bluuebunny/crossref_metadata_2025 huggingface dataset:
Einfache Darstellung von 4‐Oxo‐tetrahydropyrimidinen
Authors: Schramm Siegfried , Schmitz Ernst , Gründemann Egon
Published: January 2005

Convenient Synthesis of 4‐Oxo‐tetrahydropyrimidines

Enamino‐nitriles as 6 or 11 yield 4‐oxo‐pyrimidines (2; 2a; 10; 12a–c) on treatment with cyclohexanone in basic media. Analogous compounds are obtained using acetone, butyraldehyde, benzaldehyde or substituted benzaldehydes.
Kationische Photopolymerisation von Isobutylglycidether mit Benzoinderivat/Aryloniumsalz‐ bzw. Benzildimethylketal/Aryloniumsalz‐Initiatorsystemen
Authors: Baumann H. , Timpe H.‐J.
Published: January 2005

Cationic Photopolymerization of Isobutylglycidyl Ether with Benzoine Derivatives/Arylonium Salts‐ or Benzildimethyl Ketal/Arylonium Salts‐Initiator Systems

Radicals produced by photolysis of benzoine derivatives and benzile dimethylketale decompose arene diazonium salts (ArN2⊕), diphenyliodonium salts (Ph2I⊕) and triphenylsulfonium salts (Ph3S⊕), respectively. The carbenium ions or protons formed in this way can be used for the cationic polymerization of isobutylglycidyl ether (IBGE). The efficiency of this polymerization depends on the quantum yields of the onium salts decomposition. A reactivity ArN2⊕ > Ph2I⊕ > Ph3S⊕ was found. In the case of ArN2⊕ the salt decomposition is caused by electron transfer between the photolytically produced substituted benzylic radicals and the diazonium ions. It is assumed that by the decomposition of Ph2I⊕ and Ph3S⊕ intermediates occur, which give rise to some reversible reactions. Therefore, the quantum yields of onium salt decomposition are less than those of initiator photolysis. The rate of cationic polymerization of IBGE is also influenced by the anion of onium compounds. A sequence PF6⊖ >SbF6⊖ >BF4⊖ was found.
Nucleophile Substitutionen an Kohlensäurederivaten. XIX. Alkoholyse und Hydrolyse des Bis(trichlormethyl)carbonates
Authors: Cotarca Livius , Bacaloglu Radu , Marcu Nicolae , Târnaveanu Alexandru
Published: January 2005

Nucleophilic Substitutions at Carbonic Acid Derivatives. XIX. Alcoholysis and Hydrolysis of Bis(trichloromethyl)carbonate

The rate constants of hydrolysis and alcoholysis of bis(trichlormethyl)carbonate in xan have been determinated conductometrically. The effects of the water and alcohol concentrations, the temperature and deuterium have been studied.

By the hydrolysis and alcoholysis of bis(trichlormethyl)carbonate the nucleophilic attack of water and alcohol is the rate‐determining step, followed by a fast elimination of unstable trichloro‐methanol.
Neue Azoolefine und deren saure Spaltung zu Aryldiiminen
Authors: Kirschke Klaus , Möller Angela , Schmitz Ernst
Published: January 2005

New Azoolefines and their Acidic cleavage to Aryldiimines

3‐Amino‐1‐aryl‐3′,3′‐dimethyl‐pyrazolin‐4‐spiro‐2′‐oxiran‐5‐ones (3a–c) undergo ring opening with methoxide forming methyl 3‐amino‐3‐arylazo‐propenoates (5a–c). 5a–c are cleaved under acidic conditions. The main products of the cleavage of 5c with methanolic hydrochloric acid are nitrogen, 2, 4, 6‐trichloro‐benzen (6), 2, 4, 6‐trichloro‐aniline (9) and 2, 4, 6‐trichlorophenylhydrazine (10). Intermediates of the cleavage of 5 are aryldiimines trapped with benzaldehyde as the corresponding benzhydrazides (12a, b).
4‐Acyl‐2‐acylamino‐4,5‐dihydro‐1,3,4‐thiadiazole durch Acylierung von Thiosemicarbazonen
Authors: Andreae Siegfried , Schmitz Ernst , Seeboth Helmuth
Published: January 2005

