bluuebunny/crossref_metadata_2025 huggingface dataset:
Theoretical study of the thermal unimolecular rearrangement of fluoroethylidenes
Authors: Kakkar Rita , Walia Vibha
Published: January 2005

The thermal unimolecular isomerization of fluoroethylidenes to the corresponding fluoroethylenes has been studied by the MNDO method. It has been shown that fluorine substitution on the carbene carbon increases the activation energy in comparison with the ethylidene rearrangement. To understand the reason for this increase in the activation energy, the charge‐transfer effects have been analyzed. Fluorine substitution at other positions does not significantly affect the activation energies. The thermodynamic parameters for the reaction have been evaluated, using vibrational and rotational spectral data calculated in this work. RRKM calculations have been performed and high‐pressure Arrhenius parameters calculated. Hydrogen–deuterium kinetic isotope effects indicate that the reaction rates are altered considerably on isotopic substitution, and the change in reaction rates depends upon the position of deuterium substitution, as well as on the number of hydrogens replaced by deuterium atoms. © 1992 John Wiley & Sons, Inc.
A molecular orbital treatment of the electronic spectra of some tryptamines
Authors: Hamed Maher M. , Abu‐Eittah Rafie H. , Mobark Zein , Abdou Mohamed M.
Published: January 2005

The electronic absorption spectra of tryptamine, 5‐methoxytryptamine, 6‐fluorotryptamine, N‐acetyl‐5‐hydroxytryptamine, gramine, and melatonin were investigated. The observed transitions were π–π*, and the values of band maxima and intensity reflected an extent of interaction between the indole ring and the alkylamine side chain. Molecular orbital calculations at the level of INDO/S–CI were performed on all the studied molecules. State functions and transition energies were calculated. The correspondence between the experimental and theoretical results was satisfactory. © 1992 John Wiley & Sons, Inc.
Electronic hamiltonian of diatomic molecules in the basis of coupled momenta eigenfunctions
Authors: Kruglevsky V.
Published: January 2005

A systematic procedure has been developed to construct an electronic energy matrix for diatomics in the basis of antisymmetrized products of atomic wave functions represented as linear combinations of coupled momenta eigenfunctions. The exchange matrix element is expanded in powers of electronic interchange between atoms. General expressions of many‐electron angular coefficients have been obtained for all types of products of one‐ and two‐electron and overlap integrals in energy matrix elements. © 1992 John Wiley & Sons, Inc.
Simplified derivation of a one‐range addition theorem of the Yukawa potential
Authors: Homeier H. H. H. , Weniger E. J. , Steinborn E. O.
Published: January 2005

The one‐range addition theorem [J. Fernández Rico, R. López, and G. Ramírez, Int. J. Quantum Chem. 34, 121 (1988)] for the Yukawa potential is rederived in a very simple manner using weakly convergent expansions [E. J. Weniger, J. Math. Phys. 26, 276 (1985)] of plane waves in the Fourier representation. It is discussed in which sense the addition theorem converges. The relation to Sturmian expansions of Coulomb Green functions is pointed out. © 1992 John Wiley & Sons, Inc.
Simulated annealing to locate various stationary points in semiempirical methods
Authors: Bockisch F. , Liotard D. , Rayez J.‐C. , Duguay B.
Published: January 2005

In this work, an algorithm was developed to study the potential energy surfaces in the coordinate spaces of molecules by a nonlocal way, in contrast to classic energy minimizers as the BFGS or the DFP method. This algorithm, based on the specificities of semiempirical methods, mixes simulated annealing and local searches to reduce computation costs. By this technique, the global energy minimum can be localized. Moreover, local minima that are close in energy to the global minimum are also obtained. If the search is not only for minima but for all stationary points (minima, saddle points…), then the energy is replaced by the gradient norm, which reaches its minimum values at stationary points. The annealing process is stopped before having accurately reached the global minimum and generates a list of geometries whose energies (respectively, whose gradients) are optimized by local minimizers. This list of geometries is shortened from the nearly equivalent geometries by a dynamic single‐clustering analysis. The energy/gradient local minimizers act on the clustered list to produce a set of minima/stationary points. A targeted search of these points and reduction of the costs are reached by the way of several penalty functions. They eliminate—without energy calculation—most of the points generated by random walks on the potential energy surface. These penalty functions (on the total moment of inertia or on interatomic distances) are specific to the class of problem studied. They account for the nonrupture of either specific chemical bonds or rings in cyclic molecules, they assure that molecular systems are kept bonded, and they avoid the collapsing of atoms. © 1992 John Wiley & Sons, Inc.
Constrained self‐consistent‐field wave functions with improved long‐range behavior
Authors: Thakkar Ajit J. , Koga Toshikatsu , Matsuyama Hisashi , Archibong Edet F.
Published: January 2005

The recent suggestion that the long‐range behavior of energy‐optimized Gaussian basis sets can be improved by augmenting them with a Gaussian chosen to satisfy a constraint involving a linearly averaged position moment is explored. Calculations indicate that the high‐order moments 〈rk〉, with k > 4, in He, Be, and Li−, and 〈xkzL−k〉, with L > 4 and k ≤ L, in H2 are improved by the constraint, but that lower‐order moments and dipole polarizabilities are not. In H2, the higher moments with a given L improve by different amounts for different k, and, hence, the multipole moments do not improve. The basis‐set superposition error in HeHe and BeBe interaction energy calculations decreases if the internuclear distance is large enough. Thus, the constraint procedure improves the very long range behavior of the self‐consistent‐field wave functions. © 1992 John Wiley & Sons, Inc.
Experimental and computed bond lengths: The importance of their differences
Authors: Hargittai Magdolna , Hargittai István
Published: January 2005

