bluuebunny/crossref_metadata_2025_split huggingface dataset:
Molecular conformations and molecular shape: A discrete characterization of continua of van der Waals surfaces
Authors: Arteca Gustavo A. , Mezey Paul G. 
Published: January 2005

AbstractA characterization of molecular model surfaces is proposed. It is based on a graph associated with the van der Waals surface, defined by the detailed information on the interpenetration of van der Waals spheres of the constituent atoms. This “van der Waals graph” describes the three‐dimensional body of the molecule, and it does not coincide in general with the less informative bond graph obtainable from the molecular skeleton.The description in terms of the graph reveals clearly the changes in molecular shape induced by conformational rearrangements. The nuclear configurations can be classified by the graph associated with the molecular surface, and the graph‐theoretical analysis provides a rigorous partitioning of the configurational space based on shape properties.
A coupled‐cluster method for quasidegenerate states
Authors: Meissner L. , Jankowski K. , Wasilewski J. 
Published: January 2005

AbstractA size‐extensive, multireferences coupled‐cluster method for studies of quasidegenerate states based on the Jeziorski–Monkhorst [16] ansatz for the cluster operator (Ω = ∑ePj, where the sum is extended over the configurations spanning the model space), is presented and applied in pilot calculations. The method is referred to as multireference coupled electron‐pair method (MR CEPM), because it is assumed that the individual cluster operators can be approximated by their two‐body parts, i.e., Tj ≈ Tj(2). The linear version of this method (MR L‐CEPM) is also discussed. Both methods are applied to two simple model systems: (1) a minimum basis set model involving eight hydrogen atoms in various spacial arrangements for which the degree of quasidegeneracy can be continuously varied; (2) a model involving the C2ν insertion of Be into H2. For the first time in multireference coupled‐cluster calculations, the nonlinear parts of the equations are completely accounted for. The MR CEPM results are very encouraging for strongly quasidegenerate states. The MR L‐CEPM results are slightly below the accurate (FCI) values.
Solvent structure and dynamics: How relevant to molecular and quantum pharmacology?
Authors: Palma M. U. 
Published: January 2005

AbstractA close scrutiny of the relationship between geometric constraints and motion in a system of water molecules allows the prediction of a role of solvent dynamics in the solvent‐mediated interaction between two solutes and between different parts of one solute. The predicted mechanism is cooperative, and its operation is widely supported by experiments (not only by our group) in which solvent dynamics were modulated by isotopic or cosolvent perturbations. More recent experimental work by our group and simulation work of MCY water by Fornili et al, strengthens this evidence and visualizes how an immobilized water molecule can “seed” or “pin” a high‐connectivity patch of H bonds. Involvement of the mechanism in pattern‐specific, solvent‐mediated driving forces of significant size (additional to those due to electrostatic recognition determinants) and in functional proton‐transfer is inferred from available evidence.
Theoretical approaches to interactions between biomolecules: Legends and approximations to reality
Authors: Zahradník Rudolf , Achenbach Frank 
Published: January 2005

AbstractThere are significant differences between the conditions for chemical and biochemical reactivity. There‐fore, models for treating chemical reactions are mostly not suitable for investigating interactions and transformations of bio(macro)molecules. Common features of numerous processes occurring in vivo and in vitro (such as the role of water, ions, and colloids and the significance of Helmholtz energy surfaces) are outlined. Some characteristics of a model suitable for studying van der Waals interactions between biomacromolecules, based on Brownian dynamics and the Lifshitz theory, are described.
The influence of basis sets on wave function tails
Authors: Ishida Toshimasa , Ohno Koichi 
Published: January 2005

