bluuebunny/crossref_metadata_2025_split huggingface dataset:
Semiempirical quantum chemical treatment of the standard reduction potentials of quinone and plastoquinone in water
Authors: Mallik Buddhadeb , Datta Sambhu N. 
Published: January 2005

AbstractThe standard electrode potential for the quinone (Q)‐hydroquinone (QH2) couple in aqueous acidic media has been explicitly calculated. Molecular geometries of Q and QH2 have been optimized. Protonation of Q, i.e., the formation of QH+ and QH, have been considered. Molecular geometries of these species have been thoroughly optimized. The energy of complexation of these molecules with water have been calculated by optimizing the structures of the hydrated complexes Q · 6H2O, QH2 · 6H2O, QH+ · 6H2O. and QH · 6H2O. The ion–solvent interaction energy of QH+ · 6H2O, in turn, has been calculated by considering the complex QH+ · 6H2O…︁ 2H2O, where the two extra water molecules approach the charge center of the complex QH+ · 6H2O vertically from top and bottom of the quinonoid ring. The standard reduction potential calculated by the CNDO method, 0.8548 V, is somewhat larger than the experimental potential, 0.6998 V, at 25°C. But the INDO value, 0.7085 V, is in excellent agreement with the observed potential. The electrode potential for the plastoquinone (PQ)‐plastohydroquinone (PQH2) couple present in the aqueous pool in chloroplast has been calculated by the INDO method. The basic geometries of PQ, PQH+, and PQH2/sb have been synthesized by adopting the optimized geometries of Q, QH+, and QH2 and considering methyl substituents as well as an isoprenoid side chain containing up to 3 isoprene units with possible geometrical isomerism. The hydrated species PQ · 6H2O, PQH+ · 6H2O, and PQH2 · 6H2O are unstable compared to the isolated species PQ, PQH+, and PQH2, respectively. In fact, we have found that the hydration of PQH+ and PQH2 is much less extensive, and stability arises only when the hydroxyl groups in these two molecules are hydrogen‐bonded to water molecules. But PQH+ is also stabilized through the association of two more water molecules in the vertical direction. For this reason, we have calculated the reduction potential of the PQ/PQH2 system from the energies of the isolated molecules PQH2 and the hydrated species PQH+ · 2H2O. The computed standard reduction potential is 0.2785 V and it yields a potential of 0.07V at pH 7 at 25°C, which is in good agreement with the reduction potential 0.11 V observed for plastoquinone in the aqueous pool in chloroplast. © 1994 John Wiley & Sons, Inc.
Calculations on the spectra and nonlinear third‐order optical susceptibility of c70
Authors: Li Jun , Feng Jikang , Sun Chiachung 
Published: January 2005

AbstractThe equilibrium geometry and UV‐visible spectra of C70 were examined using semiempirical INDO/2 and INDO/CI methods. The results obtained are in good accord with experimental results. On the basis of correct electronic spectra, calculations of the nonlinear third‐order optical susceptibility (γijkl) of C70 were performed using the INDO/SDCI method combined with a sum‐over‐states expression. The calculated value for <γ> (‐2 ω, ω, ω, O) is 0.882 × 10−33 esu (ω = 1.91 μm), which is in good agreement with observation. © 1994 John Wiley & Sons, Inc.
Transition states and energy barriers from density functional studies: Representative isomerization reactions
Authors: Abashkin Yury , Russo Nino , Toscano Marirosa 
Published: January 2005

AbstractThree representative isomerization reactions (HNC → HCN, CH3NC → CH3CN, and N2H2 trans → cis and sin) have been studied using both the LCGTO–LSD and LCGTO–NLSD density functional methods and employing a new algorithm for the search and the refinement of the transition‐state structures. The inclusion of the nonlocal corrections and the use of large basis sets improve the reliability of the energetic parameters. Results are in good agreement with previous accurate first‐principle computations and available experimental data. © John Wiley & Sons, Inc.
An analysis of the integrand occurring in correlation energy functionals
Authors: Fortunelli Alessandro , Salvetti Oriano 
Published: January 2005

