bluuebunny/crossref_metadata_2025_split huggingface dataset:
Orthonormalization, polar decomposition, and transformation to an effective hamiltonian
Authors: Jørgensen Flemming 
Published: December 2004

AbstractKlein has pointed out that des Cloizeaux's orthonormalization is just another formulation of Löwdin's symmetric orthonormalization. We demonstrate that des Cloizeaux's formulation is convenient for theoretical discussions and that it is intimately connected with polar decomposition–‐which is a generalization to operators of the fact that any complex number z can be written r exp (iθ) where r is positive and θ real. We generalize two other properties for z and find an interpretation of the Carlson‐Keller theorem. Recently Lathouwers found that Löwdin's canonical orthonormalization leads to the eigenvectors of a positive operator. This operator is des Cloizeaux's. We discuss some variational theorems on this background and find the Courant‐Hilbert‐Löwdin “measure of linear independence” from a simple least squares consideration, closely related to Lathouwers' results.An orthonormalization due to Schweinler and Wigner (1970) is observed to be Löwdin's canonical. The Schweinler‐Wigner maximum characterization is referred to a simple general theorem, which also implies a new maximum characterization.In Sections 6 and 7 we consider transformation to an effective Hamiltonian from the point of view of polar decomposition and symmetric orthonormalization. It turns out that the polar part of Bloch's transformation is des Cloizeaux's. The characterizations of this by both Klein and Jørgensen are visualized in a simple way.
Molecular orbital study of the electronic absorption spectra of dithienyls
Authors: Abu‐eittah R. , Al‐sageir F. 
Published: December 2004

AbstractThe localized orbital method has been used in an SCF study of the electronic absorption spectra of the three isomers of dithienyl. Transition energies are computed for planar, perpendicular, and polyconfigurational descriptions of the molecule. Band intensities are calculated for the planar conformers. The results show that: (1) the coplanarity of the two thiophene rings in dithienyl is not significantly distorted; (2) excited states of dithienyls arise from substantial configuration interaction; and (3) the charge transfer wave function contributes significantly to the excited state.
Spin‐Dependent operators in the spin‐free quantum chemistry
Authors: Mestechkin M. M. , Klimko G. T. 
Published: December 2004

AbstractThe two‐particle spatial density matrix components introduced by McWeeny are expressed in terms of the Fock coordinate wave function, which is constructed from an arbitrary function of N spatial coordinates. The integral relations for these components are verified. The necessary matrix elements of a standard representation of the SN group are calculated.
Absorption line shape of two coupled oscillators decaying in the same bath
Authors: Hardisson A. , Mauricio F. , Velasco S. 
Published: December 2004

AbstractIn this paper the linear response theory approximation is used to calculate the electromagnetic energy absorption coefficient of two coupled oscillators decaying in the same medium. The model seems to be suitable for the treatment of the infrared spectroscopy of certain isolated molecules trapped in a rare gas crystal.
Some potential‐energy relationships for isoelectronic atomic series
Authors: Politzer Peter , Daiker Kenneth C. 
Published: December 2004

AbstractRelationships between the total energy E, the nuclear‐electronic attraction energy Vne, and the interelectronic repulsion energy Vee are derived for isoelectronic series of atoms and ions. Using these, it is possible to determine Vne and Vee for the members of such series from a knowledge of just their total energies. The method is applied to the ground and first excited states of the two‐electron series, and the ground states of the three‐ and four‐electron series, and the results are discussed.
Symmetry forcing and convergence properties of perturbation expansions for molecular interaction energies
Authors: Jeziorski Bogumił , Szalewicz Krzysztof , Chałasiński Grzegorz 
Published: December 2004

