bluuebunny/crossref_metadata_2025 huggingface dataset:
Electronic structure and optical properties of a tin‐encapsulated nickel porphyrazine compound
Authors: Liang X. L. , Ellis D. E. , Gubanova O. V. , Hoffman B. M. , Musselman R. L.
Published: January 2005

The recently synthesized metal‐encapsulated porphyrazine compound, [Sn(t‐Bu)2]4‐star‐Ni(porphyrazine)‐S8, shows very interesting structural and optical absorption features compared with other metal‐centered porphyrazines, e.g., metal phthalocyanines (Pc). Using self‐consistent‐field local density theory, we studied the ground‐state and excited‐state electronic structure of this molecule and compared it with its metal phthalocyanine analog NiPc. The theoretical optical spectra including oscillator strengths are in good agreement with experimental absorption and show that the characteristic transitions at the so‐called Soret band in NiPc are red‐shifted in the new compound. © 1994 John Wiley & Sons, Inc.
Comparison of quantum mechanical methods to compute the biologically relevant reactivities of cyclopenta‐polycyclic aromatic hydrocarbons
Authors: Rabinowitz James R. , Little Stephen B.
Published: January 2005

In computational studies to understand the interaction of polycyclic aromatic hydrocarbons (PAHs) with biomolecular systems, the semiempirical method AM1 has been used previously to determine the geometry of the PAH and its metabolites and relevant intermediates. A number of studies have shown that AM1 provides geometries for parent PAHs that are acceptably close to experimentally determined structures. However, many of the properties that determine the manner by which PAHs interact with biological nucleophiles depend on the structure of metabolites and reactive intermediates where less experimental information is available. In a previous study, we used AM1 to obtain the molecular geometries of reactive intermediates of cyclopenta‐PAHs (cPAHs) and then used single‐point Hartree‐Fock calculations, with the gaussian 3‐21g basis set, to obtain molecular energies and charge distributions, in order to predict the direction of epoxide ring opening. Recent advances in the availability of computational hardware and software have provided other, more rigorous, methods for approaching this problem. In this study, we used hartree‐fock methods in the gaussian series of programs employing the 3‐21g and 6‐31g basis sets and the local density functional method Dmol to obtain molecular geometries, energies, and charge distributions of the epoxides and the two potential hydroxycarbocations that could result from protonated ring opening, for a series of cPAHs. We have also performed the same calculations with AMSOL/SM2, a semiempirical method that adds the effect of the aqueous environment to the AM1 Hamiltonian. The division of the cPAHs into classes is not altered by these more rigorous calculations. The inclusion of water in the Hamiltonian has a greater effect on the results than using the ab initio methods to obtain the structure. © 1994 John Wiley & Sons, Inc.
An analysis of nonlocal density functionals in chemical bonding
Authors: Leeuwen Robert Van , Baerends Evert Jan
Published: January 2005

In this work, we carry out an analysis of the gradient‐corrected density functionals in molecules that are used in the Kohn–Sham density functional approach. We concentrate on the special features of the exchange and correlation energy densities and exchange and correlation potentials in the bond region. By comparing to the exact Kohn–Sham potential, it is shown that the gradient‐corrected potentials build in the required peak in the bond midplane, but not completely correctly. The gradient‐corrected potentials also exhibit wrong asymptotic behavior. Contributions from different regions of space (notably bond and outer regions) to nonlocal bonding energy contributions are investigated by integrating the exchange and correlation energy densities in various spatial regions. This provides an explanation of why the gradient corrections reduce the local density approximation (LDA) overbinding of molecules. It explains the success of the presently used nonlocal corrections, although it is possible that there is a cancellation of errors, too much repulsion being derived from the bond region and too little from the outer region. © John Wiley & Sons, Inc.
A mass formula for the energy of metal clusters
Authors: Mañanes A. , Membrado M. , Pacheco A. F. , Sañudo J. , Balbás L. C.
Published: January 2005

We obtain an analytic expression for the total energy of a metallic cluster formed by N atoms of valence v and with net charge Q, by solving variationally the extended Thomas–Fermi version of density functional theory within the spherical jellium model. The energy is expressed as an expansion (mass formula) in decreasing powers of the cluster radius RI = rsZ1/3, with Z = vN, and rs, the one electron radius of the bulk,

and the coefficients of this mass formula are functions of rs. Contributions of volume (RI3), surface (RI2), curvature (RI), constant (RI0), (1/RI), and (1/RI2) are clearly separated in the formula. The Chemical potential, work function, electron affinity, and ionization potential are easily obtained for neutral and charged clusters of any electronic density in the metallic range. A general estimation of the critical size for stability against electron detachment of negatively charged clusters is also obtained. The stability of highly charged clusters against fragmentation is also studied. © John Wiley & Sons, Inc.
Density functional theory for Jahn–Teller systems
Authors: Sukumar N.
Published: January 2005

