bluuebunny/crossref_metadata_2025_split huggingface dataset:
Ab initioMODPOT/VRDDO/MERGE calculations on energetic compounds. IV. Nitrocubanes: Mononitro to octanitro quantum chemical calculations and electrostatic molecular potential contour maps
Authors: Hariharan P. C. , Kaufman Joyce J. , Lowrey Alfred H. , Miller Richard S. 
Published: December 2004

AbstractAb initio MODPOT/VRDDO/MERGE calculations have been carried out for all the different position isomers of nitrocubane from mononitrocubane through octanitrocubane for a perfect symmetrical cubic cubane skeleton and for mononitrocubane through septanitrocubane for the almost cubic experimentally determined cubane skeleton. These calculations were carried out with our own rapid efficient ab initio programs which also incorporate a number of desirable computational strategies for calculations on large molecules. The skeletal total overlap population of the cubane skeleton (a theoretical index we showed years ago to be sensitive and predictive of stability of energetic molecular frameworks) indicates that successive nitration seems to increase the stability of the cubane skeleton. Successive nitration also seems to increase the total overlap population of the CNO2 bond. There are subtle differences depending on the exact positional isomer for a constant number of nitro groups—but the overall trend is definite. We have also generated electrostatic molecular potential contour (EMPC) maps around these nitrocubanes. These maps are indicative of preferred positions of electrophilic and nucleophilic attack as a function of the number of nitro groups or their positions. These EMPC maps can also indicate, to a first approximation, a limit on how close these molecules may be able to approach each other in a crystal.
Consistent propagator approximations
Authors: Weiner Brian 
Published: December 2004

AbstractA propagator approximation scheme is presented in the context of an abstract*‐algebra approach. The representation theory of such algebras is shown to play a crucial role in the definition of consistent approximations, i.e., approximate propagators based on model time evolutions and states. This procedure places superoperator methods of approximation on a sound Hilbert space footing. A generalization of the Fock vacuum property is introduced which leads to a simplification in the form of the model propagators. Finally a concrete example is considered that fulfills the conditions developed in this article showing that a consistent approximation to the electron propagator results in the Hartree–Fock–Boguliubov equations.
Energies of vibrating and rotating molecules by ladder operators
Authors: Huber Daniel 
Published: December 2004

AbstractA method which is well suited for calculating numerically the energies of the bent vibrating and rotating polyatomic molecule is given. By an analytical method using ladder operators the work of calculating the matrix elements of the Watson Hamiltonian is reduced to mere bookkeeping. This results in fast computation and good control of the errors introduced. To demonstrate the practical usefulness of the method, three test calculations on H2O and SO2 are reported and compared with results from the literature. It is intended to distribute the computer program developed to interested workers in the field.
Effect of hubbard interaction on solitonic excitations in polyacetylene
Authors: Kulkarni Anil D. , Alexander S. A. , Matsen F. A. 
Published: December 2004

AbstractWe have studied the effect of Hubbard interaction on solitonic excitations in the polymer trans‐polyacetylene. These excitations are modeled using the Su, Schrieffer, and Heeger (SSH) Hamiltonian. Within this framework, we have calculated the full configuration interaction and restricted Hartree‐Fock energies of solitonic excitations in trans‐polyacetylene chains with 4–10 carbon atoms. These results are extrapolated to longer chains, and it is shown that the correlation energy for these chains approaches 3%. Thus, our calculations provide a justification, hitherto absent, of the accuracy of the Hartree‐Fock calculations reported in the literature.
Least‐squares numerical Rayleigh‐Ritz and minimum‐variance methods for molecular calculations
Authors: Thole B. T. 
Published: December 2004