AbstractThe convergence properties of perturbation theories for molecular interaction energies are tested by performing high‐accuracy high‐order numerical calculations for a ground‐state hydrogen atom interacting with a proton. It is shown that a strong symmetry forcing used in the Eisenschitz‐London‐Hirschfelder‐van der Avoird (EL‐HAV) theory leads to rapidly convergent perturbation expansion whereas a weak symmetry forcing, peculiar to the Murrell‐Shaw‐Musher‐Amos (MSMA) theory, is not able to guarantee the convergence of the resulting perturbation series. The perturbation expansion introduced recently by Jeziorski and Kolos and corresponding to an intermediate symmetry forcing is shown to converge rapidly ensuring the correct asymptotic behavior of the interaction energy calculated through second order. Despite the divergence of the resulting perturbation series the MSMA theory is shown to give very useful results at the distances corresponding to the van der Waals minimum. In this region, however, virtually the same results can be obtained by using a simpler theory employing a properly symmetrized wave function of the usual Rayleigh‐Schrödinger (RS) polarization theory.
The BERKELEY system. III. General configuration‐interaction methods for open‐shell molecular electronic states
Authors: Brooks Bernard R. , Schaefer Henry F. 
Published: December 2004

AbstractA system of general “open‐ended” configuration‐interaction (CI) programs, specifically designed for the Harris Corporation Slash Four minicomputer, is described. These methods are general in the sense that an arbitrary list of configurations (linear combinations of Slater determinants) may be used, and open ended in that peripheral (i.e., disk) storage capacity determines the maximum size problem that can be solved. The largest variational calculations carried out to date using BERKELEY involve 7064 open‐shell singlet configurations (31,898 Slater determinants). Detailed timing breakdowns are presented for four test cases, two of which involve the lowest ππ* singlet state of ethylene. The other two examples are the orthogonal or bisected singlet state of trimethylenemethane and the 8B1 state of the MnCH2 complex. In the latter case, it is found that the predicted MnCH2 dissociation energy is only slightly increased by electron correlation effects.
Classical dynamics of the O + ClO → Cl + O2 and Cl + O3 → ClO + O2 reactions
Authors: Farantos S. C. , Murrell J. N. 
Published: December 2004

AbstractAn explicit function has been derived for the potential‐energy surface of the ground state of ClO3 with the six interatomic distances as variables. This surface is valid over all configurations of the atoms. The surface has been used to calculate classical trajectories for the reactions R1: O(3P2)+ClO(2Π3/2)→ O2(3∑)+Cl(2)P3/2 and R2: Cl(2P3/2)+O3(1A1)→ClO(2Π3/2)+O2(3∑). An appreciable fraction (∼1/3) of the reactive trajectories for R1 go through a long‐lived complex ClOO(2A″). The cross section decreases with increasing rotational state of the ClO; and 37% of the energy release is vibrational. The calculated rate constant at 298°K is 2.6 × 10−11 cm3/molecule sec. For reaction R2 there is no evidence of long‐lived complexes. The product ClO is predominantly found in the backward‐scattering direction. Most of the internal energy is carries off by ClO but O2 carried off most translational energy. An Arrhenius expression has been deduced from calculations at 220 and 300°K to give an A factor of 2.488 × 10−11 cm3/molecule sec and an activation energy of 1.543 kJ/mol.
Complex‐coordinate studies of helium autoionizing resonances
Authors: Moiseyev N. , Certain P. R. , Weinhold F. 
Published: December 2004

AbstractSeveral aspects of the numerical application of the complex‐coordinate method for calculating helium autoionizing resonances are discussed. A direct‐search algorithm is used to locate resonance eigensolutions that satisfy the virial theorem, and the influence of the basis set on the accuracy of the calculated positions and widths is investigated. The complex‐coordinate method is related to the stabilization method of Holøien and Midtdal.
Interpretation of electron spectra. III. Spectra of formamide, studied with HAM/3
Authors: Lindholm E. , Bieri G. , Åsbrink L. , Fridh C. 
Published: December 2004

AbstractThe molecular spectra of formamide (HCONH2) are interpreted by the use of the new semiempirical method HAM/3. Very good agreement with experiments can be obtained for the calculation of all valence ionization energies as well as for the (negative) electron affinity. In addition, the calculated excitation energies allow an unambiguous interpretation of the UV spectrum, for which previously different ordering of the triplet 3ππ* and 3nπ* transitions have been suggested.
Ab initio molecular orbital studies of sigmatropic rearrangements
Authors: Bouma Willem J. , Vincent Mark A. , Radom Leo 
Published: December 2004

