bluuebunny/crossref_metadata_2025_split huggingface dataset:
On the long way from the Coulombic Hamiltonian to the intermolecular energy surfaces: Concluding remarks at the tromsø conference, June 20–22, 1989
Authors: Löwdin Per‐olov 
Published: January 2005

AbstractAfter a brief review of the main results given at the conference, the general properties of the Coulombic Hamiltonian for a system of electrons moving in a framework of moving atomic nuclei—considered as point charges—are discussed. Since this Hamiltonian is invariant under translations and rotations, the total momentum and the total angular momentum are constants of motion, which means that it is possible to separate the motion of the center of mass and the rotation of the system as a whole. Even if these separations are simple in principle, they lead to a mixing of the electronic and nuclear coordinates that complicates the transformed Hamiltonian. The general features of this Hamiltonian are discussed both in pure quantum mechanics and general quantum theory dealing with wave functions Ψ respective density matrices ρ or system operators T. The principles of the latter are derived from five simple axioms, and it is shown that pure quantum mechanics is a special case of the general theory and that the analogy between these two approaches is essential for the “economy of thinking.” It is indicated that the general theory of the shape and topology of the energy surface 〈H〉 = TrHΓ and its critical points, as a function of the system operator Γ involving both electronic and nuclear coordinates, is a very difficult mathematical problem and that calculation of this surface even for simple molecular systems represents a formidable computational problem, which has to be solved in order to be able to understand the nature of chemical reactions from first principles.
The effect of interlayer interactions on chemisorption pattern stability: Hydrogen atoms on graphite
Authors: LaFemina John P. 
Published: January 2005

AbstractExtended Hückel band calculations, frontier crystal orbital analysis, and degenerate‐level perturbation theory are used to evaluate the effect inclusion of bulk‐layer interactions has on the relative stabilities of ordered overlayers. Five patterns of hydrogen atoms chemisorbed on unreconstructed graphite (11% coverage) are examined for the Bernal (ABABAB…) and registered (AAA…) three‐dimensional stackings. The single‐layer two‐dimensional approximation for graphite is found not to be entirely satisfactory for the type of chemisorption pattern stability questions addressed in this paper.
Explicit algebraic form of coupled cluster equations for the PPP model of benzene with an approximate inclusion of triexcited clusters
Authors: Čížek J. , Paldus J. , Vinette F. 
Published: January 2005

AbstractAn important role of model Hamiltonians for quantum mechanical description of the molecular electronic structure is discussed, and the effectiveness of the Pariser–Parr–Pople (PPP) model in calculations of molecular properties, in building of other models, and in various theoretical exploits is emphasized. The striking simplicity and transparence, yet remarkable reality and credibility, of this model is illustrated by applying the basic coupled cluster approach (CCD), as well as its extension approximating triexcited clusters (CCSDT‐1), to the PPP model of benzene. The explicit final equations are obtained using the symbolic computation language MAPLE for both the orthogonal and nonorthogonal versions of the CC formalism. These equations are presented using the interface between MAPLE and the typesetting language TROFF. In this way, the explicit CC equations are obtained in terms of basic parameters of the model, starting from the CI description and showing their equivalence with those obtained via diagrammatic techniques. A new insight into the structure of CC equations at various levels of approximation is thus achieved and a useful model for electron correlation is provided.
Transferability of the electronic structure characteristics of saturated molecules
Authors: Gineitytė V. , Shatkovskaya D. 
Published: January 2005

AbstractThe common quantum mechanical problem for the total class of saturated hydrocarbons has been stated and solved within the framework of the effective Hamiltonian method. A quantum mechanical substantiation of the transferability of the electronic structure characteristics (bond dipole moments, bond energies, and bond force constants) as well as the investigation of the origin of transferability have been considered for the stated class of chemical compounds.
The fate of the dry electron—A preliminary investigation
Authors: Mann Ady , Green Alex 
Published: January 2005

AbstractA set of electron impact cross sections for vibrational excitation of water in the gaseous phase are first presented. The associated loss function is used in conjunction with a rotational loss function to generate efficiencies for the conversion of low‐energy electron energy to rotational and vibrational excitation. An analogous calculation is carried out for intramolecular and intermolecular modes in the liquid phase leading to similar results. We also examine a simple model for the time dependence of the electron hydration process based on a square well with a time‐dependent radius and well depth. We can fit the time and wavelength dependence of the experimental absorption coefficients if we introduce another time‐dependent parameter that might be encompassed by a diffuse potential well model.
A non‐local pseudopotential in the FSGO model: Study of some organometallic systems
Authors: Kaur S. , Ray N. K. 
Published: January 2005