4‐Acyl‐2‐acylamino‐4,5‐dihydro‐1,3,4‐thiadiazoles by Acylation of Thiosemicarbazones

Thiosemicarbazones and acid anhydrides or halides react to 4‐acyl‐2‐acylamino‐4,5‐dihydro‐1,3,4‐thiadiazoles (2a–2v) in high yields. 3‐Acyl‐2‐amino‐4,5‐dihydro‐1,3,4‐thiadiazoles 5 are proved as intermediates in this conversion. In special cases they can be isolated or submitted to further acylation with an other acylating agent. 1H‐n.m.r., 13C‐n.m.r. and mass spectroscopic data of compounds 2a–2v and 5a–5e are given. The preparation of the compounds 2 can also be carried out starting with a carbonyl compound and thiosemicarbazide without isolation of the thiosemicarbazone.
Synthesis of new Thiopyranopyridines
Authors: Dunn A. D.
Published: January 2005

The synthesis of 5H‐thiopyrano[4,3‐b]pyridines, 8H‐thiopyrano[3,4‐b]pyridines, 1H‐thiopyrano[3,4‐c]pyridines and 1H‐thiopyrano[4,3‐c]pyridines from methyl pyridine esters 5–8 and nitriles 17–20 by simple photobromination, followed by reaction of the intermediate bromomethyl derivative with methyl thioglycolate and subsequent base induced cyclisation of the latter are reported.
Reaktionen von 2‐Ethoxycarbonyl‐3‐hydroxy‐N‐phenyl‐maleinimid
Authors: Augustin M. , Jeschke P.
Published: January 2005

Reaction Behaviour of 2‐Ethoxycarbonyl‐3‐hydroxy‐N‐phenyl‐maleimide

The vinylogous acid 1 gives with amines under various conditions the ammonium salts 2a–f. 2‐Ethoxycarbonyl‐3‐chloro‐N‐phenyl‐maleinimide 5 obtained by the chlorination of 1 leads to N‐substituted 2‐ethoxycarbonyl‐maleimides 6a–d and condensed heterocycles 7a, b.

The reaction of 1 with 2‐methylindol forms 8a, b or gives with methyleneactive compounds 2‐ethoxycarbonyl‐maleimide 9 and the pyridonimide 10.
Reaktionen von (Z/E)‐1‐Aryl‐4‐arylmethylen‐pyrrolidin‐2,3,5‐trionen mit methylenaktiven Verbindungen
Authors: Augustin M. , Jeschke P.
Published: January 2005

Reactions of (Z/E)‐1‐Aryl‐4‐arylmethylene‐pyrrolidine‐2,3,5‐triones with Methylene Active Compounds

The cyclization of the α,β‐unsaturated carbonyl compounds (Z/E)‐1a, b with propanedinitrile led to the corresponding 2‐amino‐3‐cyano‐4H‐pyrans 2a, b. Stable phosphorus containing compounds 3a–c were prepared by Michael type addition between methoxycarbonylmethylene triphenylphosphorane and (Z/E)‐1a, c, d in good yields.

From (Z/E)‐1a, b and dimedone or 2‐hydroxy‐maleimide Michael adducts 5a, b, 6a were obtained, which converted into 5c, 6b by alkylation with diazomethane or gave different types of heterocycles (7, 8a, b) depending on the reaction conditions.

Cyclocondensation of (Z/E)‐1b with cycloalkanones in the presence of ammonium acetate formed 4‐aryl‐1,4‐dihydro‐cycloalkeno[b]pyridines 9a–c.