State‐of‐the‐art experimental electron diffraction and computational information on the structure of alkaline earth dihalide molecules are in agreement for the shape of these symmetric triatomic molecules (linear/bent/quasi‐linear). However, the computed and measured bond lengths show differences that are not only considerably larger than the experimental error but also have the wrong sign. The physical meaning of experimental bond lengths depends on the physical techniques used in their determination and the ways of averaging over molecular vibrations. The choice of model potentials in the elucidation of experimental information is also important, especially for floppy molecules. When improved computational bond lengths become available, their comparison with experimental information will have to take account of the physical meaning of the experimentally determined bond lengths. The computed equilibrium distance (re) should be smaller than the experimental distance‐average bond length (rg). The differences may range from a few thousandths of an Å to a few hundredths with increasing temperature and, especially, with increasing floppiness of the molecule. For truly accurate comparison, experimental bond lengths should be compared with computed ones only following necessary corrections, bringing all information involved in the comparison to a common denominator. © 1992 John Wiley & Sons, Inc.
Atomic integrals containing rrr with λ, μ, ν ≥ −2
Authors: Lüuchow A. , Kleindienst H.
Published: January 2005

In this paper, the efficient evaluation of the atomic integrals I =∫rrrrrre−αr1−βr2−γr3dτ with one or two factors r is described. These integrals are necessary for a lower‐bound calculation for Li‐like systems using the method of variance minimization or Temple's formula. © 1993 John Wiley & Sons, Inc.
Direct SCF structure optimization of large molecules on networks of workstations
Authors: Vogel S. , Hutter J. , Fischer T. H. , Lüthi H. P.
Published: January 2005

A program to optimize the structure of large molecules at the Hartree–Fock level of theory running concurrently on a network of workstations is presented. Problems encountered in obtaining nearly optimal speedup and their solutions are discussed. A simple scheduling algorithm is presented that enables up to 99.5% of the code to run in parallel. © 1993 John Wiley & Sons, Inc.
Ab initio determination of polarizabilities per subunit in polymeric systems using the polarization propagator: Application to model hydrogen chains
Authors: Champagne Benoît , Mosley David H. , Fripiat Joseph G. , André Jean–Marie
Published: January 2005

A procedure based on the polarization propagator technique is used to determine the longitudinal asymptotic electric polarizabilities per unit cell of infinite periodic systems. They are computed ab initio at the random‐phase approximation level of accuracy for infinite model hydrogen chains using the STO‐3G minimal basis set. This work also focuses on the effect of the number of interacting cells taken into account in the cell index summations as well as the number of k‐points needed to perform the integration in the first Brillouin zone. Long‐range effects are shown to influence the desired response properties more strongly than do short‐range effects. Our direct method is presented as the only way to get the asymptotic longitudinal polarizabilities per unit cell of the systems presenting the largest polarizabilities, the most interesting polymers. © 1993 John Wiley & Sons, Inc.
The N4 molecule has an open‐chain triplet C2h structure
Authors: Glukhovtsev Mikhail N. , Von Ragué Schleyer Paul
Published: January 2005

The lowest‐energy N4 is computed ab initio to be the planar C2h(3Bu) open‐chain structure 13. The open‐chain N4 singlet‐state structures dissociate on geometry optimization. The tetraazatetrahedrane Td structure 1 and the tetrazete D2h structure 2 are minima at MP2/6‐31G*. However, both are higher in energy than 13 (24.1 and 21.2 Kcal/mol [UQCISD) (T)(full)/6‐311+G*//MP2/6‐31G* + ZPE(MP2/6‐31G)*, respectively]. The energy of 13 is 157.5 kcal/mol higher than that of two N2(1∑ molecules [UQCISD(T)(Full)/6‐311+G*//MP2/6‐31G*] © 1993 John Wiley & Sons, Inc.
Accurate computation of the energy spectrum for potentials with multiminima
Authors: Taşeli Hasan
Published: January 2005

The eigenvalues of the Schrödinger equation with a polynomial potential are calculated accurately by means of the Rayleigh–Ritz variational method and a basis set of functions satisfying Dirichlet boundary conditions. The method is applied to the well potentials having one, two, and three minima. It is shown, in the entire range of coupling constants, that the basis set of trigonometric functions has the capability of yeilding the energy spectra of unbounded problems without any loss of convergence providing that the boundary value α remains greater than a critical value αcr. Only the computation of the nearly degenerate states of multiwell oscillators requires dealing with a relatively large truncation order. © 1993 John Wiley & Sons, Inc.
Graph factorization: A new mode of application of a vertex‐alternation scheme
Authors: Mukherjee Asok K. , Das Debes K.
Published: January 2005

