bluuebunny/crossref_metadata_2025_split huggingface dataset:
Variational constraints on perturbed eigenvalues and their perturbation expansions. II. Perturbational inequalities
Authors: Silverman Jeremiah N. 
Published: December 2004

AbstractA large family of interlocking perturbational inequalities is derived by variational considerations for the stationary states of all systems described by a Hamiltonian linear in a real perturbing parameter λ. These inequalities constrain in many different ways the perturbation expansions of both exact and variational eigenvalues for these systems; analogous inequalities are derived for the components of the eigenvalues. A special feature of the analysis consists of obtaining inequalities applicable to the separate sums of even‐ and odd‐order perturbation energies. For lowest states of each symmetry and for positive definite perturbing operator, the interlocking effect of the inequalities becomes extremely restrictive. The inequalities are illustrated with several numerical calculations for different systems and states of the helium isoelectronic sequence. The direction of the inequalities is found to be unaffected by low‐order truncation, thus rendering them applicable to low‐order perturbation expansions. The inequalities are used to study the efficacy of low‐order perturbation theory for two‐ to ten‐electron atomic isoelectronic sequences, and to determine the functional dependence upon λ of the eigenvalues and their components for arbitrary atomic isoelectronic sequences.
Axial gaussian‐lobe orbitals: Optimization of highly angularly dependent functions
Authors: Le Rouzo H. 
Published: December 2004

AbstractHighly angularly dependent axial Gaussian‐lobe orbitals (AGLO) (up to L = 5) are presented. The angular and radial optimizations of these functions have been realized on the ground of theoretical frameworks, previously reported in the literature and somewhat extended here. The numerical difficulties that can appear in the recombination of elementary integrals over lobes are particularly investigated. It is shown that it is necessary to limit the angular accuracy, i.e., the relevant YLo character, in order to preserve the accuracy of atomic integrals. The proposed p, d, f, g, and h AGLOS satisfy this condition, and can be used with confidence in LCAO–MO–SCF calculations. Their advantages, e.g., for the treatment of large symmetrical inorganic systems containing transition metal atoms, are emphasized.
Molecular properties and basis set quality: An approach based on information theory
Authors: Maroulis G. , Sana M. , Leroy G. 
Published: December 2004

AbstractInformation theory can be used to evaluate the quality of a wave function by considering its ability to give values for some observables as close as possible to the experimental ones. A new method for improving the quality of a wave function is proposed. This paper deals exclusively with the HF (X 1∑+) molecule.
Combination of pseudopotentials and density functionals
Authors: Preuss H. , Stoll H. , Wedig U. , Krüger Th. 
Published: December 2004

AbstractSemilocal pseudopotentials have been determined for first–row (Li to Ne), second row (Na to Ar), and third‐row atoms (K, Ca). Core–valence correlation is included by adjusting the pseudopotentials to experimental energies of ions with a single valence electron. Correlation within the valence shell is taken into account by using the spin–density functional formalism. The approximations involved in this approach are tested for atomic ionization energies as well as binding energies of monohydrides and alkali diatomics, agreement with experiment is usually satisfactory, but in certain applications density functionals should be already included in the fitting of the local part of the pseudopotential. In addition, 3s/3p and 3s/2p basis sets (for first and second row, respectively), designed for use in connection with our pseudopotentials, are given; it is shown that they yield reasonable results for both SCF and correlation energies.
Unitary transformations with the unitary operators depending on projection operators
Authors: Stepanov N. F. 
Published: December 2004

AbstractUnitary transformations of operators and variables are considered for those cases when a Hermitian operator in the expression U = exp (iS) explicitly depends on the projection operators. Some simple examples of such transformations are given and the equations obtained which define projectors (among special sets of them), thus bringing forth better estimations for upper and lower bounds. The unitary transformations are also applied to the reduced resolvent operator and equations are presented for the operators U that minimize the energy functional.
Multichannel quantization and the static stark effect
Authors: Atabek O. , Lefebvre R. 
Published: December 2004