AbstractA general method is presented to find in a least‐squares sense a set of orthogonal eigenfunctions and their eigenvalues from local energy and numerical integration methods or by any other dissymmetric approach to solve the eigenvalue problem of a Hermitian operator. By this method a generalization of the minimum variance method to more than one eigenfunction is obtained, which is a variant of Scott's method. Also a new method is derived—called the minimum‐overlap method—that is a least‐squares numerical version of the standard Rayleigh‐Ritz method. Test calculations on the atoms Be and Tm and the molecules H2 and CO have been performed with both numerical Hartree‐Fock and Hartree‐Fock‐Slater methods. The least‐squares solutions are an improvement over other methods in the case of accurate basis sets. Numerical Hartree‐Fock calculations of moderate accuracy are found to be considerably faster than the analytic method.
Adiabatic SCF potential energy curves relevant to proton–oxygen molecular collisions
Authors: Staemmler V. , Gianturco F. A. 
Published: December 2004

AbstractThe large vibrational inelasticity observed in proton–O2 scattering at collision energies of ∼10.0 eV and for various angular distributions below the rainbow region is examined in terms of SCF adiabatic potential energy surfaces which exhibit strong O “character” within a specific range of relative distances between colliding partners. The possibility for this O2 distortion during encounters with protons to be responsible for the unusually large inelasticity is discussed and the time factors that are likely to play an important role during the dynamics are also considered and analyzed.
Effect of some factors on the energy band structure of proteins
Authors: Yi Xiao , Run‐Sheng Chen 
Published: December 2004

AbstractThe effects of four factors (different side chains, side‐chain disorder, conformational change, and ions and water) on the energy band structures of proteins have been investigated with the aid of the CNDO/2 crystal orbital method. The results indicate that these factors are very important. The consequences of these effects on the semiconductive properties of proteins are discussed.
A quality test for the Hartree‐Fock‐Roothaan SCF wave functions
Authors: Sordo J. A. , Pueyo L. 
Published: December 2004

AbstractA quality test for the SCF wave function based on separate examination of the one‐ and two‐electron contributions to the total energy is proposed. The test is applied to 12 different STO basis sets for the ground state of the Ni atom and its predictions compared with those deduced for the minimization of the total energy. The new test allows for a reduction in the cancellation of errors detected in some applications of the standard SCF procedure. Some relations between the quality of the basis set and its size and structure are discussed.
Orbital correlation diagrams based on multiconfigurational variation of moments I. Theory
Authors: Müller‐Remmers Peter L. , Jug Karl 
Published: December 2004

AbstractA multiconfigurational variation of moments (MCM) is presented to determine correlated orbital energies. Appropriate multiconfigurational one‐particle operators are introduced starting for closed‐shell systems from the restricted Hartree–Fock (RHF) scheme and for open‐shell system from the unrestricted Hartree–Fock (UHF) scheme. The advantage of these operators is the linear form of the correlation energy in the configuration interaction (CI) coefficients. They obey HF analogous pseudoeigenvalue equations and are therefore a generalization of the HF operator. In principle, the scheme can be extended to a multiconfiguration self‐consistent field (MCSCF) procedure for moments. In contrast to the SCF formalism they allow an orbital description of degeneracy and quasidegeneracy of the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbital. With these operators, funnels of thermally forbidden Woodward–Hoffman reactions, as well as dissociations, can be described in a physically meaningful fashion by orbitalcorrelation diagrams.
Orbital correlation diagrams based on multiconfigurational variation of moments II. Application
Authors: Müller‐Remmers Peter L. , Jug Karl 
Published: December 2004

AbstractA formalism was developed in the multiconfigurational variation of moments (MCM) framework, which yields physically meaningful orbital energies for occupied and virtual orbitals starting from self‐consistent field (SCF) calculations. This is possible through a skillful distribution of the correlation energy on the orbital energies. The application of this method is demonstrated by SINDO1 calculations on the dissociation of H2 and the following symmetry‐forbidden reactions: (1) torsion of ethylene; (2) ring opening of (a) cyclobutene, and (b) cyclopropyl cation; (3) cycloreversion of 1, 1‐dicyano‐2‐methoxycyclobutane. The allowed reactions corresponding to 2a and 2b are investigated in the SCF scheme. The energy hypersurfaces are calculated for all reactions and the MO correlation diagrams are presented and discussed.
On the electronic structure of transition‐metal complexes II. Bonding characteristics in titanium chloride systems
Authors: Hilal Rifaat 
Published: December 2004