Linear chains where the vertex weights change sign alternantly but are equal in magnitude were able to be reduced to smaller chains by a procedure analogous to that given by Coulson and Rushbrooke. The algorithm for constructing the reduced chains has been stated and proved. The results have been utilized, in conjunction with McClelland's graph‐factorization method using reflection (σ) planes, to reduce the HMO secular determinants of some chemical graphs to an extent beyond the ability of group theory. McClelland's σ‐plane algorithm, used repetitively where possible, produces factors whose sizes (nM) are equal to those (nG) of the group‐theoretic factor blocks. For linear polyacenes (LP), however, a new observation has been made: If the LP has an even number of fused rings, nM = nG; but when the LP has an odd number of fused rings, McClelland's process is effective in further reduction, i.e., nM < nG. In any case, however, the vertex alternation procedure reported in the present paper brings about further reduction. To demonstrate the utility of the present method, a sample calculation of the LUMO eigenvector graph theoretically has been shown for p‐benzoquinone and the result has been utilized to obtain an inductive effect HMO parameter of the methyl group from the charge‐transfer bands of some molecular complexes of methylated p‐benzoquinones.© 1993 John Wiley & Sons, Inc.
Dipole, quadrupole, octupole, and dipole–dipole–quandrupole polarizabilities and second hyperpolarizabilities at real and imaginary frequencies for helium in the 23S state: Dispersion‐energy coefficients for interactions between He(23S) and H(12S), He(11S), He(23S), or H2(X1∑g+)
Authors: Bishop David M. , Pipin Janusz
Published: January 2005

We have determined the dynamic dipole (α1), quadrupole (α2), octupole (α3), and dipole–dipole–quadrupole (B) polarizabilities and the second hyperpolarizability tensor (γ) for the helium atom in its lowest triplet state (23S). We have done so for both real and imaginary frequencies: in the former case, for a range of frequencies (ω) between zero and the first electronic‐transition frequency, and in the latter case for a 32‐point Gauss–Legendre grid running from zero to ħω = 20 Eh. We have also determined the dispersion‐energy coefficients C6, C8, and C10 for the systems H(12S)He(23S), He(11S)He(23S), and He(23S)He(23S) and the C, C, C, C, and C coefficients for the interaction He(23S)H2(X1∑). Our values of the higher‐order multipolar polarizabilities and of γ for the 23S state of helium are, we believe, the first to be published. © 1993 John Wiley & Sons, Inc.
Electronic‐phase transition of the Hartree–Fock solution of the infinite one‐dimensional system: Structural change in an arbitrarily doped polyacetylene chain
Authors: Tanaka Kazuyoshi , Takata Akira , Okada Mayumi , Yamabe Tokio
Published: January 2005

The concept of the electronic‐phase transition of the Hartree–Fock solution has been applied to an arbitrarily doped polyacetylene (PA) chain to examine the structural change of the polymer skeletons under such doping regimes. It has been found that the n‐doped PA chain tends to suppress the generation of the charge‐density wave (CDW) phase, whereas the p‐doped PA favors the CDW phase leading to the charged‐soliton shape. © 1993 John Wiley & Sons, Inc.
Binding regions in polyatomic molecules
Authors: Wang Xiangyang , Peng Zhouren
Published: January 2005

A justification of Berlin's theorem that concerns the binding force concept in diatomic molecules is presented based on the Hellmann–Feynman theorem and its electrostatic interpretations. The concepts of binding and the binding region first derived by Berlin for diatomic molecules are extended to polyatomic molecules; the virial of forces necessary to hold the nuclei in a molecule fixed is proved to be an exact description of the total binding effect in the molecule. The partition of molecular space into binding and antibinding regions based on the binding function, which is defined in this paper, is proposed for polyatomic molecules. The binding diagrams for some planar molecules are developed as simple applications of our theoretical considerations. © 1993 John Wiley & Sons, Inc.
Multipole moment analysis for hydrides, fluorides, and lithium compounds of first‐ and second‐row elements
Authors: Köster Andreas M. , Jug Karl
Published: January 2005

A method is presented that conserves all local dipole moments in a partitioning of overlap charge between pairs of atoms. The approximate conservation of quadrupole and higher‐moment components can be used as a criterion for the suitability of atomic basis sets. Ab initio calculations are presented for hydrides, fluorides, and lithium compounds of first‐ and second‐row elements in a systematic study of compounds and basis sets. The best suitable basis set for each series is determined by the criterion of best approximate conservation of quadrupole moment components. A linear relation between charge shifts and Mulliken electronegativity differences is established for diatomic molecules. © 1993 John Wiley & Sons, Inc.
Calculation of two‐center exchange integrals with STOs using Möubius transformations
Authors: López R. , Ramírez G.
Published: January 2005

The Möbius transformations recently proposed for integrating functions with a sharp peak close to a single boundary of integration (H.H.H. Homeier and E.O. Steinborn, J. Comput. Phys. 87, 61, 1990) are used to extend Ruedenberg's algorithm for two‐center exchange integrals to cases with very diffuse STOs. © 1994 John Wiley & Sons, Inc.
Novel approach to molecular similarity: Second‐order similarity indices from geminal expansion of pair densities
Authors: Strnad Martin , Ponec Robert
Published: January 2005

The recently introduced second‐order similarity index gAB was generalized by using the geminal and/or spingeminal expansion of pair densities that they are derived from. The comparison with previous related studies confirms that the generalization does increase the information content of new indices, especially in providing the information about the difference in the spin recoupling of pure singlet and triplet states of electron pairs. Analogously, as in previous studies, the approach was applied to the analysis of several selected pericyclic reactions and possible mechanistic implications arising from the increased information content are briefly discussed. © 1994 John Wiley & Sons, Inc.
Energy levels of small titanium oxide clusters obtained from SCF calculations
Authors: Hagfeldt A. , Lunell S. , Sieghbahn H. O. G.
Published: January 2005