Further it was shown that the bicyclic 1,4‐dihydro‐pyridine derivatives 8b and 10, which easily undergo oxidation with CrO3 or air oxygen, were intermediates in the formation of the heteroaromatic compounds 11, 12.
Tetrathiafulvalene. XXV [1]. Konjugativ verknüpfte polymere Tetrathiafulvalene (TTF)
Authors: Trinh V. Q. , Van Hinh L. , Schukat G. , Fanghäunel E.
Published: January 2005

Tetrathiafulvalenes. XXV. Conjugative Connected Polymeric Tetrathiafulvalenes (TTF)

Syntheses and properties of a conjugative connected polymeric TTF (by tetrahydroanthracene units) and phenylene‐bridged polymeric TTF are described. These polymers react with bromine, iodine or tetracyanoquinodimethane and form the radical cation salts, whose electrical conductivity is determined. The conjugatively connected polymeric radical cation salts show a higher conductivity than the twisted phenylene‐bridged polymeric radical cation salts.
Unitary group approach to general system partitioning. II. U(n) matrix element evaluation in a composite basis
Authors: Gould M. D.
Published: January 2005

An explicit segment level formalism is derived for the matrix elements of the U(n) generators in an arbitrary (multishell) composite basis. The results of this paper, which contain the usual (spin‐independent) unitary calculus approach as a limiting case, yield a more powerful and versatile algorithm than the traditional (spin‐independent) unitary group formalism.
A chemically useful definition of electron difference densities
Authors: Schwarz W. H. E. , Mensching L. , Valtazanos P. , Von Niessen W.
Published: January 2005

Molecular difference densities (DD) are conventionally constructed using spherically averaged atomic densities at the appropriate positions. For atoms in degenerate ground states, this is an unphysical choice, and artifacts dominate the DD. We suggest the extraction of both the position and the orientation of an atom with an open valence shell from x‐ray scattering or molecular density data. Subtracting the oriented atoms yields a uniquely defined, as well as chemically meaningful, DD. Covalent bonds to electronegative atoms such as O are no longer exceptional but show bond charges of normal magnitude. Lone pairs are characterized by a dipolar density shift from the bond to the back side of the atomic core.
Calculation of transition density matrices by nonunitary orbital transformations
Authors: Malmqvist Per Åke
Published: January 2005

An efficient scheme for calculating one‐ and two‐electron transition density matrices for two wave functions is described. The method applies to CAS (complete active space) wave functions and certain multireference CI expansions. The orbital sets of the two wave functions are not assumed to be equal. They are transformed to a biorthonormal basis, and the corresponding transformation of the CI coefficients is carried out directly, using the one‐electron coupling coefficients.
Butadiene and benzene π‐electron SCF ground states in the bond orbital resonance theory approach
Authors: Živković Tomislav P.
Published: January 2005

Butadiene and benzene SCF ground states are analyzed using the bond orbital resonance theory (BORT) approach. The results obtained are compared with the analogous description of these states within the VB approach. It is shown that the structural weights of BORT Kekulé structures are much larger than the structural weights of the corresponding VB Kekulé structures. For example, in the butadiene case the structural weight of the BORT Kekulé structure is over 0.941, while the structural weight of the VB Kekulé structure is only 0.221. In addition, in the BORT approach one has to consider a much smaller number of resonance structures than in the VB approach. This suggests that the description of conjugated systems should be more successful in the BORT than in the VB basis, and that formal Kekulé structures should be interpreted in the BORT and not in the VB sense.
An intermolecular potential function for the Fe(II)–H2O system from ab initio calculations
Authors: Gonzàlez‐Lafont A. , Lluch J. M. , Oliva A. , Bertrán J.
Published: January 2005

An intermolecular potential for the Fe(II)–H2O system has been determined from ab initio calculations which have been obtained with Huzinaga's MINI‐2 basis set. Interaction energies for more than 100 points of the potential energy surface were fitted to an analytical function that contains 11 adjustable parameters. The goodness of the fitting and its applicability to the study of Fe(H2O) clusters and to Monte Carlo simulations are discussed.
Transferability of atomic descriptions in molecules
Authors: Rico J. F́ernandez , Alvarez‐Collado J. R. , Paniagua M. , Ĺopez R.
Published: January 2005