AbstractAn analysis of the integrand occurring in current density functional calculations is presented, concentrating attention on correlation energy functionals and the atomic regions, i.e., the regions of space surrounding atomic centers. The analysis follows the structure of a previously proposed numerical integration scheme for three‐dimensional integrals occurring in electronic structure calculations. The scheme is based on the choice of density‐based weight functions that naturally partition the space into «atomic» volumes (in which the integration is performed in terms of spherical coordinates) and «diatomic» volumes (in which the integration is performed in terms of confocal elliptical coordinates). From this analysis, a simplified procedure for the atomic «internal» integrations is developed, whereas preliminary results are discussed for the atomic and diatomic «external» integrations. The numerical tests are performed on the C60 molecule in the symmetrical configuration. © John Wiley & Sons, Inc.
The determination of intrinsic reaction coordinates by density functional theory
Authors: Deng Liqun , Ziegler Tom 
Published: January 2005

AbstractThe first implementation of the intrinsic reaction coordinate (IRC) method within the density functional theory (DFT) framework is presented. The implementation has been applied to four different types of chemical reactions represented by the isomerization process, HCN  HNC (A); the SN2 process, H− + CH4  CH4 + H− (B); the exchange process, H˙ + HX  HX + H˙ (X  F,Cl) (C); and the elimination process, C2H5Cl  C2H4 + HCl (D). The present study presents for each process optimized structures and calculated harmonic vibrational frequencies for the reactant(s), the transition state, and the product(s) along with the IRC path connecting the stationary points. The calculations were carried out within the local density approximation (LDA) as well as the LDA/NL scheme where the LDA energy expression is augmented by Perdew's and Becke's nonlocal (NL) corrections. The LDA and LDA/NL results are compared with each other as well as the best available ab initio calculations and experimental data. For reaction (D), ab initio calculations based on MP2 geometries and MP4SDTQ energies have been added due to the lack of accurate published post‐HF calculations on this process. A detailed discussion is provided on the efficiency of the IRC algorithms, the relative accuracy of the DFT and ab initio schemes, as well as the reaction mechanisms of the four reactions. It is concluded that the LDA/NL scheme affords the same accuracy as does the MP4 method. The post‐HF methods seem to overestimate activation energies, whereas the corresponding LDA/NL estimates are too low. The LDA activation energies are even lower than the LDA/NL counterparts. The incorporation of the IRC method into the DFT framework provides a promising and reliable tool for probing the chemical reaction path on the potential energy surfaces, even for large‐size systems. IRC calculations by ab initio methods of an accuracy similar to the LDA/NL scheme, such as the MP4 scheme, are not feasible. © John Wiley & Sons, Inc.
Numerical grids for density functional calculations of molecular properties
Authors: Goursot A. , Pápai I. , Daul C. A. 
Published: January 2005

AbstractThere is an increasing number of applications using methods based on density functional theory (DFT) describing a large variety of systems of chemical interest. All these calculations are performed through evaluations of integrals of exchange and correlation contributions to the energy and the potential, which depend on the density and also on its first and second derivatives (nonlocal functionals). Within our DF program, the integrand is decomposed into single‐center components, through the introduction of weight functions, and its radial and angular dependence can be chosen. In this article, we give a brief description of these features, with compared performances of different grids for the evaluation of different integrals, such as the total number of electrons and the exchange and correlation energies. More complex properties, such as total energies and equilibrium geometries, are also studied with respect to the choice of the grid of points, in order to determine the most favorable radial and angular quadrature schemes. The incidence of this choice is analyzed in the case of the metal–metal bond of the Ru2, Rh2, and Pd2 dimers. Finally, the use of an extended grid of points is shown necessary for systems involving weak interactions, such as the Ar2 molecule. © John Wiley & Sons, Inc.
Coulomb‐Hole–Hartree–Fock functional
Authors: Clementi E. , Hofmann D. W. M. 
Published: January 2005

AbstractWe have extended to molecules a density functional previously parametrized for atomic computations. The Coulomb‐hole–Hartree–Fock functional, introduced by Clementi in 1963, estimates the dynamical correlation energy by the computations of a Hartree–Fock‐type single‐determinant wave function, where the Hartree–Fock potential was augmented with an effective potential term, related to a hard Coulomb hole enclosing each electron. The method was later revisited by S. Chakravorty and E. Clementi [Phys. Rev. A 39, 2290 (1989)], where a Yukawa‐type soft Coulomb hole replaced the previous hard hole; atomic correlation energies, computed for atoms with Z = 2 to Z = 54 as well as for a number of excited states, validated the method. In this article, we parametrized a function, which controls the width of the soft Coulomb hole, by fitting the first and second atomic ionization potentials of the atoms with 1 ⩽ Z ⩽ 18. The parametrization has been preliminarily validated by computing the dissociation energy for a number of molecules. A few‐determinant version of the Coulomb‐hole–Hartree–Fock method, necessary to account for the nondynamic correlation corrections, is briefly discussed. © 1994 John Wiley & Sons, Inc.
Ground‐state properties of CdxSn1–xTe: The role of d‐electrons
Authors: Grosch G. H. , Freytag B. , Range K.‐J. , Rössler U. 
Published: January 2005