The energy levels of small titanium oxide clusters [(TiO2)2, and (TiO2H)3, and (TiO2H)2] have been calculated using ab initio SCF methods. Both crystal and relaxed geometries have been considered. Systematic changes in the valencelevel structure resulting from geometry relaxation are found, which may be related to band‐gap changes experimentally observed for small titanium oxide particles. In addition, a TiOH local surface state is found to be well described within a limited cluster model. © 1994 John Wiley & Sons, Inc.
Electronegativity, hardness, and a semiempirical density functional theory of chemical binding
Authors: Ghosh Swapan K.
Published: January 2005

Chemical binding is modeled through interatomic charge transfer and accumulation of electron density at the bond center using the concepts of electronegativity and hardness parameters defined for the bond region. The generalized electronegativity and hardness parameters for the up‐ and down‐spin electrons are also defined within the framework of spin‐polarized density functional theory, leading to the formulation of covalent binding in molecules in terms of a two‐way flow of unpaired electrons between the atoms. The associated energy changes corresponding to these descriptions are shown to provide quite accurate predictions of bond energies for simple heteronuclear diatomic molecules. © 1994 John Wiley & Sons, Inc.
Theoretical study on metal NMR chemical shifts: Germanium compounds
Authors: Nakatsuji H. , Nakao T.
Published: January 2005

Germanium chemical shifts were studied theoretically by the ab initio molecular orbital method. The compounds studied were GeMe4−xClx and GeMe4−xHx(x = 0–4). The calculated values of the germanium chemical shifts agreed well with the available experimental values. The germanium chemical shift is due to the p‐electron mechanism that reflects the ligand electronic effect on the p‐p* excitation term in the second‐order paramagnetic term. For GeMe4−xHx, the chemical shift is almost linear to the number of the ligand, x. On the other hand, a U‐shaped dependence is predicted for the chemical shifts of the GeMe4−xClx series and is shown to be caused by the strong and nonadditive electron‐withdrawing ability of the Cl ligand. The diamagnetic contribution is relatively small for the chemical shift and is determined solely by a structural factor. © 1994 John Wiley & Sons, Inc.
Electron density distribution functions and the ASED–MO theory
Authors: Anderson Alfred B.
Published: January 2005

A diatomic charge density distribution function may be partitioned into free atom and delocalization components. Integrating the electrostatic force on one of the nuclei as the atoms join to bond yields a repulsive atom superposition energy Er(R) and an attractive delocalization energy Ed(R) which, when added, equal the Born‐Oppenheimer potential energy, E(R). Bond stretch force constants may be calculated to quite good accuracy from the Poisson equation ▽E(R) = 4πZbpa(r = R), where Zb is the charge of nucleus b at a distance of R from nucleus a and pa is the charge‐density distribution function of isolated atom a. This equation follows if the delocalization density component is rigid during molecular vibrations and Ed = c/R, where c is a constant. Ed(R) is well approximated by the change in orbital energy, δEMO(R), for the bond‐forming reaction a + b → a ‐ b as obtained from a modified extended Hückel procedure. The resulting E() = Er(R) + δEMO(R) generalizes immediately to polyatomic molecules and solids and can be used for the calculation of structures and other properties and their molecular orbital interpretation. The current understanding of the atom superposition and electron delocalization molecular orbital (ASED‐MO) theory is presented in this article. It is pointed out that a parallel exists with the density functional theory, wherein the Born‐Oppenheimer potential function may be constructed from energy points E(R) that are functionals of the molecular charge density distribution function. Calculations of molecular structures with density functional theory has also become possible only through the introduction of electron orbitals. However, the approaches are otherwise different, for density functional theory works with the total energy of the molecule and the ASED approach works with the molecular binding energy obtained by integrating the electrostatic force on a nucleus, which is zero for the isolated atoms. © 1994 John Wiley & Sons, Inc.
Wannier analysis of the Cooper pairing force
Authors: Tachibana Akitomo
Published: January 2005

It is demonstrated that in the plasma state of the homogeneous electron gas the time‐reversal electron pair is stabilized. This nature survives even in the inhomogeneous electron gas, where the Bragg reflection at the lattice points is not necessarily a stumbling block for electron pairing. Indeed, a critical radius rc exists around the lattice point, outside of which the time‐reversal scattering state of the electron pair is guaranteed. The critical radius rc has an intrinsic importance as a measure of the coherence length ζ of the Cooper pair that exhibits the time‐reversal symmetry. © 1994 John Wiley & Sons, Inc.
Extended Koopmans' theorem ionization potentials for beryllium atom shake‐up transitions
Authors: Morrison Robert C.
Published: January 2005