In a previous paper we proposed a new electrostatic description of the molecular electronic density in terms of charges, dipoles, quadrupoles, octupoles, etc. In this paper we apply this analysis to some first members of alkanic, ethylenic, acetylenic, alcoholic, aldehyde, amine, and nitrile series. Results show the clear transferability of the atomic descriptions for chemically similar atoms as well as the remarkable sensitivity of the electronic density versus changes in the basis sets, pointed out in our previous work. We discuss the calculation of molecular potentials from these atomic descriptions (and present an example) and some practical applications.
Energy expressions for atomic configurations in the L–S coupling scheme
Authors: Dinesha K. V. , Iyer Suman B. , Vishveshwara Saraswathi
Published: January 2005

The matrix elements of the spin‐free Hamiltonian between two atomic configuration state functions (CSF'S) in the L–S coupling scheme are expressed in terms of the atomic integrals Fk's and Gk's. Using these general expressions, the matrix elements have been obtained for all the atomic configurations with three valence electrons that have not been solved so far by earlier methods. The scope for applying this new approach to obtain the Auger line energies and the promotion energies of metals that involve more than two partially filled shells is indicated. The energy expressions for some of the relevant configurations are tabulated.
On the construction of the effective pair correlation function with the fixed zero value at the points where two electron positions coincide
Authors: Gritsenko O. V. , Bagaturyants A. A. , Zhidomirov G. M.
Published: January 2005

The effective pair correlation function g̃(N)(r1;r2) accounting for the contribution of the kinetic energy functional to the electron correlation energy has been constructed. This function possesses the asymptotic property g̃(N)(r1;r2) = 0 at r1 = r2 and provides an efficient formalism for a model description of the electron correlation in many‐electron systems.
Reaction rates for O + HD → OH + D and O + HD → OD + H
Authors: Garrett Bruce C. , Truhlar Donald G.
Published: January 2005

We have calculated reaction rates for the reactions O + HD → OH + D and O + DH → OD + H using improved canonical variational transition state theory and least‐action ground‐state transmission coefficients with an ab initio potential energy surface. The kinetic isotope effects are in good agreement with experiment. The optimized tunneling paths and properties of the variational transition states and the rate enhancement for vibrationally excited reactants are also presented and compared with those for the isotopically unsubstituted reaction O + H2 → OH + H. The thermal reactions at low and room temperature are predicted to occur by tunneling at extended configurations, i.e., to initiate early on the reaction path and to avoid the saddle point regions. Tunneling also dominates the low and room temperature reactions for excited vibrational states, but in these cases the results are not as sensitive to the nature of the tunneling path. Overbarrier mechanisms dominate for both thermal and excited‐vibrational state reactions for T > 600 K. For the excited‐state reaction (with initial vibrational quantum number n > 0) a transition state switch occurs for T > 1000 K for the O + HD(n = 1) → OD + H case and for T > 1500 K for the O + DH(n = 1) → OD + H reaction, and this may be a general phenomenon for excited‐state reactions at higher temperature. In the present case the switch occurs from an early variational transition state where the vibrationally adiabatic approximation is expected to be valid to a tighter variational transition state where nonadiabatic effects are probably important and should be included.
Analysis of the IGLO bond contributions to the 13C shielding tensors in the local bond frame
Authors: Facelli Julio C. , Grant David M. , Michl Josef
Published: January 2005

A new method is presented to analyze the IGLO (individual gauge for localized orbitals) bond contributions in 13C chemical shielding. The IGLO bond contributions calculated in the molecular frame are rotated to a local bond frame, in which one component is selected along the bond. This procedure removes the explicit angular dependence of the IGLO bond contributions and allows a comparison of the bond contributions in different molecules.