AbstractA non‐local core pseudopotential has been used in the framework of floating spherical Gaussian orbital (FSGO) model to study the equilibrium geometries and valence electronic structures of some organolithium and organoberyllium systems. The calculated equilibrium geometries, heats of hydrogenation, average electric polarizabilities, and magnetic susceptibilities are in good agreement with the results of the all‐electron FSGO model calculations. Valence electron wave functions obtained here have been used to predict the valence electron Compton profiles (CP) and electron momentum distributions (EMD) of the systems studied. A good correlation has been shown among the peak height of the CP (J(0)), valence electron energy (Ev), and number of valence electrons (Nv).
Monohydration selectivity mechanism of alkali cations in the potassium channel of excitable biomembranes
Authors: Kariev A. M. , Ovchinnikov A. A. 
Published: January 2005

AbstractNonempirical quantum chemical method Hartree–Fock–Roothan LCAO SCF MO in a two‐exponent Dunning basis with the use of an extended set of Gaussian functions by Huzinaga–Dunning with consideration of electron correlation according to the Meller–Plesset theory of excitations of the second order was used to study monohydrates of Li+, Na+, K+, and HCOO− ions. The indicated basis was supplemented with polarization functions of d‐type on the O atom and of p‐type on the hydrogen atom as well as with diffusion functions of p‐type on the oxygen atom. It has been found that binding energies of the water molecule with Li+, Na+ appeared to be higher and with K+ lower than with HCOO− · H2O. Potential curve shapes of K+ + H2O and HCOO− + H2O reactions are shown to be similar. The molecular mechanism of K+ channel selectivity of an excitable membrane is explained on the basis of the obtained calculations.
Exciton interactions in the spectrum of a dinaphthonorbornadiene
Authors: Strickler S. J. , Cormier Russell A. , Connolly John S. 
Published: January 2005

AbstractThe near‐ultraviolet spectrum of a dinaphthonorbornadiene is reported and is interpreted using simple exciton theory that treats this molecule as a dimer of naphthalene. It shows examples of weak, intermediate, and strong coupling for the interaction of the three observed bands of naphthalene.
Theory of ultrafast time‐resolved absorption spectroscopy
Authors: Yeh C. Y. , Chang W. L. , Ma H. , Lin S. H. 
Published: January 2005

AbstractIn this paper, the density matrix formalism has been applied to treat ultrafast time‐resolved absorption spectroscopy. We have shown that in the femto‐second (fs) pump‐probe experiments, the observed time‐resolved absorption spectra consist of the contributions from the population (i.e., incoherent contribution) and the coherence (i.e., the phase of the system). The adiabatic approximation has been used to derive the expressions for ultrafast time‐resolved spectra. We have also shown that the dynamics of the coherence will result in quantum beat. Numerical calculations have been performed to demonstrate the theoretical results.
Solvent effects in dense gases
Authors: McGlynn S. P. , Asaf U. , Kumar D. , Felps S. , Rupnik K. , Klasinc L. 
Published: January 2005

AbstractThe effects of perturber gases, partial pressures in the range 0–50 atm, on the absorption spectra of high‐n Rydbergs, n ≥ 10, in methyl iodide and benzene have been investigated. The perturbers were rare gases and H2. It has been shown that the spectroscopic energy shift Δ = Δ(ρ, n) is given by Δ = Δ°ρ, where ρ is the number density of the perturber, for n ≥ 10. The shift data can be interpreted using a Fermi model, and values of scattering lengths for perturbers can be extracted. These scattering lengths are internally consistent, independent of the absorber, and in excellent agreement with electron‐swarm results. Given that the Fermi model is nonspecific and microscopic, scattering shifts and lengths for mixtures of perturber gases can be predicted. These predictions, having been verified by experiment for He/Ar mixtures, provide a means of “tailoring” any required scattering length. Finally, it is shown empirically that the determinative scattering parameter is the polarizability of the perturber, and a universal relationship a = −0.26α + 1.18 between scattering length a, in Bohr radii, and polarizability α is found to exist.
Ab initio simulation of benzene Raman intensities
Authors: Ozkabak Ali G. , Thakur Surya N. , Goodman Lionel 
Published: January 2005