The ionization potentials were calculated for Be using the extended Koopmans' theorem (EKT) using several full configuration interaction (CI) and multiconfigurational‐self‐consistent‐field (MCSCF) wave functions as reference wave functions. The wave functions used account for 89.7–96.7% of the correlation energy. Comparisons are made with experimental values and with δCI values calculated as the difference in energy obtained from CI wave functions for Be and Be+. The best EKT IP differed from the δCI value by 0.0003 eV for the lowest IP and by 0.0006 eV for ionization into the lowest 2P state of Be+. A calculation of ionization into the second 2P state of Be+ requires diffuse orbitals that are unimportant in the wave function for the ground state of Be. This results in small natural orbital occupation numbers for natural orbitals needed in the EKT calculation. © 1994 John Wiley & Sons, Inc.
Sensitivity analysis of charge transfer systems: In situ quantities, intersecting state model and its implications
Authors: Nalewajski Roman F.
Published: January 2005

The states of reactants in the donor (base, B)‐acceptor (acid, A) systems are examined and the charge transfer (CT) in situ sensitivities, including the chemical potential, hardness, softness, and Fukui function (FF) data, are derived within the atoms‐in‐molecules (AIM) resolution. Relaxational correction to the reactant CT FF vector is identified and qualitatively examined. The previously introduced intersecting state model (ISM) of the A‐B systems is generalized beyond the N‐restricted CT energy profile and formulated in terms of the intersecting energy paraboloids of reactants, within both uncoupled (qualitative) and coupled (quantitative) formulations; here, N is the total number of electrons. The model identifies the N‐unrestricted reaction paths in the AIM electron population space, possible when the system can exchange electrons with its environment and generally corresponding to a lower activation energy. The orientation of the reactant FF vector as a function of the hardness tensor structure is qualitatively examined in a model system consisting of two populational degrees of freedom (2 df), and the resulting conclusions are used to examine the mutual orientation of the hardness ellipses of the 2 df reactants in the A‐B systems. Predicted orientations and trends in activation barriers are discussed in the context of the hard‐soft acids and bases principle. © 1994 John Wiley & Sons, Inc.
Electron correlations and transport effective mass in narrow‐band systems and the Hubbard model
Authors: Arai Tadashi
Published: January 2005

The Hubbard model is rederived from a tight‐binding band calculation, showing that, as long as parameters are properly chosen, the model is justified for calculating electronic properties of narrow‐band systems. The treatment is extended by including correlations and it was found that bound solutions called dimers exist. The concept of dimers is found to be very powerful for understanding the unusual properties of heavy fermion systems. However, a Mott‐Hubbard‐like model may be required to calculate properties of high Tc cuprates. © 1994 John Wiley & Sons, Inc.
A “canonical functions” approach to the eigenvalues of a system of two coupled Schrödinger equation
Authors: Fakhreddine Khaled , Kobeissi Hafez
Published: January 2005

The problem of the determination of eigenvalues for two coupled Schrödinger equations is considered. A new method to solve this problem is presented. This method replaces the use of the wave functions (with unknown initial values) by eight canonical functions αij and βij (i = 1,2; j = 1,2) having well‐defined initial values at an arbitrary “origin” r0. These functions are collected in four couples; each one is the solution of the given coupled equations. For a given E, an “eigenvalue functions” D(E) is defined by an analytical expression depending on αij (r) and βij (r) at r = 0 and r = ∞ only. The successive eigenvalues En of the given system are precisely the successive intersection of the graph D(E) with the E‐axis. The present method eliminates the conventional use of wave function initial values as well as the conventional problem of the prior guess of the limit points; it determines these points automatically. It eliminates also the use of trial values for E and the need of iterations for its correction. The numerical application of a standard example used by Friedman and co‐workers (1990) shows that the eigenvlues computed by the present method are highly accurate for low and high levels; the average relative discrepancy between computed and exact levels is about 3.4 × 10 −15 (this discrepancy never exceeds 1.6 × 10 −14), which is almost the precision of the computer. © 1994 John Wiley & Sons, Inc.
Quantum chemistry study on B14, B, and B14H
Authors: Li Qian Shu , Gu Feng Long , Tang Au Chin
Published: January 2005

The structure of B14, B, and B14H in octahedral symmetry has been investigated by ab initio calculations at the STO‐3G and 4–31G levels. The relationship of molecular orbitals among them has been analyzed and it can be found that the number of valence bonding orbitals of high borane obeys the Wade rule. The similarities and difference between boron clusters and carbon clusters are also discussed. © 1994 John Wiley & Sons, Inc.
Full configuration interaction study of the ground state of closed‐shell cyclic PPP polyenes
Authors: Bendazzoli Gian Luigi , Evangelisti Stefano , Gagliardi Laura
Published: January 2005

Full configuration interaction (FCI) calculations are reported for the closed‐shell cyclic polyenes CNHN (N = 6, 10, 14, 18) in the Pariser–Parr–Pople (PPP) approximation, as a function of the hopping parameter β. A wide range of values of β is considered, from a highly correlated situation, β = 0, to a very weakly correlated limit, β = −10 eV. An estimate of the role of higher than two‐body connected cluster components was done, through a partial (i.e., limited to two‐body terms in the exponent) cluster analysis performed on the FCI wave function. The comparison with the approximate coupled pair theory that accounts for quadruply excited clusters [see P. Piecuch and J. Paldus, Theor. Chim. Acta 78, 65 (1990)] shows a good agreement in the whole range of the hopping parameter, particularly when the contribution of connected triples excitations is also taken into account. © 1994 John Wiley & Sons, Inc.
A theoretical investigation of some novel pilocarpine prodrugs
Authors: Ek holm Michaela , Konschin Henrik
Published: January 2005