AbstractThe electronic structure of [TiCl4]n systems (n = 0, −1, −2, −3, and −4) has been investigated by the INDO method. Calculations show the stability of the tetrahedral structure over the square‐planar one, for any value of n, in accord with experimental observation. The occupation of the 4p orbitals of the titanium atom was found to increase regularly as the dihedral angle decreases and reaches a maximum at the square‐planar conformation. Energy partitioning analysis enabled much deeper understanding of bonding characteristics of the studied systems. It has been shown that the one‐atom energy component may be a very sensitive measure of the stability of the central metal atom in its various oxidation states.
Theoretical investigations of the electronic states of porphyrins. III. Low‐lying electronic states of porphinatoiron(II)
Authors: Rawlings Diane C. , Gouterman Martin , Davidson Ernest R. , Feller David 
Published: December 2004

AbstractAb initio configuration interaction calculations are reported on the lowest quintet, triplet, and singlet states of FeII(P). Due to the large number of states found, a catalog of the low‐lying states is presented. Novel triplet and quintet charge‐transfer states are reported as low as 1.3 eV. These states are d5 (S = 5/2) on the iron low‐spin‐coupled to the radical anion excited porphyrin ring (S = 1/2 or 3/2). Oscillator strengths originating from each of three low‐energy triplet states are reported.
Correlation contributions to the radical cation states of linear even polyenes: An open‐shell RHF‐CNDO/S(CI) analysis of the optical and photoelectron spectra
Authors: Bigelow Richard W. 
Published: December 2004

AbstractConfiguration interaction wave functions are calculated for the low‐lying radical cation states of trans‐butadiene, hexatriene, and octatetraene within the open‐shell RHF‐CNDO/S(CI) approach. The consequences of various one‐electron contributions to the interpretation of existing photoemission and radical cation optical spectra are emphasized. Electron correlation is shown to be essential to achieve adequate energy and intensity profiles assuming photoelectron or optical excitation. The excitation energies and transition amplitudes (optical and photoemission) are also found to be sensitive to the molecular geometry. The present results are consistent with previous interpretations that photoionization measurements probe the neutralmolecule alternating single‐double bond‐length structure, whereas optical excitation samples an ion‐state–state–induced “relaxed” reference configuration having a weakened bond‐length alternation. Calculated trends in the spectroscopic properties are extrapolated to extended members of the even‐polyene series.
Intermolecular interactions: Elaboration on an additive procedure including an explicit charge‐transfer contribution
Authors: Gresh N. , Claverie P. , Pullman A. 
Published: December 2004

AbstractAn additive procedure is derived for the computation of intermolecular interactions, in which an explicit expression for the charge‐transfer energy contribution ECT is implemented. In the total interaction energy,

the electrostatic terms EMTP and Epol are calculated as in our previous treatment. The dispersion contribution is calibrated by reference to variation‐perturbation computations on model systems and the repulsion contribution is computed as a sum of bond—bond, bond—lone pair, and lone pair—lone pair interactions. Tests of the procedure are given for representative hydrogen‐bonded systems.
Direct evaluation of spin representation matrices and ordering of permutation‐group elements
Authors: Rettrup Sten 
Published: December 2004

AbstractA direct and general method is presented for constructing the orthogonal spin representation matrices (irreps) of the permutation group corresponding to the Yammanouchi‐Kotani coupling scheme. For arbitrary permutations the irreps are constructed directly from the Young tableaus by a process which is, in general, only quadratic in the number of spin eigenfunctions, but which in actual computations becomes linear on vector computers for moderate sizes of the matrices. We also introduce a graphical representation of the group elements and a universal lexical ordering of permutations. The methods have been implemented and computational examples are presented.
Comparison of theory and experiment for excited singlet states of the H2 molecule
Authors: Dressler K. , Wolniewicz L. , Quadrelli P. 
Published: December 2004