The results provide a new method to study the electronic basis of shielding interactions. The problems associated with the multiple gauge origins used in the IGLO method are discussed in their relationship to the bond contribution analysis.
MBPT and coupled cluster calculation on the neon atom with numerical orbitals
Authors: Adamowicz Ludwik , Bartlett Rodney J.
Published: January 2005

Our diatomic coupled cluster method with numerical orbitals has been applied to the Ne atom. Second, third, and full fourth order MBPT correlation corrections as well as coupled cluster correlation energies are reported.
Orthogonal Waller–Hartree spin eigenfunctions
Authors: Lim Tiong‐Koon
Published: January 2005

It was shown recently by the present author that the double symmetrization and the double antisymmetrization are essential in the spin‐dependent and the spin‐free formalisms, respectively, to perform the exclusion of all the unnecessary spin eigenfunctions and the selection of a unique set of linearly independent spin eigenfunctions. The double antisymmetrized Wigner matric basis and the Wigner double symmetrized matric basis are presented in this article for N up to 6. The double symmetrization or the double antisymmetrization also results in a direct expansion method for the calculation of the orthogonal spin coefficients; this direct method does not require the knowledge of the wave functions of the N − 1 electron system. The modified method of Graebenstetter can also be used to calculate these orthogonal spin coefficients.
The structure and properties of flavins: Molecular orbital study based on totally optimized geometries. I. Molecular geometry investigations
Authors: Hall Lowell H. , Orchard Bradley J. , Tripathy Sukant K.
Published: January 2005

Molecular orbital calculations (MINDO/3) using energy minimized molecular geometries were performed on oxidized and reduced lumiflavin and related methylated isoalloxazines, including cationic and anionic species. Close agreement with experimental geometry, photoelectron spectra, and NMR data supports the importance of optimized geometries for these molecular systems and provides the basis for interpretation of chemical and biological properties. Oxidized forms are shown to be most stable in the planar configuration but also highly flexible about the N(5)—N(10) axis; only 1 kcal/mol is required for a 10° bend. N(10) is generally out of the plane slightly; also, C(9)‐methyl substitution introduces nonplanarity. The unsubstituted isoalloxazine is computed to be 0.76 kcal/mol (ΔH) less stable than its isomer, alloxazine. Calculations were also performed on enol as well as quinone‐methide tautomeric forms.

Reduced flavin geometry depends on methyl substitution pattern: N(10) substituted forms are bent with typical fold angles around 155°, whereas the unsubstituted reduced form is planar. Both oxidized and reduced forms are also flexible.

Proton affinities were calculated for protonation and deprotonation of oxidized and reduced forms. Protonation of oxidized forms is favored at N(1) by 10–12 kcal/mol and produces somewhat nonplanar isoalloxazinium ions. In addition, ΔH for the two‐electron reduction of lumiflavin is estimated to be −19.7 kcal/mol.

In this paper investigations of geometric aspects are presented along with introductory and background material. Orbital structure and electron distribution studies are presented in paper II.
Explicit Galois resolvents for sextic equations
Authors: Hurley A. C. , Head A. K.
Published: January 2005

Several techniques for calculating the Galois resolvents of polynomial equations are discussed and implemented. In particular, the method of power sums, in conjunction with the symbolic algebra program muMATH, is used to derive a complete set of explicit algebraic resolvents for the general sextic equation. A simple example, drawn from the theory of crystal elasticity, illustrates the utility of these results in answering the question “When is a polynomial equation (with multinomial coefficients) solvable?”.
Chiral‐achiral molecular coupling: The effect on the chiral partner
Authors: Craig D. P. , Thirunamachandran T.
Published: January 2005

Pair coupling between a chiral molecule and an achiral molecule can induce weak circular dichroism in the achiral partner, as is well known in induced circular dichroism. Here the effect of the same coupling on the chiral partner is analyzed. The effect is an increase or decrease in the rotatory strength that may be detectable under conditions where the effect is enhanced by a near‐resonance.
Electron pair correlation in atoms and molecules
Authors: Nesbet R. K.
Published: January 2005

Accurate methods for computing energies and electronic properties of atoms and molecules have been derived from direct treatment of localized pairs of electrons. The conceptual development and implementation of such methods is reviewed.
The extended Koopmans' theorem Fock operator
Authors: Matos J. Mauricio O. , Day Orville W.
Published: January 2005

