions. © 1995 John Wiley & Sons, Inc.
The PPP model of alternant cyclic polyenes with modified boundary conditions
Authors: Bendazzoli G. L. , Evangelisti S. 
Published: January 2005

AbstractThe extension of the PPP Hamiltonian for alternant cyclic polyenes to noninteger values of the pseudomomentum by imposing modified boundary conditions is discussed in detail. It is shown that a computer program for periodic boundary conditions can be easily adapted to the new boundary conditions. Full CI computations are carried out for some low‐lying states of the PPP model of altemant cyclic polyenes (CH)N, (N even) at half‐filling. The energy values obtained by using periodic (Bloch) and antiperiodic (Möbius) orbitals are used to perform energy extrapolations for N → ∞. © 1995 John Wiley & Sons, Inc.
Theoretical studies on the structure and electronic properties of 3‐isothiazolones
Authors: Morley John O. , Charlton Michael H. 
Published: January 2005

AbstractThe structure of 4,5‐benzo‐3‐isothiazolone and 5‐phenyl‐2‐(E)‐styryl‐3‐isothiazolone have been calculated both at the semiempirical AM1 level and by using the ab initio 4‐31G basis set. Neither method gives a satisfactory account of the bond lengths and angle at the sulfur atom in comparison with crystallographic data, but the inclusion of polarization functions on the sulfur atom alone gives a marked improvement in the theoretical results. The conformations, structure, and electronic properties of a series of other antifungal 3‐isothiazolones have been calculated by the same method. Although structural effects do not appear to play an important role in the mechanism of the inhibition of fungi by the 3‐isothiazolones described here, the electronic properties calculated in terms of the atomic charge at sulfur and the LUMO energy appear to relate to their known reactivity with nucleophiles and their observed biological activity. © 1995 John Wiley & Sons, Inc.
Searching for transition states: The line–then–plane (LTP) approach
Authors: Cárdenas‐Lailhacar Cristián , Zerner Michael C. 
Published: January 2005

AbstractA procedure for finding transition states ( TS) that does not require the evaluation of the second derivatives during the search is proposed. The procedure is based on connecting a series of points representing products Pi and reactants Ri and taking conservative steps along the difference vector from Pi toward Ri and from Ri toward Pi until the two points coalesce. Although the points Po and Ro represent the product and reactant specifically, other Pi and Ri are determined by minimization in hyperplanes perpendicular to Pi −1 and Ri−1. We apply this technique to six well‐known potential functions and compare these results with those obtained from other well‐known procedures. © 1995 John Wiley & Sons, Inc.
Ab initio energies and tunneling lifetimes of the doubly charged AH2+ (A = Mg—Ar) diatomics
Authors: Nefedova V. V. , Boldyrev A. I. , Simons J. 
Published: January 2005

AbstractPotential energy curves for the ground and low‐lying excited states of the AH2+ (A = Mg—Ar) dications have been calculated using high‐level ab initio methods with large atomic orbital basis sets. Quasi‐bound potential energy curves with local minima and deprotonation barriers have been found for most of the dications studied. The energies, tunneling lifetimes, and widths of the quasi‐bound states have been calculated by numerical solution of the radial Schrodinger equation using the Numeov method. All these dications except ArH2+ have low‐lying states which support quasi‐bound vibrational states. The ArH2+ dication has a 2∏i potential energy curve with a minimum so shallow that it does not support any quasi‐bound vibrational states. Results of our calculations are compared with previous ab initio calculations and available experimental data. © 1995 John Wiley & Sons, Inc.
Acidity of first‐ and second‐row hydrides: Effects of electronegativity and hardness
Authors: De Proft F. , Langenaeker W. , Geerlings P. 
Published: January 2005

AbstractA study is made on the influence of the electronegativity and hardness of a group X on the gas‐phase acidities of the first‐ and second‐row hydrides HX. It is shown that these two density functional theory (DFT)‐based global properties are sufficient for the correct description of the gas‐phase acidity sequences, the group hardness being the more important factor and describing the influence of polarizability effects in the charged forms of the acid–base equilibrium. Various calculated properties of the traditional quantum chemical type (Mulliken charges and the MEP) and DFT‐based (local softness), associated with the neutral form of the acid, were found to contain a certain periodicity in their description of the gas‐phases acidities. The gas‐phase acidities (Δ Hacid and ΔGacid values) were also obtained theoretically via quantum statistical thermodynamical calculations at the Hartree–Fock level with the 6‐31 + G* basis set; these calculated acidities were in fair agreement with the experimental ones. © 1995 John Wiley & Sons, Inc.