AbstractThis paper presents a survey of published and unpublished ab initio calculations of the vibrational structures of the ten lowest electronic singlet states of the hydrogen molecule up to the H(n = 1) + H(n = 2) dissociation limit. The data are based on adiabatic potential functions (clamped‐nuclei electronic energies and nuclear‐mass‐dependent diagonal corrections). Nonadiabatic coupling has been treated ab initio within the five states. of 1Λ symmetry (X,EF, GK, HH̄) and 1Σ I.1Πg. The accuracies of the theoretical energies are determined by comparisons with experimental data for H2, HD, and D2. The level shifts and predissociation probabilities of the excited 1Σ states, generated by nonadiabatic coupling with the discrete and continuous vibrational structure of the ground state, and radiative properties have also been calculated.
Use of coupled‐cluster based linear response theory and multireference hermitian MBPT to IP calculations of HF
Authors: Roy S. , Sengupta S. , Mukherjee D. , Mukherjee P. K. 
Published: December 2004

AbstractWe report in this paper the results of outer and inner valence IP calculations for the HF molecule using two different many‐body methods for the direct evaluation of energy differences. The first is the nonperturbative coupled‐cluster based linear response theory (LRT) and the second is the hermitian open‐shell many‐body perturbation theory (MBPT). A Huzinaga‐Dunning (9s5p→ 5s3p/3s) basis has been used. LRT uses an “ionization operator” S as in the equation of motion method (EOM) to generate the ionized states from a coupled‐cluster type of ground state. S is chosen to consist of single ionization and ionization‐cum‐shake‐up operators, thus treating the Koopmans as well as the shake‐up states on equal footing. LRT would thus be capable of computing both the outer and the inner valence regions with equal facility. This is borne out by the results. For the open‐shell MBPT, the model space is chosen to be spanned by the singly ionized determinants. The convergence of the results for the inner valence region is slow, and the results obtained from the [2, 1] Pade' approximants are presented. Unlike the LRT, the inner valence region is not reproduced with full complexity in MBPT, indicating that it is essential to modify the theory by way of expanding the model space to contain the shake‐up determinants also.
LCGTO‐LSD calculations for CH2
Authors: Radzio E. , Salahub D. R. 
Published: December 2004

AbstractLCGTO‐LSD calculations for ground (3B1) and excited (1A1) states of methylene, CH2, have been performed. Various exchange‐correlation potentials and a variety of basis sets (including f functions) have been used. For both states the LSD optimized geometry agrees well with both experimental and the most advanced ab initio results. The correct ground state is found, and the 1A1‐3B1 energy separation was found to be ∼ 15 kcal/mol, using the “best” local exchange‐correlation potential (VWN), the experimental value being ∼ 9 kcal/mol. This result compares favorably with the Hartree‐Fock limit separation of 25 kcal/mol. The Kohn‐Sham exchange potential leads to a gap of ∼26 kcal/mol.
Charge distributions and chemical effects. XLII. Atomic charges and their role in energy calculations
Authors: Fliszár S. 
Published: December 2004

AbstractA physically meaningful charge partitioning rooted in Mulliken's scheme meets with success, provided the usual equipartitioning of overlap populations involving dissimilar atoms is abandoned in favor of a constraint rendering all alkane carbons as similar as possible to one another. Charge analyses are to be carried out after configuration interaction involving reasonably large optimized basis sets. At this level, both the relative ordering and the magnitude of theoretical atomic charges in hydrocarbons are precisely those required for energy calculations of alkanes and alkenes featuring atomic charges in an explicit manner.
Invalidity of the ubiquitous mass‐velocity operator in quasirelativistic theories
Authors: Farazdel Abbas , Smith Vedene H. 
Published: December 2004

AbstractThe general problem of relativistic corrections to the kinetic energy in quasirelativistic theories, is discussed and related formulas are developed. It is shown that the well‐known mass‐velocity operator, Hmv = (−α2/8)p4, is incorrect and does not provide any proper relativistic corrections.
What do kinetic‐energy anisotropies tell us about chemical bonding? I. Diatomic hydrides
Authors: Sharma B. S. , Thakkar Ajit J. 
Published: December 2004