By expanding the wave function of a system of N particles in terms of products of functions of one and (N‐1) particles, the one‐particle, nonlocal operator F̂EKT (extended Koopmans' theorem) is determined. It is shown that although this operator is nonhermitian, its eigenvalues and eigenfunctions represent the ionization energies and occupied orbitals, respectively. The eigenfunctions of F̂EKT are the one‐particle functions that enter into the expansion of the wave function of the system as partners of the (N‐1)‐particle wave functions. The eingenvalues are also one‐particle energies that, multipled by the orbital occupancy probalities, enter the expression for the total N‐particle energy of the system.
Approximate solutions of Heisenberg Hamiltonians
Authors: Sanchez‐Marin J. , Malrieu J. P. , Maynau D.
Published: January 2005

Even when each atom of a 2n‐center cluster or molecule only brings one active electron in one atomic orbital, the size of the Heisenberg Hamiltonian matrix increases as C. Simple truncations of this matrix would result in size consistence defects, as evident from the isomorphism between Heisenberg and configuration interaction (CI) matrices. Geometry‐dependent Heisenberg Hamiltonians derived from accurate ab initio calculations on the two‐center systems have proved to be very efficient for conjugated hydrocarbons and for alkali metals; in order to apply this approach to intermediate size systems (10–20 centers), a rational procedure is proposed consisting of the selection of a truncated set of determinants (of low energy) and a dressing of the truncated Hamiltonian matrix under the perturbation of the other determinants. The second order dressing is analogous to a so‐called “shift Bk procedure” or Generalized Degenerate Perturbation theory and is weakly dependent on an E0 parameter. Tests performed on various 8‐ and 10‐atom systems show the accuracy of the procedure. An iterative selection of the truncated basis set and proper choices of the E0 values allow one to obtain the whole lower part of the spectrum. The calculated geometries are satisfactory. Some preliminary applications are reported concerning the C12H14 dodecahexene linear chain, perfectly fitting with previous extrapolations.
Molecular electrostatic fields from bond fragments
Authors: Nagy Péter , Ángyán János G. , Náray‐Szabó Gábor , Peinel Gustav
Published: January 2005

We apply completely transferable, strictly localized molecular orbitals for the calculation of molecular electrostatic fields. This approach, derived from our previous bond fragment method for the calculation of molecular electrostatic potentials, reduces computational efforts drastically. The fields around small molecules containing first‐ and second‐row atoms are systematically overestimated as compared with ab initio calculations with a minimal STO‐3G basis set. However, deviations can be corrected by a simple multiplicative factor, which means that the overall shape of the potential and field around the molecule is correctly reproduced. Our approximate field can be used to determine possible hydration sites around molecules as proposed earlier by Peinel and coworkers. Application of the method is illustrated on the formamide molecule.
Electron correlation described by extended geminal models: The EXGEM4 and EXGEM5 models
Authors: Røeggen I.
Published: January 2005

Within the framework of the general extended geminal model, two new approximate models EXGEM4 and EXGEM5 are introduced. The models are tested against full CI calculations on the water molecule for three different nuclear configurations and a full CI potential energy curve for the LiH molecule in the ground state. On the basis of these calculations, it is suggested that the models will yield electronic correlation energies with an accuracy of 1–2% of the corresponding full CI result.
Accurate electron density and one‐electron properties for the beryllium atom
Authors: Esquivel Rodolfo O. , Bunge Annik Vivier
Published: January 2005

An accurate analytical electron density for the beryllium atom is obtained by using a fast and systematic method recently developed and tested for the neon atom. Asymptotic conditions both at the nucleus and at large distances are obeyed. A point‐by‐point comparison between our density and the one obtained from an almost “exact” configuration interaction wave function shows that differences are less than 0.5% for r between 0 and 5 bohrs and less than 1 % up to 9 bohrs. The accuracy of the density is also assessed by comparing results of density moments and x‐ray scattering factors.
Polyatomic SCF calculations with numerical orbitals. I. Introduction and computational experiments on the CYBER 205
Authors: Alexander S. A. , Monkhorst H. J.
Published: January 2005

We discuss a procedure for calculating numerical Hartree–Fock orbitals that can be applied to polyatomic systems. This approach is formulated in momentum space to avoid Coulomb singularities and uses fast Fourier transforms to solve integral convolutions. Results for a number of simple systems are presented.
The double many‐body expansion of potential energy surfaces from interacting 2S atoms
Authors: Varandas A. J. C.
Published: January 2005