A new class of pilocarpine double prodrugs of the bispilocarpic acid diester type has been investigated. Molecular orbital and molecular mechanics calculations have been done to obtain information about the conformational preferences and general physicochemical properties. Geometries of the structures were obtained with the force field implemented in the CHEM‐X program and using the semiempirical quantum chemical program AMPAC and the AM1 method. A correlation between the van der Waals volume, van der Waals area, and log P of the bispilocarpic acid diester structures is observed. The structures of these biscompounds are compared with the structures of the corresponding monopilocarpic acid diesters. Electrostatic and electronic properties of the compounds will be discussed in the context of the interaction between the prodrugs and the hydrolyzing esterase enzymes. © 1994 John Wiley & Sons, Inc.
Protonation‐induced conformational flipping in hypermodified nucleic acid base N6−(N−glycylcarbonyl) adenine
Authors: Tewari Ravindra
Published: January 2005

The influence of accepting the hydrogen bond by N(3) of adenine on the conformational preferences of the N(6) substituent in modified nucleic acid base N6−(N−glycylcarbonyl)adenine (gc6Ade) has been modeled by the protonation of N(3). The preferred orientation of the glycylcarbonyl substituent changes on the protonation of N(3). The preferred conformation for the N(3) protonated base is the planar, intramolecular bifurcated hydrogen‐bonded structure involving the interaction of the ureido N(11)H with the N(1) of the purine and the O(13b) of the amino acid carboxyl. Another conformation of nearly equal stability, having the internal hydrogen bonding of O(13b) with the N(6)H, is also predicted. Such protonation or hydrogen‐bonding‐induced conformational flipping may enable the structural reorientation of the anticodon loop required for the functioning of tRNA. © 1994 John Wiley & Sons, Inc.
An extension of Frost–Musulin and Möbius–Zimmerman diagrams
Authors: Zhou Zhongxiang
Published: January 2005

The eigenspectrum of a Möbius graph is a complete inversion of a Hückel graph for odd‐numbered monocycles. For even‐numbered monocycles, the Coulson‐paired levels (xi, – xi) in a Hückel graph become the degenerate levels (x′1, x′1) in a Möbius graph. Here, we present the proof of a general theorem stating that the same eigenspectral relationship is found in a much wider class of graphs. © 1994 John Wiley & Sons, Inc.
Simulation of diffusion‐limited aggregation and reactions over its surfaces
Authors: Lee Shyi‐Long , Lee Chung‐Kung
Published: January 2005

Modifications of Witten–Sander diffusion‐limited aggregation (DLA) were proposed. Effects caused by releasing boundary shape, nonfixed seed, and rotating biased drift on the morphology of the resulting DLA were examined. The diffusion‐limited reaction on surfaces of DLA were also studied by performing a multifractal scaling analysis. © 1994 John Wiley & Sons, Inc.
Cooperativity and electron correlation effects on hydrogen bonding in infinite systems
Authors: Suhai Sándor
Published: January 2005

Structural and electronic properties of hydrogen‐bonded infinite chains of hydrogen cyanide and formamide molecules have been investigated by the ab initio crystal orbital method using several, partly highly polarized, atomic basis sets of increasing size at the Hartree–Fock (HF) level and by including electron correlation effects in the second order of Møller–Plesset perturbation theory. The results obtained show that hydrogen bonding in molecular crystals of the type investigated is a highly cooperative phenomenon, both from the structural and energetic points of view. Comparison with clusters of up to four monomers demonstrate how various structural parameters converge toward their limiting values in the infinite system. The results obtained for infinite HCN chains show an excellent agreement with those observed for solid HCN, whereas the infinite formamide chain proves to be a reasonable model for the corresponding liquid phase. © 1994 John Wiley & Sons, Inc.
Electron correlations and pairing in low‐dimensional systems
Authors: Ukrainskii I. I.
Published: January 2005

The many‐electron system in one and two dimensions are studied within the geminal approach. The analytical expressions for the wave functions and ground‐state energies are obtained for a number of 1‐D and 2‐D systems: conjugated polymers, organic conductors, 2‐D conductors with square lattices, and others. It is shown that electron excitations of a kink type can exist in 2‐D systems with mixed valency. In this case, the correlation pairing of current carriers arises as a result of correlation effects leading to superconducting properties of the system. © 1994 John Wiley & Sons, Inc.
Molecular dynamics simulations of n‐Alkane melts confined between solid surfaces
Authors: Winkler Roland G. , Gerstmair Anja , Reineker Peter , Matsuda Tsunetoshi , Yoon Do Y.
Published: January 2005

Molecular dynamics simulations of polyethylene chains (CnH2n+2 for n = 13, 16, 28, 60) have been carried out to investigate both equilibrium and dynamic properties of polymer melts confined between flat solid surfaces. We observe an oscillatory monomer density in the direction normal to the solid surfaces, which depends on the size of the monomers (i.e., on the volume density of the system). The packing manner of monomer segments, segment orientation, and local conformations of chains are found to be independent of chain length. In addition, preferential interfacial adsorption of chain ends is observed. The chains are flattened close to the surface and many molecules assume essentially two‐dimensional train configurations even in the case of C60H122 melts. The apparent self‐diffusivities of the centers of mass of the molecules depend on their distance from a surface. Molecules adjacent to a surface exhibit a reduced mobility perpendicular to the surface and an increased one parallel to it. The self‐diffusion constant parallel to a surface depends strongly on the size of the monomers. An increase of the united atom diameter by 10% reduces the diffusion constant by a factor of three, in good agreement with the experimental value. © 1994 John Wiley & Sons, Inc.
Density functional studies on hydrogen‐bonded complexes
Authors: Kieninger Martina , Suhai Sädor
Published: January 2005