AbstractAb initio molecular‐orbital theory with the STO‐3G and 4‐31 G basis sets has been used to study the 1,3‐sigmatropic hydrogen rearrangements: propene→propene, formic acid→formic acid, and vinyl alcohol→acetaldehyde, and the1,5‐shifts:1,3‐pentadiene→ 1,3‐pentadiene and β‐hydroxyacrolein→ β‐hydroxyacrolein. Transition states have been determined using gradient procedures. Improved descriptions of the energies of the reactions have been obtained using 3 × 3 configuration interaction. In accord with expectations based on orbital‐symmetry considerations, the calculated barriers are considerably greater for 1,3‐ than for 1,5‐shifts. The forbidden pathway for the degenerate 1,3‐shift in propene is predicted to require less activation energy than the allowed pathway, a result that can be rationalized in terms of interactions with subjacent and superjacent orbitals.
First‐order properties and the Hellmann–Feynman theorem in the case of a limited CI wave function
Authors: Nerbrant P.‐O. , Roos B. , Sadlej A. J. 
Published: December 2004

AbstractTwo distinct approaches to the calculation of first‐order properties with a limited CI wave function are discussed. One is based on the Hellmann–Feynman theorem and the other on the direct evaluation of the total energy derivative at zero perturbation. Corrections to the Hellmann–Feynman expectation value are given for the CI wave function consisting of a single determinant reference state and all single and double replacements of this. These corrections are the extended Brillouin matrix elements and involve interactions between the zeroth‐order wave function and triply substituted configurations. The usefulness of these matrix elements for the generation of MC SCF orbitals and for the calculation of cluster corrections to the wave function is briefly discussed. The formulas for the Brillouin matrix elements expressed in terms of one‐ and two‐electron integrals have been automatically generated using the syntax of the algebraic program SCHOONSCHIP.
Expansion and completeness theorems for operator manifolds
Authors: Dalgaard Esper 
Published: December 2004

AbstractSome expansion and completeness theorems for operator manifolds, which are currently being employed in propagator theory, are derived. It is shown that excitation or ionization operators satisfying the conditions Q|0〉 = |Λ〉 and QΛ|0〉 = 0 for general excited states |Λ〉 and reference state |0〉 may be expanded uniquely in particular sets of basis operators. These results are then used to discuss rigorous expressions for fermion propagators.
Electron and nuclear density matrices and the separation of electronic and nuclear motion
Authors: Goscinski O. , Palma A. 
Published: December 2004

AbstractThe so‐called parametric dependency of the electronic wave function in the Born–Oppenheimer approximation is discussed. Considering a function beyond this approximation, a new set of equations is derived with parametric and nonparametric dependency. Nuclear and electron reduced density matrices are introduced in this derivation. They are used in a formulation of the problem of separation of nuclear and electronic motion.
Solitons, bioenergetics, and the mechanism of muscle contraction
Authors: Davydov A. S. 
Published: December 2004

AbstractThe properties of exciton and soliton excitations in one‐dimensional molecular systems and in α‐helical protein molecules are investigated. It is shown that collective excitations—solitons, corresponding to a combination of vibrational excitations in peptide groups and a local deformation of molecules—are possible in α‐helical protein molecules. These excitations move along the molecule without energy losses and are perfect energy carriers. A qualitative description of the shortening in the length of muscular fibers is given using the concept of solitons occurring under the hydrolysis of ATP molecules at the ends of thick fibers contained in the sarcomeres of muscular fibers.
Organization of phospholipids in biological membranes
Authors: Govil Girjesh , Hosur Ramakrishna V. 
Published: December 2004

AbstractPreferred conformations of phospholipids have been predicted through quantum‐chemical techniques and classical potential functions. An essential condition for a conformation to exist in a biomembrane is that it should be possible for it to organize in the form of a bilayer. Taking into consideration the conformational flexibility of the polar head group, organization at the lipid–water interface has been considered. The biological implications of such an organization in terms of formation of “hydrophobic channels” is discussed. Quantum‐mechanical investigations on the transport phenomenon have shown that the “selectivity” of biological membranes is connected with the “organization.” Calculations of the quantum‐mechanical transmission coefficients for different model potential profiles indicate that minor differences in the height of potential barriers in certain regions can lead to significant changes in transmission coefficients. The “directional selectivity” of substrates (differences in transmission coefficients for flow in and out of the cell) can be explained on the basis of differences in membrane organization. These results have some important consequences in the evolutionary process in biological membranes.
Ab initio molecular fragment calculations with pseudopotentials
Authors: Gáspár R. , Gáspár R. 
Published: December 2004