AbstractWave function tails are analyzed quantitatively by investigating the dependence of exterior electron density (EED) on basis sets; the EED is defined as the integrated electron density outside the repulsive molecular surface. Ab initio MO calculations with large scale basis sets were performed to establish the benchmark order of EED values for valence orbitals of some simple molecules. It is found that very popular basis sets, such as 4‐31G, which are determined by energy optimization, are inferior in describing the wave function tails to some similar size basis sets, such as MIDI‐4, which are obtained by least‐squares fit to near Hartree‐Fock atomic functions. Further the EED values for atomic 2s functions are shown to be unfavorably smaller than those for atomic 2p functions when the same value is used for the exponent α in the GTO basis sets. This indicates that the frequently used constraint αs = αp is not appropriate for describing wave function tails with medium‐size basis sets. Deficiencies in the energy‐optimized basis sets are found to become more serious for molecules including heavier atoms.
Atomic energy levels from configuration interaction calculations with relativistic corrections
Authors: Fraga S. , Fabian E. San , Sordo J. A. , Campillo M. , Climenhaga J. D. , Klobukowski M. 
Published: January 2005

AbstractIt is shown that configuration interaction calculations, with inclusion of the relativistic corrections, constitute an appropriate approach for the prediction of atomic energy levels and that results of experimental accuracy are possible given the availability of large‐scale, fast computers. The results obtained for He through F emphasize both the practical difficulties to be encountered and the possibility of predictions with less than 1% error.
Electronic structure, theoretical electronic properties of tetraphenyldithiapyranylidene DIPSϕ4 molecule, and evaluation of the charge transfer occurring with doping agents like TCNQ
Authors: El Khatib N. , Chermette H. , Maitrot M. 
Published: January 2005

AbstractAn extensive theoretical calculation of the tetraphenyldithiapyranylidene DIPSϕ4 molecule by means of the MSXα method is presented. The results obtained permit one to interpret the optical spectrum and the XPS spectrum as well as the charge transfer between DIPSϕ4 and planar molecular semiconductor compounds such as TCNQ. To our knowledge, this is the first theoretical determination to date of such a charge transfer ratio from separate parameter‐free calculations on large‐size molecules.
An algorithm for R matrix calculations for atom–diatom reactive scattering
Authors: Linderberg Jan 
Published: January 2005

AbstractSome aspects of R matrix theory that derive from a finite element method implementation of a hyperspherical coordinate formulation of the quantum mechanical three‐body problem are analyzed with a view of designing suitable algorithms. Propagation of the R matrix from a hyperspherical surface to asymptotically appropriate wave fronts is given special attention.
Hydrogen bonding in potassium hydrogen carbonate studied by compton scattering on single crystals
Authors: Bräuchler M. , Lunell S. , Olovsson I. , Weyrich W. 
Published: January 2005

AbstractCompton spectra excited by 241Am radiation have been measured on thin single‐crystal slices with the scattering vector oriented parallel and perpendicular to the pair of hydrogen bonds in the (HCO3) dimers as well as to the plane of the dimers. the reciprocal form factors extracted from the spectra are strongly anisotropic with two extra zero passages in the direction of the hydrogen bonds. the results are in very satisfactory agreement with theoretical data that are calculated for the dimer with and without additional point charges simulating the neighboring ions in the crystal. The calculations have been done with Gaussian basis sets of double‐zeta quality within the Hartree–Fock approximation. The theoretical reciprocal form factors of the monomer HCO and the dimer show only small differences in the directions perpendicular to the hydrogen bonds, whereas the differences in the hydrogen bond direction are remarkable and account for the experimentally observed features. In comparison to this effect the influence of the neighboring ions by their charges is much smaller, which is even true when taking into account the influence of the neighboring dimers in the crystalline stack in more detail by symmetrical orthogonalization of the combined wave functions. The same orthogonalization procedure applied to a pair of monomers in the spatial arrangement of the dimer yields a reciprocal form factor that in the experimentally reliable range above 1.8 Å accounts for most of the dimerization effect in the direction of the hydrogen bonds. Thus also for this prototype of paired hydrogen bonds (cf., carboxylic acids, DNA, and RNA), it confirms our earlier experimental finding on liquid water now unambiguously, namely that, in the same way as the cohesion in ionic crystals, hydrogen bond formation in bulk matter is distinctly dominated by electrostatic attraction, which is compensated by repulsion owing to the Pauli principle.
Transport of excitation energy in a three‐dimensional doped molecular crystal
Authors: Priyadarshy Satyam , Datta Sambhu Nath 
Published: January 2005