AbstractA coupled channel formulation for the static Stark effect on the hydrogen atom is used to obtain directly the complex resonance energies through an iterative matching procedure. The formulation makes use of a complex coordinate. This solves the initialization problem in a very simple way, since the boundary conditions can be those used for bound states. A comparison is made with the results of Reinhardt and collaborators who expand the complex rotated resonance wave function in a basis of integrable functions.
Algebraic characterization of bridged polycyclic compounds
Authors: Mekenyan Ov. , Bonchev D. , Trinajstić N. 
Published: December 2004

AbstractAn approach based on the topological distance matrix is used for algebraic characterization of bridged polycyclic compounds. The classical bridged structures which have external bridges between cycles were examined together with the more complicated three‐dimensional polycyclic systems regarded as containing internal bridges. Thirteen rules are given for characterizing the main types of structural rearrangements in these compounds. The important topological characteristic of (poly)cyclic systems, the molecular cyclicity, is examined in the polycyclic condensed, spiro‐ and bridged structures, respectively.
Effect of molecular size on carbonic anhydrase inhibition by sulfonamides
Authors: Kumar Kamal , Bindal Mahesh C. , Singh Prithvi , Gupta Satya P. 
Published: December 2004

AbstractThe molecular size is found to play an important role in carbonic anhydrase (CA) inhibition by sulfonamides. Significant correlation is obtained between the inhibitory power of meta‐substituted analogs and the van der Waals volume (Vw) of the substitutents. This provides a theoretical basis to map the active sites in enzyme and find the nature of sulfonamide‐receptor binding. It is inferred that in addition to well known binding of sulfamyl group with Zn2+ ion of the enzyme, the meta substituent of sulfonamides interacts with some secondary binding site and this interaction is argued to be of van der Waals type.
Specificity and molecular mechanism of abortificient action of prostaglandins
Authors: Kothekar V. 
Published: December 2004

AbstractThe paper reports comparison of electrostatic charge and energy distribution on the basis of the CNDO/2 method for six forms of prostaglandins–PGF2α, PGF1β, PGE1, PGE2, PGB1, and PGA1–having diverse physiological action. The isopotential mapping done in three dimension showed that the lower value of electrostatic potential and proximity of the two low energy regions around O9 and O11 in PGE2 and PGF2α is probably responsible for their higher abortificient activity. We also compare here the variation of the long‐ and short‐range interaction between ring–chain and chain–chain portion of different forms and compared them with the variation in their action.
Ab initioSCF study of guanidine and substituted guanidines
Authors: Sapse Anne Marie , Snyder Grace , Santoro Angelo V. 
Published: December 2004

AbstractProton affinities, geometries with optimized parameters, and net atomic charges are reported for guanidine‐, methyl‐, amino‐, and fluoro‐substituted guanidines. The results are obtained using the ab initio SCF method as implemented by the GAUSSIAN‐70 computer program with a 6–31G basis set. Basicity is discussed in terms of electron distribution and intramolecular attraction forces.
Proton transference in hydrogen bonded systems. I. Uracil tautomerism
Authors: Mondragón A. , Blake I. Ortega 
Published: December 2004

AbstractWe have made a detailed analysis of the adequacy of different theoretical approaches to the study of uracil tautomerism. We have considered the effect of molecular relaxation and the ability of several methods to reproduce it. We have also considered their performance in predicting the tautomerization energy barrier. We found that semiempirical methods are not adequate for predicting the energy value but that they can be used for obtaining geometry optimizations. We also found a strong discrepancy between the energy values predicted by a minimal basis set and a 2ζ basis set, the last one being 24.9 kcal/mol. The implications of these findings are discussed and the results compared with experimental evidence.
Level set topology of the nuclear charge space and the electronic energy functional
Authors: Mezey Paul G. 
Published: December 2004

AbstractElectronic energy level set topologies in the abstract nuclear charge space Z of molecular systems are defined and analyzed. Two theorems, one on the general convexity of level sets in Z, another on homotopies of boundaries of level sets, induced by nuclear geometry variations in the nuclear configuration space R, are proven. The applications of the two theorems are illustrated by examples of various molecules and ions.
“Stable” quasistationary state: New solution of the time‐dependent Schrödinger equation
Authors: Tachibana Akitomo 
Published: December 2004