The double many‐body expansion (DMBE) method has been applied to the many‐valued ground‐state potential energy surfaces from three and four interacting 2S atoms. New forms are suggested where the degeneracy manifold of the two Riemann sheets of the potential surface can be made identical to that predicted from the London equation that is correct to first‐order. Considerable flexibility can be introduced through adjustable coefficients to fit ab initio data, whereas the potential energy terms can be chosen to maintain the correct analytic properties at the conical intersection. Hence, the new forms may be used on the extrapolation of the potential energy from the lowest‐ to the upper‐Riemann sheets. Long‐range effects are semiempirically described from the general formalism of the DMBE method.
On the linearly driven parametric oscillator
Authors: Fernández Francisco M. , Castro Eduardo A.
Published: January 2005

The time‐dependent Schrouml;dinger equation for the oscillator H = α(t)p2 + β(t)x2 + γ(t)x is exactly solved. The time evolution operator is easily obtained by means of an operator algebra and the quantum‐mechanical equations of motion. The problem is reduced to solving the classical equations of motion. The method is shown to apply to multidimensional oscillators.
Renormalized inner projection—a case study: The octic oscillator
Authors: Vinette Francine , Cizek Jiri , Vrscay Edward R.
Published: January 2005

The upper and lower bounds of a harmonic oscillator with an octic perturbation are studied with the use of renormalized inner projection. It is shown that this relatively simple technique works even in the infinite coupling constant limit. Symbolic computation is very convenient and useful in these types of problems, where only a finite number of operations are required.
An analytic method for three‐center nuclear attraction integrals: A generalization of the Gegenbauer addition theorem
Authors: Weatherford Charles A.
Published: January 2005

A completely analytic method for evaluating three‐center nuclear attraction integrals for STOS is presented. The method exploits a separation of the STO into an ‘evenly loaded’ solid harmonic and a OS STO. The harmonics are translated to the molecular center of mass in closed finite terms. The OS STO is translated using the Gegenbauer addition theorem; 1s STOS are translated using a single parametric differentiation of the OS formula. Explicit formulas for the integrals are presented for arbitrarily located atoms. A numerical example is given to illustrate the method.
A survey of optimization procedures for stable structures and transition states
Authors: Head John D. , Weiner Brain , Zerner Michael C.
Published: January 2005

We examine a variety of methods for obtaining the stable geometry of molecules and the transition states of simple systems and summarize some of our findings. We find the most efficient methods for optimizing structure to be those based on calculated gradients and estimated second derivative (Hessian) matrices, the later obtained either from the Broyden–Fletcher–Goldfarb–Shanno (BFGS) quasi‐Newton update method or from approximations to the coupled perturbed Hartree–Fock method. For uncovering transition states we find particularly useful a variety of the augmented Hessian theory used to uncover regions of the potential energy hypersurface with one and only one negative eigenvalue of the Hessian matrix characterizing the catchment region of the transition state. Once this region is found we minimize the norm of the gradient vector to catch the nearest extreme point of the surface. Examples of these procedures are given.
A practical solution to the “unknown normalization” problem
Authors: Macías A. , Martín F. , Riera A. , Yáañez M.
Published: January 2005

We analyze the theoretical basis of a procedure to determine an unknown normalization factor in discretized wave functions, which we have successfully used in a series of calculations of resonance widths for atomic and molecular systems. By reducing this determination to that of a suitable interpolation function for the energy eigenvalues, the problem is easily solved when atomic basis sets are chosen according to simple rules. Illustrations of our procedure are presented for atomic, molecular, and model systems; renormalized wave functions are compared with the exact ones for these model systems. The resulting method of renormalized continuum wave functions has a wide range of application in the study of long‐lived quasibound states (predissociation, autoionization, photoionization, unimolecular reactions, etc.).
Construction of linearly independent relativistic symmetry orbitals for finite double‐point groups including time reversal symmetry
Authors: Meyer J.
Published: January 2005