AbstractC6H6 Raman scattering activities calculated from harmonic model ab initio Hartree–Fock 6–311 ++ G(d, p) polarizability derivatives (and harmonic force fields calculated at the same level) accurately simulate experiment (to within 1% for the a1g modes). Accurate predictions are also made for the e2g modes (to within 5% for ν7 and ν9, and more poorly for ν6 and ν8 [in Fermi resonance with ν6 + ν1]) and for the e1g out‐of‐plane mode, ν10. Only the ν6 in‐plane CCC bending mode scattering activity is found to be anomalous. Systematic variation of the basis set indicates that the benzene scattering activities and depolarization ratios are strongly dependent on inclusion of both carbon and hydrogen atom diffuse functions in the basis set. Predictions are also made for 12C6D6 and for unmeasured intensities in 13C6H6. Measurements of a1g mode scattering activities in the latter molecule are predicted to be useful in testing the harmonic Hartree–Fock Raman intensity model.
High‐resolution infrared analysis of the ν17 band of pyridine
Authors: Thiel Yvonne , Walters Valerie A. , Wiberg Kenneth B. , Colson Steven D. 
Published: January 2005

AbstractA method for analyzing asymmetric top rovibrational bands displaying both blended and resolved features is described. The two‐phase computational procedure uses a modified version of the asymmetric rotor‐band contour program FASTPLOT to generate a preliminary set of upperstate spectroscopic constants. The parameters are subsequently refined by employing the assigned line‐fitting formalism of the ASYROT program using both resolved and blended features. The technique is detailed in a comprehensive analysis of the ν17 band of pyridine. Inclusion of quartic centrifugal distortion constants was found to satisfactorily model a high‐resolution (0.004 cm−1) spectrum of this band, yielding a standard deviation of 0.00137 cm−1. The variation in the rotational parameters with vibrational quantum number is examined in terms of possible weak rovibrational perturbations to the ν17 state. An ab initio calculation of the ν17/ν27 Coriolis coupling constant indicates the observed results are consistent with the interaction of these two vibrational states.
Ab initio MRD‐CI calculations for breaking a chemical bond in a molecule in a crystal or other solid environment. III. Me2NNO2 decomposition of dimethylnitramine in a large crystalline environment
Authors: Chapman Douglas A. , Roszak Szczepan , Keegstra Phillip B. , Hariharan P. C. , Kaufman Joyce J. , Buenker Robert S. 
Published: January 2005

AbstractRecently we extended our strategy for MRD‐CI (multireference double excitation‐configuration interaction) calculations, based on localized/local orbitals and an “effective” CI Hamiltonian, for molecular decompositions of large molecules to breaking a chemical bond in a molecule in a crystalline or other solid environment.Our technique begins with an explicit quantum chemical SCF calculation for a reference molecule surrounded by a number of other molecules in the multipole environment of more distant neighbors. The resulting canonical molecular orbitals are then localized, and the localized occupied and virtual orbitals in the region of interest are included explicitly in the MRD‐CI with the remainder of the occupied localized orbitals being folded into an “effective” CI Hamiltonian. The MRD‐CI calculations are then carried out for breaking a bond in the reference molecule. This method is completely general in that the space treated explicitly, as well as the surrounding space, may contain voids, defects, deformations, dislocations, impurities, dopants, edges and surfaces, boundaries, etc.Dimethylnitramine is the smallest prototype of the energetic R2N—NO2 nitramines, such as the 6‐member ring RDX or the 8‐member ring HMX. Decomposition of energetic compounds is initiated in the solid by a breaking of the target bond. Thus, it is crucial to know the difference in energy between breaking a bond in an isolated energetic molecule versus in the molecule in a solid. In the present study, we have carried out MRD‐CI calculations for the Me2N—NO2 dissociation of dimethylnitramine in a dimethylnitramine crystal. The cases we investigated were one dimethylnitramine molecule (surrounded by 53 and 685 neighboring dimethylnitramine molecules represented by multipoles), three dimethylnitramine molecules, and three dimethylnitramine molecules (surrounded by 683 neighbors). All multipoles were cumulative atomic multipoles up through quadrupoles. The MRD‐CI calculations on dimethylnitramine required large numbers of reference configurations from which were allowed all single and double excitations.
A partial recurrence relation for reduced class coefficients of the symmetric group
Authors: Katriel Jacob 
Published: January 2005

AbstractAn expression for the product of a single‐cycle class [(1)N ‐ P(p)]N and an arbitrary class [(1)l1(2)l …︁ (N)lN]N of the symmetric group has recently been conjectured. This expression involves a sum over a relatively small number of reduced class sums, depending on p indices. A further conjecture is formulated and demonstrated, according to which reduced class coefficients (RCCS) involving cycles whose length is expressed by means of a single index can be related to corresponding coefficients in the product of [(1)N ‐ P+1(p ‐ 1)]N with an arbitrary class sum. Consequently, the problem of evaluating the general class sum product reduces to that of obtaining a relatively small set of fundamental RCCS containing no single‐index cycles. The conjectures mentioned can be used to evaluate the product [(1)N ‐ p(p)]N · [(1)N ‐ q(q)]N in terms of fundamental RCCS that can all be obtained from the product [(r)]r · [(r)]r, where r = min(p, q). For the latter product, we use a result due to Boccara.
SCF theory of molecular interactions
Authors: Grant A. J. , Pickup B. T. 
Published: January 2005