AbstractPseudopotential theory is introduced into the ab initio FSGO molecular fragment method. Pseudopotential molecular fragments using pseudopotentials of double‐zeta quality are characterized, and a method of their assembly into larger molecules is presented. Core–valence electron separation is achieved at both levels of the molecular calculations. Heteroatom incorporation into the method is also considered. Applications of the pseudo‐FSGO molecular fragment method to hydrocarbons and simple molecular systems containing heteroatoms are discussed. Results are compared to those of the original FSGO method and experiment.
Role of electron correlation in the quantum‐mechanical calculations of the coulomb interaction energy in the DNA base pairs
Authors: Kuprievich V. A. , Kudritskaya Z. G. , Klymenko V. E. 
Published: December 2004

AbstractThe intermolecular electronic correlation contributions to the Coulomb component of the nucleic acid base interaction energy are estimated. The Coulomb energy is evaluated with the use of atomic monopoles, which are determined from the π‐electronic densities calculated by the SCF method and by employing partially or completely optimized APSG wave functions. When the correlation is thus taken into account, a systematic decrease in atomic charges occurs; this effect is considerable only if an optimized orbital set is used. As a result, the Coulomb interaction energy due to the π‐electronic atoms decreases from −1.13 to −0.85 kcal/mol for the AT pair and from −7.15 to −4.61 kcal/mol for the GC pair.
Multimolecular clusters: Their isomerism and effective characteristics evaluated by quantum chemistry
Authors: Slanina Z. 
Published: December 2004

AbstractA method is described for processing the enthalpy, entropy, and Gibbs free energy terms of formation of the individual isomers of n‐particle clusters obtained by means of quantum‐chemical calculations, to enable a comparison of these partial theoretical characteristics with the overall experimental ones. The general scheme of weighting treatment is illustrated by examples based on recent quantum‐chemical results of studies of isomeric forms of (NO)2, (H2O)5 and (D2O)5, and CH3OH · 3H2O and CH3OH · 6H2O clusters.
Geometry of ammonia molecule in the lowest triplet state estimated theoretically
Authors: Jeziorek Danuta , Żurawski Bronislaw 
Published: December 2004

AbstractThe geometrical parameters of the ammonia molecule in the lowest‐lying triplet state have been estimated by means of the SCF method and by making an extrapolation to take into account electron correlation effects. The planar configuration has been obtained as the most probable geometrical arrangement (3 A2″).
Electrostatic isopotential maps for large biomolecules
Authors: Náray‐Szabó G. 
Published: December 2004

AbstractA new method for the calculation of electrostatic isopotential maps, based on completely transferable bond orbitals, is proposed. According to the simple form of the potential term the amount of computational work is proportional to the first power of the number of valence electrons. Slater‐type atomic orbitals are used to construct the bond orbitals, thus all integrals can be calculated explicitly. The subtilisin charge‐relay system is studied as an example. It is found, in agreement with the results of Beppu and Yomosa, that the two protons transfer in a stepwise manner.
Theoretical studies on substrate binding to the active site of carbonic anhydrase
Authors: Sawaryn Andrzej , Andrzej Sokalski W. 
Published: December 2004

AbstractThe role of some amino acids and metal ions in the catalytic activity of carbonic anhydrase (carbonate dehydratase EC 4.2.1.1) has been investigated. The additional stabilization of the transition state complex was used as the qualitative measure of the effect of molecular surrounding on CO2 hydration reaction calculated within the approximate CNDO/2 approach. The effect of the molecular environment has been simulated by inclusion into the SCF LCAO MO Hamiltonian, a term representing the interaction with a set of point charges and atomic dipoles centered on experimentally determined position of Zn2+ ion and all atoms of histidine 94, 96, 119 and threonine 199, 200 (or histidine 200 in the case of carbonic anhydrase B). The possible molecular mechanism of CO2 hydration inside the active site has been also discussed.
Intermolecular charge transfer in model hydrogen‐bonded systems—biological implications
Authors: Chojnacki Henryk 
Published: December 2004