AbstractWe report here a theoretical formulation of the transport of excitation energy in a three‐dimensional molecular crystal containing one impurity. The excitation is assumed to be localized in the jth site at time t, and the expression for the probability of finding the excitation at some other site j′ at a later time t′ is derived. The probability is given by the correlation function \documentclass{article}\pagestyle{empty}\begin{document}$ \left\langle {\hat P_j (t)\hat P_j (0)} \right\rangle $\end{document}, where \documentclass{article}\pagestyle{empty}\begin{document}$ \left\langle {\hat P_m } \right\rangle $\end{document} represents the site projection operator, |m〉 〈m|. In our derivation we neglect the interaction among excitons of different bands, account for the presence of the impurity by adding a small perturbation term to the pure crystal Hamiltonian, and calculate the exciton solutions through first order. We consider a general impurity; that is, the trap depth is nonvanishing and may even be complex. The exciton–phonon interaction is taken to be linear in lattice displacement vectors; we assume that the short time behavior of \documentclass{article}\pagestyle{empty}\begin{document}$ \left\langle {\hat X} \right\rangle _{{\rm phonon}} $\end{document} gives the dominant contribution to the physical property X being studied and solve the dynamical problem by using a time‐dependent effective potential consisting of fluctuations around the equilibrium average exciton–phonon interaction. Several limiting cases are briefly discussed.
On surface states of transition metal carbides, nitrides, and oxides: (001), (011), and (111) surfaces
Authors: Pick Štěpán , Tomášek Mojmír 
Published: January 2005

AbstractThe simple semiquantitative approach to the calculation of the energy of surface states, proposed recently by the authors, is applied to high‐symmetry surfaces of selected transition metal carbides, nitrides, and oxides. The results are compared with recent experimental and theoretical data. The necessity to include the changes in the potential at the crystal surface is indicated.
A classical path approach to reactive scattering
Authors: Billing G. D. , Muckerman J. T. 
Published: January 2005

AbstractA recently proposed (semiclassical) classical path method for treating reactive scattering is reviewed. This method exploits properties of hyperspherical coordinates which allow a collision coordinate to be defined without reference to the arrangement of the particles. The coordinate space is divided into classical and quantal subspaces such that the description of the “arrangement” is quantal. Selected results are presented for the three‐dimensional D + H2 → HD + H reaction.
A theoretical study of protonation of triatomic silicon–carbon compounds
Authors: Largo‐Cabrerizo A. , Flores J. R. 
Published: January 2005

AbstractAb initio molecular orbital methods are employed to study the low‐lying states of C3H+, SiC2H+, Si2CH+, and Si3H+. Special attention is paid to a comparative study between C3H+ and Si3H+. In both cases a 3B2 state is found to lie the lowest at the HF level, although inclusion of correlation effects favor a linear structure (1Σ+ state) for C3H+, which lies 25 kcal/mol below the 3B2 state at the MP4 level, and a bent structure (1A′ state) for Si3H+, which lies just 2 kcal/mol below the 3B2 state. The proton affinities of C3, SiC2, Si2C, and Si3 are estimated at different levels of theory. Both protonation at carbon and silicon atoms are considered for SiC2 and Si2C. It is found that C3 comparatively has a low proton affinity. On the other hand, Si3 has a relatively high proton affinity compared with the protonation at silicon atom for both SiC2 and Si2C. These results are discussed on the basis of electronic structure arguments.
Ab initio MO calculations of hyperfine coupling constants. A basis set study for 19F, 35Cl, 19F, and 35Cl
Authors: Karna S. P. , Grein F. 
Published: January 2005

AbstractHyperfine coupling constants (HFCC) of the 19F and 35Cl atoms and the 19F and 35Cl radical anions have been calculated by the unrestricted Hartree–Fock (UHF) method using polarization and diffuse functions with contracted double‐zeta as well as uncontracted basis sets. The Adip values are fairly insensitive to changes in the basis set and show good accordance with experimental and other theoretical studies. The isotropic HFCCS aN of 19F, 19F, and 35Cl show strong dependence on d functions and the state of contraction of the s, p set. Spin‐projected UHF wave functions lead to better agreement with experiment.
Lie algebraic approach to the scattering system He + H2
Authors: Ding Shiliang , Yi Xizhang , Deng Conghao 
Published: January 2005