The first discussion of the dynamics of Jahn–Teller systems in terms of the electronic density as the fundamental variable was given by W.J. Clinton in 1960, where the degenerate electronic configuration of a Jahn–Teller molecule was interpreted in terms of the infinite number of ways in which the charge distribution can be oriented for the same energy. The moving nuclear framework serves as the perturbation necessary to define the orientation of the charge density, with no activation energy required to put the charge cloud into motion. Recently, this notion of the electronic charge cloud in a Jahn–Teller molecule sweeping out the potential surface over which the nuclei move has found mathematical expression in our work in terms of a generalized electronic current density in nuclear‐coordinate space [N. Sukumar and B.M. Deb, Int. J. Quantum Chem. 40, 501 (1991)]. The introduction of the electronic phase as a function of both electronic and nuclear coordinates, in addition to the electronic density, is a crucial component of this formulation. In the present work, the density‐based treatment is extended to the nonadiabatic situation, with the Born couplings interpreted as nonadiabatic currents in parameter space. Abelian and non‐Abelian gauge transformations of these currents are discussed. © John Wiley & Sons, Inc.
Direct minimization of the energy in density functional theory
Authors: Fantucci Piercarlo , Polezzo Stefano
Published: January 2005

The minimization of the energy functional of the first‐order density matrix γ(r,r') is achieved using unitary transformations applied to γ. Equivalently, such transformations can be carried out also on one‐electron orbitals (natural orbitals) and their occupation (integer or non‐integer) numbers. The conventional local density approximation based on the electron density p(r) is then considered as a special case. The direct minimization of the energy functional of p with respect to the parameters of the unitary transformation leads to stationary conditions that are all equivalent to the Kohn–Sham equations. Preliminary numerical tests show that the proposed algorithms for the direct minimization of the energy work in a satisfactory manner. © John Wiley & Sons, Inc.
The dependence of 10Dq upon the metal–ligand distance, R, for transition‐metal complexes. What is its microscopic origin?
Authors: Moreno M. , Barriuso M. T. , Aramburu J. A.
Published: January 2005

Experimental results on 3d Oh complexes in insulators reveal that 10Dq α R−n, where R is the metal–ligand distance and n is close to five. This strong dependence determines the Huang–Rhys factor, S(A1g), associated to the symmetric A1g mode of the first excited state of complexes like MnX and CrX (X = halide) and makes it possible to measure R changes down to ∽ 10−3 Å. This work is devoted to understanding, within a molecular orbital framework, the microscopic origin of such a dependence, which is related to the corresponding one displayed by the transferred spin densities fσ, fs, and fπ. The analysis is focused on MnF. As a main result, it is shown that though fσ ≫ fs the interaction between d(eg) orbitals and 2s orbitals of F− is not only primarily responsible for the R dependence of 10Dq but also makes a significant contribution to the 10Dq value itself. The present work thus shows that the significant dependence of 10Dq upon R is ultimately related to the strong dependence of fs and the isotropic superhyperfine constant As upon R displayed by the experimental results of several 3d impurities. © John Wiley & Sons, Inc.
Predictive chemical kinetics: Density functional and hartree–fock calculations on free‐radial reaction transition states
Authors: Green William H.
Published: January 2005

Density functional theory (DFT) using gradient‐corrected «nonlocal» functionals is used to calculated the thermochemistry and barrier heights for several types of peroxyl radical isomerizations currently being studied by kineticists. The calculations are generally in good agreement with experimental data, where such data are available. An important exception is that the O—H bond strengths in hydroperoxides are all predicted to be too weak by about 7 kcal/mol. The calculated reaction barriers are a few kcal/mol lower than the experimental estimates, but comparable in accuracy to the much more computationally expensive second‐order Møller–Plesset (MP2) predictions. Various theoretical methods converge on a 42 ± 2 kcal/mole barrier for CH3OO → H2CO + OH. The DFT calculations can be used to predict reaction barriers in cases where no reliable experimental data are available. The effects of the choice of basis set and correlation functional are explored. Improvements needed to make these calculations most valuable to the chemical kineticist are discussed. © 1994 John Wiley & Sons, Inc.
Density functional theory applied to the excited states of coordination compounds
Authors: Daul Claude
Published: January 2005