AbstractThe electronic structure of the model hydrogen‐bonded systems has been studied at the all‐valence level in relation to the charge transfer mechanism. It is concluded that together with the intermolecular proton transfer an electronic charge transport occurs for hydrogen bonds of 2.70–3.00 Å in length, i.e., when the proton motion within the bond is anticipated. For elucidation of transport properties of one‐dimensional hydrogen‐bonded systems the tunneling–hopping model is preferred instead of the band theory. The importance of the proposed mechanism of the charge transfer for biological processes has been emphasized.
Mechanism of protein folding
Authors: Ptitsyn O. B. , Finkelstein A. V. 
Published: December 2004

AbstractThe first stage of protein self‐organization—the formation of a fluctuating secondary structure in the unfolded protein chain—is considered. The stereochemical theory is presented enabling one to calculate helix‐coil and β‐structure‐coil equilibrium constants. It is shown that the most probable localization of fluctuating α‐ and β‐structure in the unfolded protein chain corresponds to the native localization of these structures. The formation of large α‐ and β‐structural blocks is observed, each of them including several native α‐helices or β‐strands.
Relationship between spatial structure and pharmacological activity of a series of β‐adrenomimetics
Authors: Zhorov B. S. , Govyrin V. A. 
Published: December 2004

AbstractAll equilibrium conformations of a series of N‐alkyl‐α‐alkyl derivatives of noradrenaline and their synthetic precursors—phenylaminoketones—have been calculated. It has been shown that spasmodic changes in bronchodilating activity of the catecholamines, observed with the increase in α‐alkyl substituent size, may arise from the difference in the ratio of diastereoisomers produced by reduction of aminoketones.
Regularities of the processes of radiationless conversion in polyatomic molecules
Authors: Plotnikov V. G. 
Published: December 2004

AbstractThe processes of radiationless conversion in aromatic and heteroaromatic molecules are investigated theoretically. The values of constant rates of internal conversion are calculated. The theoretical estimation of constant rates of S–T conversion aromatic hydrocarbon molecules and in their carbonyl‐, thiocarbonyl‐, and nitroderivatives and in azaheterocyclic molecules are given. The S‐T conversion probability between the states of different orbital nature (nπ* and ππ*) is equal to ca. 1010–1011 sec−1 that is two to four orders higher than the conversion probability between the states of the same orbital nature. It is shown that the process of T‐S conversion may be described in the second and in the highest orders of perturbation theory. The luminescent characteristics of molecules are connected with the relative position of electron states of different orbital nature and multiplicity and conversion characteristics.
Calculations of 1H NMR coupling constants for conformational studies of isomeric pentofuranosyl nucleosides
Authors: Jaworski Andrzej , Ekiel Irena 
Published: December 2004

AbstractThe SCF FPT method in the INDO approximation of molecular orbital theory is employed for calculations of 1H NMR cisoidal coupling constants in model compounds. Obtained results are used for finding conformational states of α‐xylo‐, β‐lyxo‐, and α‐2′‐deoxyribonucleosides in solution.
Excitons and Bose–Einstein condensation in living systems
Authors: Mishra R. K. , Bhaumik K. , Mathur S. C. , Mitra S. 
Published: December 2004

AbstractThe problems connected with the transformations of energy in the living systems are reviewed. Possibility of Bose–Einstein condensation by input of radiant energy over a critical level is considered as a route for overcoming the barriers of activation energy under the conditions of a pump and a thermalizing environment. Molecular force fields constitute the pump. Under our scheme excitons constitute the major fraction of bosons to so condense. Instantaneous dipoles of London theory are then examined as excitons. Lastly an energy packet from a quantized dipolar field is suggested rather than the concept of “conformons.” Questions of charge and of other modes of energy transfer are briefly discussed.