AbstractIn this article, the Hamiltonian for the scattering of the He + H2 system is given by using the interaction potential V(X, Y, Θ) determined by experiments and the semiclassical method. From this Hamiltonian we find a dynamical algebra h6. The statistical expectation of the energy and the transition probability of H2, Pn→m, are derived; therefore, selection rules have been found easily.
Electron–Electron interaction energy in homonuclear diatomic molecules
Authors: Piccitto G. , Pucci R. , March N. H. , Grassi A. 
Published: January 2005

AbstractThe derivative of the electron–electron potential energy Uee with respect to internuclear separation R is studied for light homonuclear diatomic molecules at equilibrium. It is readily related to nuclear–nuclear potential energy Unn, the force constant K, and the electron–nuclear potential energy Uen. An approximate expression, based on the simplest form of density functional theory, is then used to eliminate dUen/dR|Re. The result thus obtained for dUee/dR|Re transcends an earlier proposal of Kryachko by including a term 2/3ReK, with K the force constant. Numerical tests at SCF–RHF level are presented for nine homonuclear diatomic molecules.
Reaction field effects on the electronic structure of carbon radical and ionic centers
Authors: Karelson Mati , Tamm Toomas , Katritzky Alan R. , Szafran Miroslaw , Zerner Michael C. 
Published: January 2005

AbstractWe examine solvent effects on carbon radical and ionic centers of HCXY by including a self‐consistent reaction‐field into the AM1 and MNDO electronic structure models to mimic dielectric effects. We find that such concepts as merostability are principally solvent effects, and that, as expected, molecules with large dipoles or with charge assymmetry are stabilized more by solvent than those with atoms that are more electrically neutral. Of some importance in this study is the finding that conformation is also dependent on solvation and that change in geometry must be considered if an accurate estimate is to be made of energy differences such as those examined in the calculations of merostabilization.
Coherence in disordered condensed matter. III: H+ and OH− ionic conductance and proton transfer reactions in aqueous solutions
Authors: Chatzidimitriou‐Dreismann C. A. , Brändas E. J. 
Published: January 2005

AbstractThe physical context of the high ionic conductivity of the H+ and OH− ions in aqueous solutions is considered. Quantum dynamical aspects of the mobility of these ions are investigated, especially within the framework of the complex scaling method (CSM). It is argued that the high ionic mobilities under consideration are due to quantum delocalization of the ions as well as mass interference of the H+ ions with water protons. This effect is formally described with the aid of the coherent‐dissipative structures, which become spontaneously created in the solution owing to the thermal motion. In particular, (1) a novel connection between the proton transfer rate (as measured by NMR) and the mobilities of the considered ions (as determined by conductivity measurements) is derived. Additionally, (2) a relation connecting the activation energies of the above reaction rates with the ionic mobilities is found. The above relations contain no fitting parameters. Comparisons with experimental results is made, which also reveals the predictive power of the present CSM treatment in the field of microdynamical processes in condensed phases.
Theoretical study of transitions involving three rydberg series in two‐valence electron systems: MgI and its electronic sequence
Authors: Martin Inmaculada , Barrientos Carmen , Gutiez Inmaculada 
Published: January 2005

AbstractOscillator strengths for transitions involving levels of three spectral series, 3s2 1S → 3snp1P0, 3s3p 1P0 → 3snd 1D, and 3s3p 1P0 → 3sns 1S, have been computed and analyzed for several ions of the MgI isoelectronic sequence (MgI‐TiXI). The quantum defect orbital (QDO) formalism has been used throughout, and the results are comparable to those of more complex theoretical procedures.
Quasiparticle band structure and exciton spectrum of hexagonal boron nitride using second‐order Møller‐Plesset many‐body perturbation theory
Authors: Vračko M. , Liegener C.‐M, , Ladik J. 
Published: January 2005