Coordination compounds are usually symmetrical molecules with degenerate orbitals. Hence, the individual multiplet states arising from open‐shell configurations can, in general, not be expressed by a single determinant. We have therefore exploited symmetry to the largest possible extent in order to simplify the relation between the multiplet splitting and single‐determinant energies and thus developed a new method based on vector coupling to keep the computational effort to a minimum. A system of computer programs working on both mainframe and personal computers has been developed, carrying out for any desired point group the required group theoretical manipulations. The description of the method is illustrated by considering three practical examples. © 1994 John Wiley & Sons, Inc.
The effects of nonlocal gradient corrections in density functional calculations of hydrocarbon radical hyperfine structures
Authors: Eriksson Leif A. , Malkin Vladimir G. , Malkina Olga L. , Salahub Dennis R.
Published: January 2005

Ground‐state equilibrium geometries and hyperfine structures of a number of organic neutral and charged radical compounds are computed using the linear combination of Gaussian‐type orbitals–density functional theory method. In addition to the local spin‐density approximation, we also use two different nonlocal (gradient corrected) schemes for the calculations of the exchange and correlation potentials. The different functional forms are found to generate slightly different total and unpaired spin‐density distributions in the molecules, and as a result, the computed isotropic hyperfine coupling constants vary markedly. The smallest variations are found for the hydrogens, where the results are generally in satisfactory agreement with experiment. For the carbon atoms, however, large differences in isotropic coupling constants are observed. The anisotropic hyperfine structures are generally very well described at all levels of theory. © John Wiley & Sons, Inc.
Electronic structure of paramagnetic clusters of transition metal ions. 3. Magnetic properties and scattered wave description of the electronic structure of the hexanuclear octahedral cluster [Fe6(μ3−S)8(PEt3)6] (BPh4)2
Authors: Bencini Alessandro , Uytterhoeven Myriam G. , Zanchini Claudia
Published: January 2005

Spin‐polarized Xα–SW calculations of [Fe6(μ3−S)8(PH3)6]2+ as a model of the cluster [Fe6(μ3−S)8(PEt3)6] (BPh4)2 have been performed. The highest occupied energy levels are well separated from empty levels, and up to a maximum of eight electrons can be unpaired, giving a maximum spin state with S = 4. This electronic state is consistent with the magnetic data of [Fe6(μ3−S)8(PEt3)6](BPh 4)2, which have been interpreted using the Heisenberg–Dirac–Van Vleck exchange spin Hamiltonian. The S = 4 state arises from the magnetic coupling between five low‐spin (Si = 1/2) and one intermediate‐spin (S = 3/2) iron(III) center. © 1994 John Wiley & Sons, Inc.
Geometrical and spectroscopical characterizations of some complex entities of aluminum(III) with fluoride ions by LDF‐based calculations
Authors: Bouyer Frédéric , Picard Gérard , Legendre Jean‐jacques
Published: January 2005

The structure of cryolite is investigated with a theoretical approach based on LDF calculations. In fact, experimental techniques for structural studies are difficult to perform in cryolite melts because of hard experimental conditions (high temperature, corrosiveness, etc.). Use of the DMol software allows us to determine the stabilities and the vibrational frequencies of AlF, AlF, and AlF isolated complexes. The results obtained compared with published experimental works confirm that AlF should be considered in the dissociation scheme of cryolite as previously evidenced by other authors. © 1994 John Wiley & Sons, Inc.
Symmetry breaking and its influence on the correlation energy for CF and CF ions
Authors: Garcia de la vega J. M. , San fabian E.
Published: January 2005

We have studied symmetry breaking in three open‐shell systems: CF (D2d and C2v) and CF (D3h) molecular ions. These different Hartree–Fock solutions are employed as starting points to calculate the correlation energy of these ions with perturbative, configuration interaction, and density functional methods. When symmetry‐broken or symmetry‐adapted wave functions are used, the correlation energy obtained with each method changes the order of stability of CF for a determined symmetry. Density functional methods produce higher correlation energies although they do not alter the order of stability of Hartree–Fock calculations. The behavior of correlation energy with different methods and the characteristics of the symmetry of wave functions are compared. A study of appearance energies for three different channels of the decomposition reaction of ionized carbon tetrafluoride are considered by using different methods with symmetry‐broken or symmetry‐adapted wave functions to calculate correlation energies. © 1994 John Wiley & Sons, Inc.
Investigations of hydrogen‐bonded systems: Local density approximation and gradient corrections
Authors: Kaschner R. , Seifert G.
Published: January 2005