AbstractWithin the framework of density functional theory (DFT), we calculate the ground‐state electronic properties of CdxSn1–x Te using norm‐conserving pseudopotentials in connection with the local density (LDA) and virtual crystal approximation (VCA). Our particular interest is in the influence of the Cd‐4d and the Sn‐4d electrons by comparing results obtained with pseudopotentials, which either consider the d‐electrons explicitly or in the frozen core.
In the mixed crystal system CdxSn1–x Te, the transition from a ten‐ (x = 0) to an eight‐electron (x = 1) system is realized, which is accompanied by a change of the crystal structure from rock salt (SnTe) to zinc blende (CdTe). By calculating the ground‐state energies, we find the equilibrium lattice constant as the function of x and the bulk modulus, as well as the crossover value of x for the transition from rock‐salt to zinc blende. The calculated lattice constants and bulk moduli are in much closer agreement with experimental data if the d‐electrons are taken into account explicitly, whereas the crossover is rather insensitive with respect to the d‐electrons. In view of the electronic charge density, we demonstrate the decrease of ionicity for increasing x. © 1994 John Wiley & Sons, Inc.
Application of second‐order density functional methods to the calculation of the LiFH potential energy surface
Authors: Suárez Cristina , Aguado Alfredo , Tablero César , Paniagua Miguel 
Published: January 2005

AbstractSecond‐order density functional methods are used to introduce the electron correlation in Hartree‐Fock (HF) ab initio electronic energy calculations of three‐dimensional potential energy surfaces (PES). We analyze the behavior of these methods in PES calculations by applying them to the Li + FH reaction, which has been considered a prototype of the elementary atom–diatom reactions. This system has been studied also by the usual techniques, allowing a point‐by‐point (for a total of 317 grid points) comparison for the lowest 2A' adiabatic state. In particular, we compare the results obtained using the HF, Møller–Plesset (MP3 level), and configuration interaction (CISD and MRDCI levels) methods with the corresponding results obtained using the Colle–Salvetti (CS) and Moscardó‐San Fabián (MSF) procedures using the HF results as the starting point. We found that the CS and MSF procedures support the prediction of a shallow well in the entrance channel that deepens slightly away from collinearity and disappears for a bond angle Θ < 74°. We also found that the constrained saddle‐point positions remain essentially constant from Θ = 180°–90° and are clearly in the exit channel as for the MRDCI approach (corresponding to the best results). In conclusion, there is a good overall agreement, but there is a question in which this agreement is less pronounced: the heights of the saddle points including the transition state. In particular, the transition‐state height is about 3 kcal/mol higher than the more accurate value obtained with the MRDCI approach. However, the second‐order density functional methods have been capable of reducing the HF barrier from 18 to 9 kcal/mol (all of these values obtained by spline interpolation), the latter value being very similar to the CISD result. © 1994
John Wiley & Sons, Inc.
Ionization potentials of atoms calculated with a nonlocal exchange and a local correlation functional
Authors: Cordero N. A. , Gritsenko O. V. , Rubio A. , BalbáS L. C. , Alonso J. A. 
Published: January 2005

AbstractWe calculate the first ionization potential of atoms (3 ⩽ Z ⩽ 54) by means of a nonlocal density functional approach and the results are compared with those obtained using other recent density functional approaches based on density gradients. In these calculations, we use a nonlocal weighted spin‐density approximation of exchange effects and a local spin‐density approximation of Coulomb correlation, both based on novel forms of the pair‐correlation functions. We also calculate the total energies of the He, Be, and Ne isoelectronic series. © 1994 John Wiley & Sons, Inc.
Density functional treatment of water–carbon dioxide van der waals complex
Authors: Abashkin Yuri , Mele Franca , Russo Nino , Toscano Marirosa 
Published: January 2005