Results of density functional calculations will be reported on a variety of hydrogen‐bonded complexes, ranging from weak to strong hydrogen bonds. The charged bimolecular NH3NH complex and the dimers of water and methanol were investigated using a local approximation of the exchange‐correlation potential and two different nonlocal potentials with gradient corrections. In the case of the water dimers, the dependence of the results on the extension of the atomic basis set has also been investigated. The equilibrium structures of all complexes have been determined. Dipole moments, hydrogen‐bond lengths, and hydrogen‐bonding energies, calculated with corrections for the basis‐set superposition error using the counterpoise method, have been found to agree well with the corresponding experimental results. © 1994 John Wiley & Sons, Inc.
Calculation of excitation and photoionization spectra by quasi‐degenerate perturbation theory
Authors: Lisini Adriana , Decleva Piero
Published: January 2005

The ability of the QDPT CI method, in which the quasi‐degenerate perturbation theory is applied within the configuration interaction (CI) approach, in dealing with the calculation of excitation and photoionization spectra is shown through an overview of comparisons between the QDPT CI values and the full CI ones in the same basis set. A direct comparison with the experimental data is given for the core photoelectron spectrum of Ne and the valence photoelectron spectra of Ar and HCl. The quality of the results obtained is very satisfactory, both as regards the energies and the wave functions, indicating the validity and the flexibility of the method that can confidently be applied also in cases where strong correlation effects are present. © 1994 John Wiley & Sons, Inc.
Hypervirial theorem and matrix elements for the Coulomb potential
Authors: López‐Bonilla José L. , Morales Jesús , Rosales Marco A.
Published: January 2005

It is well known that hydrogen like atoms may be represented as Morse oscillators. Our aim in this article was to show how this result and the hypervirial theorem lead to a computational method with which the matrix elements for the Coulomb potential can be easily calculated. Also, a closed analytical expression for 〈n2l2|rk|n1l1〉 is obtained. © 1995 John Wiley & Sons, Inc.
On bound states in two‐body systems
Authors: Frolov Alexei M. , Smith Vedene H.
Published: January 2005

The Hamiltonian of an arbitrary two‐body nonrelativistic system is written in closed form in terms of generators of the noncompact O(2, 1) Lie algebra. The bound‐states problem in such systems is investigated with the help of the self‐adjoint irreducible representations of this algebra (positive discrete series). It is shown that the initial Schrödinger equation can be reduced to a simple eigenvalue problem, in which all matrices can be determined easily from the basic properties of the O(2, 1) algebra and by applying the explicit form of the potential interaction. Based on such algebraic considerations, we obtain a number of useful relations between the expectation values of the potential and kinetic energies for the so‐called prethreshold states, i.e., those bound (E < 0) states for which E ≈ 0. © 1995 John Wiley & Sons, Inc.
General criteria for assessing the accuracy of approximate wave functions and their densities
Authors: Núñez Marco A.
Published: January 2005

By means of examples, Löwdin showed that L2 convergence of approximate wave functions ψn to the exact ψ using the single limit limn→∞〈ψn, Aψn〉 = 〈ψ,Aψ〉 is not sufficient to compute accurate expectation values. It is shown that L2 convergence is indeed a sufficient condition to compute accurate expectation values using iterated limits limm→∝︁ limn→∝︁〈ψn, Aψm〉 = 〈ψ, Aψ〉 instead of a single limit. Practical conditions that guarantee the stability of single‐limit calculations are given. It is also shown that the L2 covergence of wave functions implies the convergence in the L1(R3)‐norm of their corresponding densities. This permits us to prove Weinhold's conjecture that the rate of convergence of densities are greater than that of wave functions. The results are extended to the momentum space, and their equivalence with those of position space is shown. Properties of Lp spaces are used to introduce the Cauchy criterion that permits us to check the convergence in norm of approximate wave functions and their densities, as well as to estimate exact errors. This is illustrated by a numerical example. © 1995 John Wiley & Sons, Inc.
Determination of the acid dissociation constants of p‐benzohydroquinone by the INDO method
Authors: Datta Sambhu N. , Mallik Buddhadeb
Published: January 2005

The stepwise acid dissociation constants for p‐benzohydroquinone (QH2) in aqueous media have been explicitly calculated for the first time, with the INDO parametrized SCF–MO method. We have optimized the geometries of QH2, QH−, and Q2− and of the QH2 · 6H2O, QH− · (H3O+) · 5H2O, and Q2− · (H3O+)2 · 4H2O systems that model the solvated species. The presence of the associated water molecules (and hydronium ions) account for the stabilization due to hydrogen bonding as well as for a part of the effect of interaction of these molecules with the respective reaction fields in an aqueous medium. To simulate the first solvation shell in a more complete manner, four more water molecules have been considered to be placed above and below the quinonoid ring and the optimized geometries of the resulting hydrated species, QH2 · 10H2O, QH− · (H3O+) · 9H2O, and QH− · (H3O+) · 8H2O, have been determined. The standard free‐energy changes calculated for the dissociation of QH2 into QH− and H+ is 0.0251 Hartree (65.9 kJ mol−1) and that of QH− into Q2− and H+ is 0.0285 Hartree (74.8 kJ mol−1). Experimentally observed dissociation constants for these two steps correspond to free‐energy changes of 0.0214 Hartree (56.2 kJ mol−1) and 0.0248 Hartree (65.1 kJ mol−1), respectively. © 1995 John Wiley & Sons, Inc.
Distortion of the Fermi surface of an anisotropic two‐dimensional Fermi gas
Authors: Wu Jin Zhong , Goodman Bernard
Published: January 2005