The applicability of the local density approximation (LDA) and of corresponding gradient corrections (for the exchange and correlation energy) for the treatment of the hydrogen bond is investigated. As test systems, we consider the water dimer and the H2O…︁HX complexes (X = F, Cl, Br): Using an LCAO scheme, their equilibrium geometries and interaction energies are ćalculated and compared with experimental data and with other calculations. We obtain that the LDA gives the geometries in qualitative agreement with other data, whereas the energies are overestimated. The use of the gradient corrections (GC) according to Becke and Perdew leads to a significant improvement of the geometry, and especially of the interaction energies. The calculations indicate further that LDA + GC should also be able to describe weaker intermolecular interactions than the usual hydrogen bond. Finally, a short discussion of the charge distribution and the dipole moments of the H2O…︁HX complexes is performed. © 1994 John Wiley & Sons, Inc.
Density functional theory: An effective theoretical tool for the study of σ radicals
Authors: Barone Vincenzo , Adamo Carlo , Russo Nino
Published: January 2005

The structures, harmonic force fields, and hyperfine coupling constants of a number of representative σ radicals have been investigated by the density functional approach including nonlocal corrections. Scaling of CH bond lengths and stretching constants leads to accurate structures and force fields. Hyperfine coupling constants reach, at least, the level of the most sophisticated conventional post‐Hartree–Fock models. This suggests that the density functional approach is a promising theoretical tool for the study of relationships between structure and spectroscopic properties of large free radicals. © 1994 John Wiley & Sons, Inc.
Bonding of acetylene to copper atom, dimer, and trimer
Authors: Fournier René
Published: January 2005

The bonding of acetylene to copper atom, dimer, and trimer was investigated with a Kohn–Sham density functional approach. Full geometry optimization yielded the equilibrium structures of various CunC2H2 species. Gradient corrections were included in the calculation of binding energies (BE). The Cu—C2H2 complex was found to have a Cs structure and a BE of 10 kcal/mol. Three isomers of Cu2C2H2 have similar total energies: a C2v end‐bonded structure with a BE of 18 kcal/mol, and two 1,2‐dicupro ethylene isomers—a cis form with a BE of 12 kcal/mol and a trans form with a BE of 15 kcal/mol. Two stable C2v isomers of Cu3C2H2 were found. In both isomers, the Cu3 ring relaxes from its isosceles structure, with two short bonds (2.247 Å) and one long bond (2.478 Å), and adopts a nearly equilateral geometry. In one isomer of Cu3C2H2, the acetylene is bonded to one apex of the Cu3 ring with a BE of 29 kcal/mol. In the other, it is bonded to two copper atoms of one side of the Cu3 ring with a BE of 33 kcal/mol. © 1994 John Wiley & Sons, Inc.
Self‐consistent calculations of total energies and charge densities of solids without solving the band‐structure problem
Authors: Cortona Pietro , Monteleone Andrea Villafiorita
Published: January 2005

We have used a recently proposed method allowing a direct calculation of the total energy and of the charge density of a crystal to determine the equilibrium lattice parameter, the cohesive energy, and the bulk modulus of MgO and CaO. The results that we have obtained are compared with those of a variety of band‐structure calculations as well as with the experimental data. © 1994 John Wiley & Sons, Inc.
On the electronic structure of MoO: Spin‐polarized density functional calculations of spectroscopic properties of low‐lying quintet, triplet, and septet states
Authors: Brocławik Ewa , Salahub Dennis R.
Published: January 2005

Density functional theory (DFT) calculations for the ground state and four excited quintet, two septet, and two triplet states of the molybdenum oxide molecule are reported. Equilibrium geometries and other spectroscopic constants are determined for these states and compared both with recent spectroscopic measurements and other theoretical calculations, where available. Experimental assignments of the 5II ground state and excited 5Σ+, 5Σ−, 5Δ, and B' 5II states are confirmed; also candidates for low‐lying triplet3 Δ and 3Σ− and septet 7II and 7Σ+ are presented. Theoretical calculations for 5Σ−, B' 5II, and 3Σ− states are reported for the first time. The results are in many cases in better agreement with experiment than are other calculations, already at the simplest level of approximation within DFT, which confirms that this method is a useful tool for investigation of transition‐metal compounds. © 1994 John Wiley & Sons, Inc.
Applicability to atoms of a large set of correlation energy functinals
Authors: San fabián E. , Moscardó F. , Pérez‐jordá J. M.
Published: January 2005