AbstractThe new concept of the stability of the quantum mechanical state is introduced for the study of the time‐dependent process of the quasistationary state. A stable solution of the time‐dependent Schrodinger equation predicts the presence of the new sharply localized long‐living state at the most repulsive part of the effective potential.
Conformational analysis by bond orbitals with delocalization corrections: Rotation of the ser‐195 side chain in α‐chymotrypsin
Authors: Surján Peter R. , Náray‐Szabó Gábor , Mayer István 
Published: December 2004

AbstractA recently developed procedure for calculation of tails of localized molecular orbitals is applied in order to obtain approximate rotational potential curves. In order to calculate reliable rotational barriers, tails, originating from direct interactions between bonding and antibonding strictly localized orthogonal orbitals, have to be determined. This finding permits us to develop a fast approximate SCF semiempirical procedure offering a reliable tool for studying conformational problems in rather large (bio)molecules. As a first example, a model of the active site of α‐chymotrypsin was examined. According to our calculations, a hydrogen bond between Ser‐195 and His‐57 should exist.
Calculation of diamagnetic susceptibilities from the Thomas–Fermi–Amaldi theory for ions
Authors: Donnamaria Maria C. , Castro Eduardo A. , Fernández Francisco M. 
Published: December 2004

AbstractThe importance of the Fermi–Amaldi correction for the Thomas–Fermi theory for ions is analyzed. An analytical trial electron density with a proper limiting behavior has been chosen and the Thomas–Fermi–Amaldi energy expression is minimized in a variational way for several closed‐shell ions. The obtained optimum electron densities have been used for calculating the diamagnetic susceptibilities, and a comparison with other theoretical results and available experimental data is performed.
SCF and MC–SCF Study of CO2 with bond angle variation
Authors: Combs Leon L. , Lunell Sten 
Published: December 2004

AbstractMulticonfiguration (MC) SCF calculations are reported for CO2 for bond angles between 60° and 180°. The ground state configuration is found to be …︁5a4b∣b∣a for small bending angles and …︁6a3b∣b∣a for large bending angles, the change in ground state character occurring at a bond angle of about 100°. The force constant for bending obtained from the MC–SCF function is about 8.0% lower than the corresponding SCF value, and in considerably better agreement with experiment.
Molecular geometry of benzosemiquinone radicals: Radicals from resorcinol and its derivatives
Authors: Shinagawa Yasuko , Koyama Seiji , Shinagawa Yoshiya 
Published: December 2004

AbstractThe observed proton hyperfine coupling constants of radicals from resorcinol and 2‐carboxylic, 5‐carboxylic, and 5‐methyl resorcinols in acidic and alkaline solution were reproduced by means of the INDO (intermediate neglect of differential overlap) with molecular geometry adjusting method. The bond lengths of CO were longer in acidic solution than in alkaline solution and the total energies were lower in acidic solution than in alkaline solution. The assignments of the coupling constants of the radicals from resorcinols were |A2| > |A5| in alkaline oxidation and |A2| < |A5| in acidic oxidation.
Theoretical determination of the X2Σ+ and A2Π potentials of CsO using relativistic effective core potentials
Authors: Laskowski Bernard C. , Langhoff Stephen R. , Siegbahn Per E. M. 
Published: December 2004

AbstractTheoretical potential energy curves are computed for the X 2Σ+ and A 2Π states of CsO using a relativistic effective core potential and a large valence Gaussian basis set. Seventeen electrons are correlated by a CI(SD) calculation from each HF reference. We find the X 2Σ+ state lower by 497 and 726 cm−1 at the HF and CI(SD) levels. Our calculated ωe of 312 cm−1 for the X 2Σ+ state agrees well with experimental values deduced from studies in matrices.
Potential energy surface determinations for nonrigid molecules: Application to acetone
Authors: Smeyers Yves G. , Bellido M. Nieves 
Published: December 2004