We report here the first quantitative theory of the change of anisotropic Fermi surfaces (FS) due to the dynamical e–e interaction. The new FS is constructed as a self‐consistent solution of the Dyson equation. This incorporates effects of “anomalous” diagrams, absent in the usual perturbation theory, but which are responsible for changing the FS. Calculations are presented for 2D electrons. Correlations reduce the anisotropy. © 1995 John Wiley & Sons, Inc.
Why irreversibility? The formulation of classical and quantum mechanics for nonintegrable systems
Authors: Prigogine Ilya
Published: January 2005

Nonintegrable Poincaré systems with a continuous spectrum (so‐called large Poincaré systems, LPS) lead to the appearance of diffusive terms in the frame of classical or quantum dynamics. These terms break time symmetry. They lead, therefore, to limitations to classical trajectory theory and of wave‐function formalism (Schrödinger's equation). These diffusive terms correspond to well‐defined classes of dynamical processes (i.e., so‐called vacuum‐vacuum transitions). The diffusive effects are amplified in situations corresponding to persistent interactions. As a result, we have to include, already, in the fundamental dynamical description the two basic aspects, probability and irreversibility, which are so conspicuous on the macroscopic level. We have to formulate both classical and quantum mechanics on the Liouville level of probability distributions (or density matrices). For integrable systems, we recover the usual formulation of classical or quantum mechanics. Instead of being primitive concepts, which cannot be further analyzed, trajectories and wave functions appear as special solutions of the Liouville–von Neumann equations. This extension of classical and quantum dynamics permits us to unify the two concepts of nature that we inherited from the nineteenth century, based, on the one hand, on dynamical time‐reversible laws and, on the other, on an evolutionary view associated to entropy. It leads also to a unified formulation of quantum theory, avoiding the conventional dual structure based on Schrödinger's equation, on the one hand, and on the “collapse” of the wave function, on the other. A dynamical interpretation is given to processes such as decoherence or approach to equilibrium without any appeal to extra dynamic considerations (such as many‐world theory, coarse graining, or averaging over the environment). There is a striking parallelism between classical and quantum theory. For large Poincaré systems (LPS), we have, in general, both a “collapse” of trajectories and of wave functions. In both cases, we need a generalized formulation of dynamics in terms of probability distributions or density matrices. Since the beginning of this century, we have known that classical mechanics had to be generalized to take into account the existence of universal constants. We now see that classical as well as quantum mechanics also have to be extended to include unstable dynamical systems such as LPS. As a result, we achieve a new formulation of “laws of physics” dealing no more with certitudes but with probabilities. This formulation is appropriate to describe an open, evolving universe. © 1995 John Wiley & Sons, Inc.
On the Dirac–Kepler problem: The Johnson–Lippmann operator, supersymmetry, and normal‐mode representations
Authors: Dahl Jens Peder , Jøorgensen Thomas
Published: January 2005

The relativistic Kepler problem is discussed, with emphasis on the exact supersymmetry of the problem. It is shown that the supersymmetry is generated by the Johnson–Lippmann operator. Two related operators are found to generate new supersymmetries in an extended function space. Each of these supersymmetries may be disguised as radial supersymmetries. The radial supersymmetries are discussed and it is shown that each of them defines a normal‐mode representation of the hydrogen‐atom radial functions. Thus, one obtains two different, but equivalent, analytical expressions for these functions. The expressions are well known, but are rederived here in the light of the new understanding. Finally, the nonrelativistic image of the relativistic supersymmetry is constructed and its generators shown to be identical with those recently presented in the literature. © 1995 John Wiley & Sons, Inc.
Configuration interaction studies using biorthogonal approach to VB basis
Authors: Kadolkar C. , Sarma C. R. , Rettrup S.
Published: January 2005

In the present article, we have attempted a systematic procedure for use of biorthogonal techniques to the configuration interaction studies in molecules using nonorthogonal valence bond (VB) orbitals. The procedure developed is integral‐driven and a program based on this has been developed. Test runs of the program have been carried out in case of full and truncated configuration spaces. © 1995 John Wiley & Sons, Inc.
Splitting of the characteristic polynomial (CP) using a computational technique to obtain the factors of the mirror plane and two‐, three‐, and n‐fold symmetric graph
Authors: Patra Swarna M. , Mishra Rama K.
Published: January 2005

All possible quadratic factors from the characteristic polynomial are generated. Selective extraction of the factors are smoothly achieved. Factors of the mirror plane and two‐, three‐, and n‐fold rotationally symmetric graphs are obtained without using any rule for the fragmentation pattern. The relationship among the irreducible representations of the two‐, three‐, and n‐fold operators are established through our computer program. © 1995 John Wiley & Sons, Inc.