An extensive study of the application of a large set of correlation energy functionals to atomic systems is presented. Hartree–Fock densities and, sometimes, generalized‐valence‐bond wave functions are used. The functionals have been grouped into four sets: (I) local density and local spin density, (II) local spin density with self‐interactioon correction, (III) local spin density with gradient correction, and (IV) methods found on the correlation factor approach. In this bench mark, we report the results of calculations made for the ground states of the first and second rows, their anions and cations, and the two‐ and four‐isoelectronic series. © 1994 John Wiley & Sons, Inc.
A density functional study of pseudotetrahedral metal–nitrosyl complexes
Authors: Bohr F. , Chermette H. , Ruiz‐lópez M. F.
Published: January 2005

Local and nonlocal density functional computations have been carried out to study the electronic structure and the equilibrium geometry of the isoelectronic series Cr(NC)4, Mn(NO)3(CO), Fe(NO)2(CO)2, and Co(NO)(CO)3 and model compounds Fe(NO)2L2 (L = Cl, HCN, NH3, PH3, and C2H4). The structure of Fe(NO)2(C4H6) is also described. The discussion is focused on structural modifications through a change of ligand, in particular those concerning the metal‐nitrosyl conformation (linear vs. bent). Though this is a preliminary study of metal–nitrosyl properties by DFT methods and more computations are required to analyze the mechanism of homogeneous catalysis processes, our results support the hypothesis that structural reorganization from linear to bent metal–nitrosyl plays a key role in some reactions, such as in butadiene dimerization. © 1994 John Wiley & Sons, Inc.
An insight into optical and EPR properties of AgCl and AgF complexes through MS–Xα and SCCEH calculations
Authors: Valiente R. , Aramburu J. A. , Barriuso M. T. , Moreno M.
Published: January 2005

MS‐Xα and SCCEH calculations on the Ag2+ complexes AgF and AgCl (displaying an elongated D4h symmetry) have been carried out for a better understanding of their experimental optical and EPR properties. As salient features, the present work supports that the unpaired electron in AgCl spends a little more time on ligands than on Ag2+, in agreement with the previous analysis of EPR and optical data for KCl:Ag2+. Furthermore, the five experimental optical transitions observed in that case are reasonably assigned. The first transition (observed at 12,500 cm−1) is assigned to a jump involving the 5a1g orbital built mainly (∽70%) from 3p orbitals of axial ligands, a fact that reflects the distinct level scheme for AgCl when compared to that for more ionic complexes. Calculations on AgF and AgF performed as a function of the equatorial Ag2+ –F− distance led to a reasonable understanding of experimental gyromagnetic and superhyperfine tensors displayed by Ag2+ in fluorides. The different relative decrease undergone by g‖– go (8%) and g ⟂ – go (28%) on passing from CsCdF3:Ag2+ to RbCdF3:Ag2+ is shown to be consistent with the formation of AgF and AgF complexes, respectively, related to the different substitutional position of Ag2+ in such lattices. The decrement of about 8.5% experienced by both g‖ – go and g⟂ – go values on going from CsCdF3:Ag2+ to NaF:Ag2+ is pointed out to reflect the different electrostatic potential (exerted by the rest of the lattice upon the complex) seen by AgF embedded in NaCl or perovskite‐type lattices. © 1994 John Wiley & Sons, Inc.
The chemisorption of hydrogen on Cu(111): A dynamical study
Authors: Forni A. , Wiesenekker G. , Baerends E. J. , Tantardini G. F.
Published: January 2005