A formalism is developed for the construction of relativistic symmetry‐adapted molecular basis functions under consideration of time reversal invariance. The theory is applicable to the finite double point groups Cn, Cnh, Sn, Cnv, Dn, Dnd, Dnh, T, Th, Td, O, and Oh. It is based on the LCAO method. A projection operator technique is employed to construct molecular symmetry orbitals from atomic orbitals. The search for linearly independent basis function is simplified by means of group theoretical considerations.
Hall transformation of exp( – r): Correction of the results
Authors: Ten Hoor Marten J.
Published: January 2005

Owing to an error in his determination of the repulsion integral and the kinetic energy. Hall's results [1] for the ground state of atomic two‐electron systems are wrong. Here these results have been corrected. For Z ≤ 5, the results have been obtained by determining the pertinent integrals numerically: but for Z ≥ 5, these can be found sufficiently accurately from Z expansions.
Linear integrability of wave functions
Authors: Koga Toshikatsu , Thakkar Ajit J.
Published: January 2005

Finiteness is usually imposed as a condition for physical admissibility of a wave function. Examination of this condition in both position‐ and momentum‐space shows that finiteness of the wave function at the origin in one space implies “linear” integrability in the reciprocal space, except in some pathological cases. Some implications of this result are discussed.
Extrapolation of the total energy of polymers to the bulk limit using generalized padé approximants
Authors: Cioslowski J. , Bessis D.
Published: January 2005

Generalized Padé approximants are used to extrapolate the total energy of polymers described by alternant Hamiltonians to the bulk limit. The method provides an upper bound to the energy. The origin of (quasi) periodic oscillations of the energy per unit cell as the function of the number of unit cells is enlightened through analysis of the moments of the Hamiltonian.
Optical activity of small peptide chains with right‐handed α‐helix structure
Authors: Volosov Andrey
Published: January 2005

The optical activity of oligopeptides in the conformation of the right‐handed α‐helix was calculated by the direct semiempirical quantum chemical CNDO/OPTIC method. The oligomers of glycine and alanine from dimer up to pentamer were considered. The comparison with results obtained using the model of interacting groups (MIG) based on the perturbation approach was carried out. The calculational results show that the circular dichroism (CD) of oligopeptide α‐helices is essentially different from the CD of the peptide polymers in the same conformation. The comparison between results obtained by the CNDO/OPTIC method and by the MIG leads to doubts about the reliability of the use of the MIG to calculate rotatory strengths of nπ* transitions of oligopeptides in α‐helix conformation.
Molecular conformations and molecular shape: A discrete characterization of continua of van der Waals surfaces
Authors: Arteca Gustavo A. , Mezey Paul G.
Published: January 2005

A characterization of molecular model surfaces is proposed. It is based on a graph associated with the van der Waals surface, defined by the detailed information on the interpenetration of van der Waals spheres of the constituent atoms. This “van der Waals graph” describes the three‐dimensional body of the molecule, and it does not coincide in general with the less informative bond graph obtainable from the molecular skeleton.

The description in terms of the graph reveals clearly the changes in molecular shape induced by conformational rearrangements. The nuclear configurations can be classified by the graph associated with the molecular surface, and the graph‐theoretical analysis provides a rigorous partitioning of the configurational space based on shape properties.
A coupled‐cluster method for quasidegenerate states
Authors: Meissner L. , Jankowski K. , Wasilewski J.
Published: January 2005

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

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

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

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

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

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

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

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

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

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

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

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

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

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

The derivative of the electron–electron potential energy Uee with respect to internuclear separation R is studied for light homonuclear diatomic molecules at equilibrium. It is readily related to nuclear–nuclear potential energy Unn, the force constant K, and the electron–nuclear potential energy Uen. An approximate expression, based on the simplest form of density functional theory, is then used to eliminate dUen/dR|Re. The result thus obtained for dUee/dR|Re transcends an earlier proposal of Kryachko by including a term 2/3ReK, with K the force constant. Numerical tests at SCF–RHF level are presented for nine homonuclear diatomic molecules.