AbstractIn the present work, the problem of the determination of the potential energy surface for nonrigid molecules is examined in the case of the double rotation of the methyl groups in acetone. From the symmetry adapted functional form for the potential, the minimum number of configurations to be calculated is deduced in order to have a reliable surface. With this consideration in mind, the potential energy surface of acetone is determined in some Hartree–Fock semiempirical (CNDO/2) and ab initio procedures with different standard basis sets. In addition, ab initio calculations are performed using different sets of floating Gaussian functions in order to introduce some polarization effects in the wave function. Finally, the influence of the electronic correlation effects in the barrier height, and the role of the possible relaxation of the structure during the rotation is discussed.
Avoided crossing of molecular excited states and photochemistry: Butadiene and unprotonated Schiff base
Authors: Bonačić‐Koutecký V. , Persico M. 
Published: December 2004

AbstractConsequences of the twisting motion around the CC bond of butadiene and around CC, CN, and CC bonds of the small unprotonated Schiff base (allylideneimine) on low‐lying singlet and triplet states have been investigated employing large scale CI treatments. Characterization of the important features of the electronic wave functions in terms of VB‐like ionic and covalent contributions in different twist intervals has been carried out. Importance of the zwitterionic singlet states with large charge separation in two different parts of molecule attached to the relaxed bond versus low‐lying covalent excited state has been discussed. Photochemical implications of different minima on the energy hypersurfaces of the excited states with different features of electronic wave functions have been proposed.
Water chain intervention in hydrolytic and tautomeric reactions
Authors: Bertrán J. , Lledós A. , Revetllat J. A. 
Published: December 2004

AbstractWater chain intervention in the hydrolysis of methyl fluoride and in some tautomeric 1,3 proton transfer reactions has been studied. The results show that in solution the process are neither unimolecular nor bimolecular as in the gas phase, but are multimolecular, where the geometric parameters of the solvent intervene in the reaction coordinate.
Electrostatic and relaxation effects in ionization of molecular dimers and intermolecular complexes
Authors: Peel J. Barrie 
Published: December 2004

AbstractThe shifts in ionization energies which occur when a molecule is incorporated as an asymmetric dimer or in an intermolecular complex are analyzed theoretically. MOSCF calculations with 4–31G basis sets were performed on closed‐ and open‐shell states of (HF)2, H2O·HF, and their valence–hole ions, as well as on the heterodimers incorporating the higher homologues CH3F, CH3OH, and (CH3)2O. The analysis concerns the influence of electrostatic, polarization, and charge transfer effects associated with complexation on the initial molecular state of each monomer system, as well as monomer–dimer differences in the electronic relaxation mechanism considered as a final state effect in the ionization process. The calculated ionization energy shifts which agree well with the experimental data available for (CH3)2O·HF, show that the shifts are dominated by electrostatic effects, but some effects arising from differences in molecular size and electric polarizability of the monomers can be discerned.
Time‐dependent Schrödinger equations: Symmetry breaking, separation of variables, and nonlinear effects
Authors: Truax D. Rodney 
Published: December 2004

AbstractNonlinear equations are important for the description of dissipative processes and soliton‐related phenomena. One of the principal aims of the theory of symmetry breaking in quantum mechanics is to provide a systematic method for introducing interactions which reduce the symmetry of a system. Such a method is applied here to compute the general form of the symmetry‐breaking interaction F(x,t, Ψ, Ψ*) for the time‐dependent Schrödinger equation in one spatial dimension

For F = 0, it has been shown that the Lie algebra of space–time invariances of this equation is S1 = s1 (2,IR)□w1, the Schrödinger algebra. Following the method of Boyer, Sharp, and Winternitz, all conjugacy classes of subalgebras of S1 are given. For each subalgebra, the most general form of the interaction term F(x,t, Ψ, ψ*) is constructed. The potential F then reduces the symmetry from S1 to the considered subalgebra. Furthermore, the one‐dimensional subalgebras of S1 obtained above partition S1 into orbits of operators. To each orbit there corresponds a coordinate system in which the above equation separates variables. A partial resolution of the solutions of the above equation has been obtained by exploiting this relation. The algebraic approach to symmetry breaking yields a rich variety of interaction terms F, which appear to generalize the nonlinear Schrödinger–Langevin–Kostin equation for nonconservative systems and the so‐called nonlinear Schrödinger equation. A criterion in terms of the existence or nonexistence of Bäcklund transformations is conjectured in order to distinguish between dissipative and soliton equations.
Comments on the Bose condensation of phonons in the biological systems
Authors: Chatterjee R. , Tuszyński J. A. , Paul R. 
Published: December 2004