Ab initio band‐structure calculations within a density functional formalism were performed to compute the binding energy curves of atomic hydrogen with the high‐symmetry adsorption sites of the (111) surface of copper. For a two‐layer slab of Cu atoms and H coverage equal to 0.25, the binding energies are 2.25, 3.12, and 3.24 eV, for on‐top, bridge, and threefold sites, so that the chemisorption of H2 on Cu(111) is exothermic for threefold and bridge sites, but endothermic for on‐top sites. Starting from these results, an LEPS potential for the interaction of H2 with the Cu(111) surface was built. In this model potential, the most favored approaches correspond to a H2 molecule parallel to the Cu surface, and for them, the activation barrier is located at the corner between the entrance and the exit channels of the reaction, and its lowest value is 0.6 eV. The LEPS potential was used in quasi‐classical trajectories calculations to simulate the adsorption of a beam of H2 molecules on Cu(111). The results show that (a) when H2 is in the ground vibrational state the dissociative adsorption probability Pa increases from 0 to .90 along a roughly sigmoidal curve by increasing the collision kinetic energy from 0.4 to 1.3 eV, and (b) the vibrational energy can be as effective as the translational one in promoting dissociative chemisorption, in agreement with the experimental results. © 1994 John Wiley & Sons, Inc.
SystematicLSDinvestigation on cationic boron clusters: B(n 2–14)
Authors: Boustani Ihsan
Published: January 2005

The geometrical structures and properties of small cationic boron clusters B(n= 2–14) have been investigated using the local spin density (LSD) formalism. The nonlocal correction has also been calculated. The linear search for minima on the potential energy surface has been performed using analytical gradients of theLSDtotal energy. Most of the final structures of the cationic boron clusters prefer planar or quasi‐planar nuclear arrangements and can be considered as fragments of a planar surface or as segments of a sphere. The calculated adiabatic ionization potentials of Bnexhibit features that are analogous to those of measured ionization potentials for boron clusters. Most of the calculated normal modes of the cationic clusters have frequencies that are around 1000 cm−1and have strong infrared intensities, and they correspond quite well with analogous properties of solid boron. © 1994 John Wiley & Sons, Inc.
First principles calculation of the optical properties and stability of hydrogenated silicon clusters
Authors: Hirao Masahiko , Uda Tsuyoshi
Published: January 2005

Optical properties and stability of hydrogenated silicon clusters are investigated using density functional pseudopotential calculation. The dipole transitions between the band‐edge orbitals are allowed, in contrast to the indirect gap in bulk silicon. Evolution of a small amount of hydrogen atoms enhances the dipole transition, increasing the photoluminescence intensity. Further dehydrogenation creates gap states due to dangling bonds, which may decrease the photoluminescence intensity via nonradiative recombination processes. The Stokes shift is also estimated by calculating the relaxed structure of the excited state within the local density approximation. © 1994 John Wiley & Sons, Inc.
Natural polyelectron population analysis
Authors: Parrondo Ramóan M. , Karafiloglou Padeleimon , Marcos Enrique Sánchez
Published: January 2005

A method to perform a polyelectron population analysis of correlated molecular orbital wave functions on the basis of natural atomic orbitals (NAOs), as given by Weinhold, is presented. The method allows calculations of the probabilities of finding various types of electronic events occuring in some target AO positions, including the contributions of ionic and covalent resonance structures. This method is general and neither the theory nor the developed algorithm limit the number of electrons and holes that can be considered. Thus, the analyzed MO wave function can be a usual CI or a MCSCF one, and apart from Weinhold's NAOs. any other type of orthogonal AOs can be used as analyzers, provided that these AOs are linear combinations of the SCF‐AOs. Numerical applications are given for ethylene, formaldehyde, butadiene, and acroleine, by adopting various AO basis‐set levels (STO−4G, 4–31G, and 6–31G) and by analyzing correlated wave functions (CISD). Improvements in the polyelectron populations when increasing the quality of AO basis sets and the corresponding valence NAOs are revealed by several examples. Furthermore, it is shown that the electroegativity of oxygen in acroleine only has an effect on contributions of ionic and covalent resonance structures, but not on delocalization of the double bonds. 1993 John Wiley & Sons, Inc.
Evaluation of cross sections for X‐ray and high‐energy electron scattering from molecular systems
Authors: Wang Jiahu , Smith Vedene H.
Published: January 2005

An analytical and computationally efficient formalism for the evaluation of rotationally averaged cross sections for X‐ray and high‐energy electron scattering from molecular systems is given. Its application for both SCF and CI wave functions is discussed. © 1994 John Wiley & Sons, Inc.
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

A 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

An 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

In 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

The 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

The 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

Atomic 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

This 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

The 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

A 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

Based 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

The 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

The 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

By 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

The 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

The 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

Conformational 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

We 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

Calculations 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.