AbstractMany‐body perturbation theory up to second‐order in the Møller‐Plesset partitioning has been used to calculate the quasiparticle band structure of hexagonal boron nitride, treated as a periodic two‐dimensional system. It was found that correlation leads to an essential narrowing of the fundamental gap. The exciton spectrum has been calculated using the first order irreducible vertex part with Hartree‐Fock and quasiparticle band structure data. It was found that the exciton binding energy amounts to about 10% of the fundamental gap (for the first singlet excitation).
From PPP‐MO theory to all‐valence electron calculations of ionic and excited states in organic molecules
Authors: Montero Luis A. , Alfonso Liz , Alvarez J. Raul , Perez Eduardo 
Published: January 2005

AbstractPPP MO theory initiated in 1953 a reliable SCF approach to excited states in multielectronic systems. Further developments, such as Del Bene‐Jaffé's CNDO/S method, modified Pople's CNDO all‐valence electron (AVE) approach to improve the results but maintained some basic troubles of this theory which were not in the PPP method. A new approach to SCF‐MO‐NDO calculations with a priori chosen parameters and configuration interaction of singly excited states is developed in this paper from the ground considerations of the PPP method and it is called CNDOL. Thus, AVE NDO approximations have been modified in order to take into account independent monocentric terms in the Fock matrix elements for each kind of azimuthal quantum number atomic orbitals in the basis set. The only input data are Slater exponents, valence state ionization potentials, and electron affinities of each kind of atomic orbital. No parameters are empirically adjusted to fit results to experimental or previous accurate theoretical data. CNDOL is suitable for obtaining theoretically consistent results in polyatomic systems which are too large to be treated by more sophisticated procedures. Applications to hydrocarbons are shown which compare PPP results with the new AVE approach derived from it here.
Extrapolation of finite cluster and crystal‐orbital calculations on trans‐polyacetylene
Authors: Weniger E. J. , Liegener C.‐M. 
Published: January 2005

AbstractSequences of total energies of trans‐polyacetylene were obtained by either finite cluster or crystal‐orbital calculations. Several different extrapolation methods were employed to improve the convergence of these two sequences. The properties of the extrapolation algorithms are discussed with emphasis on efficiency and reliability for short strings of physical input data. Four decimal digits were gained by extrapolation in the case of the finite cluster energies, but, at most, 2 decimal digits in the case of the crystal‐orbital energies. The extrapolation of the finite cluster energies gave results that are almost as good as the results produced by the extrapolation of the crystal‐orbital energies, and that are better than the unextrapolated crystal‐orbital energies.
The TST–CEQ calculations on the heavy‐light‐heavy Cl + HCl reaction
Authors: Ju Guan‐Zhi , Chen De‐Zhan 
Published: January 2005

AbstractIn this paper, the TST–CEQ calculated results including the reactive cross sections and rate constants for the Cl + HCl reaction at the specified vibrational state of the HCl are presented. It is shown that the oscillatory behavior of the system in the three‐dimensional world does not appear, and the larger the vibrational quantum number of HCl, the smaller the cross sections ¯QTST–CEQ (E, ν) are at a given total energy used in the calculations, but the larger the reactive rate, constants KTST–CEQ (T, ν) are at a given temperature. The KTST–CEQ (T, v) is the same as when first published, and the average reactive rate constants KTST–CEQ (T) are close to the ones by Garrett and Schatz et al.
Theoretical description of nuclear quadrupole coupling in light diatomic molecules
Authors: Vojtík J. , Paidarová I. , Špirko V. , Śavrda J. , Petráš M. 
Published: January 2005

AbstractA practical procedure for calculation of nuclear quadrupole coupling constants for light diatomic molecules is discussed. The procedure is based on a molecular wave function that explicitly describes nuclear motion. The approach is capable of yielding quadrupole coupling constants for excited rovibrational levels of diatomic molecules in their ground and excited electronic states. An application of the procedure to the X1Σ+g and B1Σ+u states of HD and D2 is presented.
Squeezing transformation caused by frequency jumps for solving the problem of two coupled oscillators
Authors: Fan Hong‐Yi 
Published: January 2005

AbstractFor the Hamiltonian
equation image
a unitary operator that is closely related to the squeezing transformation caused by frequency jumps is found and its coordinate representation provides an alternative approach for calculating the wave function of the eigenstate of ℋ︁, whereas its normally ordered form is convenient for deriving the resolvent kernel in a coherent‐state basis.