AbstractLCGTO–LSD and LCGTO–NLSD methods have been tested for the study of water–carbon dioxide weakly bound binary complex. Different local and nonlocal exchange‐correlation energy functionals and many grid radial points have been used. Results show that both nonlocal corrections and a large number of radial points in the grid are mandatory for well reproducing the experimental data. © 1994 John Wiley & Sons, Inc.
Quantum mechanical valence study of a bond‐breaking–bond‐forming process in triatomic systems
Authors: Nalewajski Roman F. , Formosinho Sebastiãto J. , Varandas AntÓnio J.C. , Mrozek Janusz 
Published: January 2005

AbstractThe recently introduced set of the quadratic, two‐electron covalent and ionic valence indices is used to investigate the bond‐breaking–bond‐forming (BB‐BF) process in an atom exchange reaction between H2 and X (X = H, F—I) as well as in the O2—H system. Valence changes accompanying selected charge reorganizations are examined within the three‐orbital model and valence diagrams for symmetric transition states (TSs) are given. The UHF valence data for Li2O and CO2 and the H—H—X, O—O—H, and O—H—O (ABC) TSs (collinear and angular) are reported and compared to valence data in the separated fragments limits (SFL), AB and BC. The overall valence, ν(ABC), and the total (ionic plus covalent) diatomic valences, νAB and νBC, are used as measures of the overall bond‐order in a concerted BB–BF reaction, to test the postulate of the bond‐energy–bond‐order (BEBO) model. In collinear TSs of H2X, ν ≊ −1, i.e., one bonding electron pari, is found to be roughly preserved, whereas in the angular H2X and in collinear O—H—O TSs, the effect of increased valence at the saddle‐point is observed, relative to that of diatomic fragments (reactiants or products). For the angular O—O—H TS, a similar increase in | ν (ABC)| relative to both O2 and OH SFLs is detected; smaller changes relative to the O2 data are found in the collinear TS. This observation is in agreement with earlier predictions from the intersecting‐state model. The relative diatomic valences, ν/ν and ν/ν, are shown to conserve the overall relative bond multiplicity around 1 in both collinear and angular TSs of the H2—X systems. © 1994 John Wiley & Sons, Inc.
Improved tight‐binding method
Authors: Telezhkin V. A. , Rafalovich A. A. 
Published: January 2005

AbstractAn improved tight‐binding method, taking into exact account the kinetic energy oerator, is proposed. The potential part of the Hamiltonian is represented by its matrix elements on some set of localized functions (like the LCAO method). The spectrum and the density matrix are determined by means of solving the nonlinear eigenvalue problem. © 1994 John Wiley & Sons, Inc.
Two types of Kramers rate equations for reactions in condensed media
Authors: Christov S. G. 
Published: January 2005

AbstractTwo cases of bilinear coupling of a particle to a medium are compared. They differ in that one of the mediums does not modify the particle potential, whereas the other does. Two corresponding kramers‐type rate equations for adiabatic reactions are derived directly from a dynamic rate theory and interpreted from the view of the stochastic theory. Both equations become identical in the weak coupling limit. Their relations to transition‐state theory are also discussed. Corresponding results for nonadiabatic reactions are considered. © 1994 John Wiley & Sons, Inc.
The coordination chemistry of the catalytic zinc ion in alcohol dehydrogenase studied by ab initio quantum chemical calculations
Authors: Ryde Ulf 
Published: January 2005

AbstractThe coordination chemistry of the zinc ion in the active site of alcohol dehydrogenase has been studied by the ab initio Hartree–Fock method. Geometry optimizations were performed using analytical gradients and basis sets of double‐zeta quality. Correlation effects were included at the MP2 level. The active site was modeled by Zn(HS)2XL(H2O)0–2, where X denotes ammonia or imidazole and L denotes water, methanol, ethanol, or the corresponding aldehydes or anions. It is shown that with uncharged L‐ligands the four‐coordinate complexes are about 20, 17, and 40kJ/mol more stable than are the corresponding three‐, five‐, and six‐coordinate complexes, respectively. If the L‐ligand is negatively charged, only the four‐coordinate complexes are stable. These results suggest that the active‐site zinc ion in alcohol dehydrogenase prefers a coordination number of four during the catalytic reaction, especially when the nonprotein ligand is negatively charged. Ligand exchange at the zinc ion is likely to proceed by an associative mechanism with intermittent formation of a five‐coordinate complex. The results lend no support to mechanistic proposals attributing an important catalytic role to a negatively charged five‐coordinate hydroxide or alkoxide ligand. © 1994 John Wiley & Sons, Inc.
Self‐consistent methods for the treatment of low‐lying molecular vibrations
Authors: Schmidt P. P. 
Published: January 2005