AbstractFollowing the conjecture of Fröhlich that the Bose condensation of phonons is the dominant biological effect, Wu and Austin derived from the Fröhlich Hamiltonian an expression for the relaxation time which is inversely proportional to the temperature by using the technique of Green's function. In this paper, we have generalized the Wu and Austin expression by improving their approximations with the superconducting canonical transformation and obtained the relaxation time depending exponentially on the inverse temperature. Moreover, we have checked our expression by using a different method based on the “continued fraction expansion” and also employed this technique to derive the dielectric constant for this system.
Ab initio study of proton transfers including effects of electron correlation
Authors: Scheiner Steve , Szczȩlśniak Malgorzata M. , Bigham Larry D. 
Published: December 2004

AbstractProton transfers in a number of systems are investigated using ab initio molecular orbital methods. Calculations are carried out with several different basis sets ranging in size from 4–31G to 6–311G**. Electron correlation is included using Møller–Plesset (MP) perturbation theory to second and third orders. Enlargements of the basis set invariably lead to higher energy barriers to proton transfer, while substantial reductions result from inclusion of correlation effects. Application to (HOHOH)− of third‐order MP theory with a triple‐valence basis set augmented by polarization functions on oxygens and the central proton, denoted MP3/6–311G*(*), leads to excellent agreement with the results of Roos et al. whose calculations involved an extensive CI treatment with a large basis set. For equivalent hydrogen bond lengths, the transfer barrier in the cation (H2OHOH2)+ is nearly identical to that for the (HOHOH)− anion while the barrier in (H3NHNH3)+ is somewhat smaller. The reduction of the SCF barrier height resulting from inclusion of correlation is greater for (O2H3)− than for the above cations. The lowest energy structure of (O2H5)+ contains a symmetric hydrogen bond in which the proton is located midway between the two oxygens whereas asymmetric H bonds are found in the equilibrium geometries of (N2H7)+ and (S2H5)+. The difference in energy between the symmetric and asymmetric configurations of (O2H3)− is extremely small.
Energetics and electronic rearrangements of proton transfer in (H3NHOH2)+
Authors: Scheiner Steve 
Published: December 2004

AbstractProton transfer between N and O in the hydrogen‐bonded system (H3NHOH2)+ is studied by ab initio molecular orbital methods. Potential energy curves are calculated at the hartree–Fock level using the 4–31G basis set for hydrogen bond lengths R(NO) varying from the equilibrium value of 2.664 to 3.10 Å. Short hydrogen bonds are associated with asymmetric single‐well potentials in which the minimum corresponds to the NHO configuration. For longer R(NO) separations, the potential is of double‐well form, including both NHO and NHO as minima. It is found that the height of the energy barrier to proton transfer is sensitive to both stretches and bends of the hydrogen bond. Continuous changes in the electron density are monitored at various stages of proton transfer via density difference maps and Mulliken population analyses. The initial loss of density from the proton‐accepting molecule during the first half of the transfer is accelerated during the second half. A correlation is drawn between the energetics of transfer in a number of systems and the net charge lost by the proton‐acceptor group.
Integral evaluation algorithms and their implementation
Authors: Hegarty Dermot , Van Der Velde G. 
Published: December 2004

AbstractThree different algorithms for the calculation of many center electron‐repulsion integrals are discussed, all of which are considered to be economic in terms of the number of arithmetic operations. The common features of the algorithms are as follows: Cartesian Gaussian functions are used, integrals are calculated by blocks (a block being defined as the set of integrals obtainable from four given exponents on four given centers), and functions may be adopted to R(3). Adaption to molecular point group symmetry is not considered. Tables are given showing the minimum number of operations for a selection of block types allowing one to identify the theoretically most economic, and the corresponding salient features. Comments concerning the computer implementations are also given both on sealar and vector processors. In particular, the Cyber 205 is considered, a vector processor on which we have implemented what we believe to be the most efficient algorithm.