AbstractThe kinetic‐energy anisotropies of fifteen diatomic hydrides AH with A = H, Li, Be, B, C, N, O, F, Na, Mg, Al, Si, P, S, Cl are calculated from self‐consistent‐field wave functions constructed from extended basis sets of Slater‐type orbitals. It is found that there is no consistent ordering of the bond‐parallel and bond‐perpendicular components of the kinetic energy with respect to separated atom values. An analysis of the orbital contributions reveals that nonbonding π orbitals make large contributions to the total kinetic‐energy anisotropy. This makes it difficult, if not impossible, to deduce anything about the nature of the chemical bond from the total anisotropy. However, certain dimensionless orbital kinetic‐energy anisotropies are useful for interpretative studies because, in free atoms, these quantities have fixed values that depend only on the symmetry of the orbital.
Counterpoise corrections to the components of bimolecular energy interactions: An examination of three methods of decomposition
Authors: Bonaccorsi Rosanna , Cammi Roberto , Tomasi Jacopo 
Published: December 2004

AbstractCounterpoise corrections for the basis set superposition error to the components of the bimolecular interaction energy are defined for three methods of decomposition. The results for the case of the NH3 + BH3 interaction are presented and discussed.
Study of photoelectron spectra of some XPY3 molecules (X = O, S and Y = Cl, Br) by the SCF‐Xα‐SW method
Authors: Pan Y. K. , Xin M. , Li D. , Chiu Lue‐Yung Chow 
Published: December 2004

AbstractIn this paper we extend our previous studies to investigate the ionization energies of some XPY3 molecules (X = O, S and Y = Cl, Br). The calculated orbital energies agree very well with reported experimental ionization energies. The molecular orbital orderings obtained coincide with recent experimental orbital assignments. The results are also compared with previous ab initio and semiempirical calculations for OPCl3, OPBr3, SPCl3, and SPBr3 molecules. The comparison indicates that the present results show improved agreement with experiment and clarify certain ambiguities in the earlier assignments.
Combining perturbation theory techniques with variational CI calculations to study molecular excited states
Authors: Buenker Robert J. 
Published: December 2004

AbstractThe accurate treatment of molecular excited states is based on a matrix solution of the electronic Schrödinger equation in a suitable AO basis which relies heavily on the existence of a good approximate description of a given eigenfunction prior to the solution of a secular equation. The manner in which one or more roots corresponding to excited states can be traced from a rather simple starting point to the final results obtained from a large‐scale CI treatment is outlined. In such calculations it is very useful to have a measure of the energy‐lowering capability of configurations which differ by at most a double excitation from a series of reference configurations which represent the dominant contributions to one or more electronic states. The efficient computation of the required Hamiltonian matrix elements for this purpose, as well as for the construction of the final secular equations, can be achieved through the use of the table CI method, which does not rely on any special relationship between configurations to complete its work. Employing CI expansions of length no greater than 10000‒20000 terms is very advantageous for the calculation of special properties of the resulting wavefunctions, especially those of spin‐dependent quantummechanical operators. With the help of perturbative corrections it is possible to obtain energy results with such methods which are accurate to within 0.01 hartree of the full CI limit [R. A. Phillips, R. J. Buenker, P. J. Bruna, and S. D. Peyerimhoff, Chem. Phys. 84, 11 (1984)], whose error is small compared to that inherent in the use of conventional double‐zeta‐plus‐polarization AO basis sets, therefore making further reduction of the CI errors almost inconsequential. Numerous examples for the calculation of the energy and properties of molecular excited states are surveyed.
Optimal spacing and weights in diffusion monte carlo
Authors: Vrbik Jan , Rothstein Stuart M. 
Published: December 2004

AbstractWe describe how to select optimal values of the time step τ and how to allocate computer time in order to minimize the error bar of the unbiased energy estimate (τ = 0 limit) in diffusion Monte Carlo simulations (DMC). Several polynomial models are considered, appropriate for both linearly biased and quadratically biased algorithms. An efficient nonoptimal design is also reported, which has the advantage of checking the model's adequacy. Further, we compare the efficiency of linearly biased and quadratically biased DMC, and we estimate the efficiency of DMC versus Green's function Monte Carlo.