AbstractThe subject of this article is the self‐consistent‐field (SCF) treatment of low‐lying molecular vibrations in molecules subject to solvent effects and light atom migration. The analyses use a Cartesian Gaussian basis and Gaussian functional expansions of potential energy operators. The objective of the work was to establish approximate and practical methods of analysis of vibrational degrees of freedom in molecules that build on and compare well with the highly accurate treatments of vibrations in small molecular systems of the past decade. An application to a system in which hydrogen bonding contributes the major anharmonic effect illustrates the method. © 1995 John Wiley & Sons, Inc.
The evaluation of matrix elements in the analysis of anharmonic molecular vibrations: Functional taylor series expansions
Authors: Schmidt P. P. 
Published: January 2005

AbstractThis article presents methods for computing Cartesian Gaussian matrix elements using a Taylor series for general potential energy operators that admit well‐behaved radial derivatives. These operators arise in the analyses of anharmonic vibrations in molecules. Application to the evaluation of matrix elements for hydrogen associated two wells illustrates the method. © 1995 John Wiley & Sons, Inc.
Unified treatment of two‐center Coulomb, hybrid, and exchange integrals over Slater‐type orbitals
Authors: Guseinov I. I. , Yassen R. F. 
Published: January 2005

AbstractUsing Neumann expansion for 1/r12 in elliptical coordinates a combined formula has been obtained for two‐center Coulomb, hybrid, and exchange integrals with Slater‐type orbitals. © 1995 John Wiley & Sons, Inc.
Ensemble representable densities for atoms and molecules. I. General theory
Authors: Cassam‐Chenaí P. 
Published: January 2005

AbstractA method to obtain ensemble representable densities from experimental diffraction data is proposed. The method uses ab initio molecular densities instead of the commonly employed one‐electron orbital densities, and as a result, few parameters need to be optimized in the fitting procedure to the experimental structure factors. The optimized coefficients can provide information about intra‐ and intermolecular electronic correlations, spin‐orbit coupling, etc. This work also provides new explicit formulas to determine the rank of a fermionic wave function, i.e., the rank of the one‐fermion density matrix. © 1995 John Wiley & Sons, Inc.
A method of calculation of the matrix elements between the spin‐projected nonorthogonal Slater determinants
Authors: Igawa Akira 
Published: January 2005

AbstractIn the resonating Hartree–Fock calculation, we have to calculate the matrix element between spin‐projected nonorthogonal Slater determinants (S dets). The matrix element between coherently spin‐rotated S dets are given by a linear transformation of the spin‐projected ones. Using the inverse transformation, we get the projected matrix elements from the coherently spin‐rotated ones. By appropriately choosing the angles of the spin rotation, the required computational time is considerably reduced. © 1995 John Wiley & Sons, Inc.
Theoretical studies on the inner‐sphere reorganization energies for the self‐exchange reactions of gas‐phase diatomic molecules HA (A = Mg, Al, Si, P, S, Cl)
Authors: Bu Yuxiang , Zhang Yuhua , Qiu Linlin 
Published: January 2005

AbstractOn the basis of the recently proposed accurate calculation scheme of the inner‐sphere reorganization energies (RE) of the reactants in gas‐phase electron‐transfer xprocesses, the inner‐sphere RE values for the AH + AH+ (A = Mg, Al, Si, P, S, Cl) self‐exchange systems are calculated in terms of an ab initio Hartree–Fock self‐consistent‐field MO method at different basis‐set levels (6‐31G**, 6‐31 +G**, DZ, and DZP). The structural parameters involved are also determined via the perturbation theory and the Dunham expansion of the Morse function and compared with the experimental values. Dissociation energies are corrected by electron correlation at the MP2/6‐31G* level. Results of the inner‐sphere REs obtained from different models via ab initio calculations for these systems discussed here are in full agreement with the corresponding experimental data. © 1995 John Wiley & Sons, Inc.
Numerical Hartree–Fock results for atoms Cs through Lr
Authors: Koga Toshikatsu , Watanabe Shinya , Thakkar Ajit J. 
Published: January 2005

AbstractAccurate nonrelativistic numerical Hartree–Fock results are reported for the heavy atoms Cs (Z = 55) through Lr (Z = 103) in their ground states. © 1995 John Wiley & Sons, Inc.