AbstractPerturbation theory is used to analyze the interactions between two closed‐shell systems in the SCF approximation. The effects of orbital overlap are included. The dominant terms through second order are identified, including electrostatic, penetration, polarization, charge transfer, and SCF dispersion effects. The basis set limit is studied so that basis set superposition contributions may be identified. Calculations are presented for HeHe and Na+H2O.
The generalized Slater–Condon rules
Authors: Verbeek Jacob , Van Lenthe Joop H. 
Published: January 2005

AbstractBy relating the blocking structure of the relevant matrix of overlap‐integrals to its cofactors, the Slater–Condon rules for the evaluation of an element of a matrix representation of an electronic Hamiltonian in a Slater determinant basis are generalized to the case where not all orbitals are orthogonal. This yields a set of 33 rules, which allows for an efficient implementation of the valence bond theory.
Quantum‐Chemical modeling of catecholamine storage including continuum solvent effects
Authors: Góamez‐Jeria J. S. , Parra‐Mouchet J. , Morales‐Lagos D. 
Published: January 2005

AbstractA model for catecholamine storage in vesicles is analyzed within the SCRF‐CNDO/2 approach including continuum solvent effects. The model considers the approach of cationic norepinephrine (NE) to a positively charged guanidinium moiety. Ion‐pair formation is found for the whole range of dielectric constants. Even though stable states of H‐bonded partners are found for large dielectric constants, this process is ruled out to occur because it involves too high energies. It appears that the medium's polarity is determinant in lowering the energy barrier between the ion‐pair complex and the separated partners. Thus, as the medium dielectric constant increases, the equilibrium between the two states is enhanced.
Normalized irreducible tensorial matrix expansions applied to an effective sp‐type Hamiltonian for the PES of water
Authors: Ellzey M. L. 
Published: January 2005

AbstractAny matrix can be expanded on a basis of SU(2) normalized irreducible tensorial matrices, NITM, defined in terms of 3‐j symbols or coupling coefficients of SU(2). The NITM transform under rotations according to Wigner's matrices. If one dimension of an NITM is odd and the other even, the tensor has half‐integer rank. A simple NITM basis consists of all NITM having the same numbers of rows and columns as the expanded matrix. A compound NITM basis consists of two or more simple bases, each spanning a corresponding block in the expanded matrix. The choice of NITM basis for expanding an effective Hamiltonian matrix is a crucial step in formulating a model. To illustrate the use of a compound NITM basis, including nonsquare NITM, an effective sp‐type overlap‐free superposition Hamiltonian is constructed and applied to the photoelectron ionization potential spectrum of water.
Second‐order correlation potential in the Kohn–Sham approximation for atoms
Authors: Boada R. L. , Karasiov V. V. , Labzowsky L. N. 
Published: January 2005

AbstractA new type of correlation functional derived from the second‐order expression for the correlation energy of an atom is proposed. The derived correlation potential contains one free parameter, which is determined by fitting the known pair correlation energy. The calculations with this potential in the Kohn–Sham approximation give rather accurate values for the matrix elements of different operators.
Second‐order perturbational treatment of normal coordinates in the string model for the hydration reaction of formaldehyde
Authors: Tachibana Akitomo , Fueno Hiroyuki , Yamato Masaya , Yamabe Tokio 
Published: January 2005

AbstractA perturbation theory for normal coordinates of nonadiabatic solvation is presented by means of the “string model” of chemical reactions. The dynamic normal coordinate is introduced for the perturbational treatment of the “intrinsic” normal coordinates that are orthogonal to the reaction path. The reaction is defined as the intrinsic reaction coordinate (IRC) that is treated as a string. The string is thrown in the external force field that acts as a nonadiabatic source of perturbation. As an application of the present treatment, the effect of a water molecule for hydration reaction of formaldehyde is calculated. A second‐order perturbation effect for the enhancement of the reaction rate is found.
A quantum chemical investigation of pyrolysis reactions of glyoxylic acid ethylester
Authors: Bøhm Stanislav , Skancke P. N. 
Published: January 2005

AbstractTwo possible reaction paths for the pyrolysis of the ethylester of glyoxylic acid have been studied by ab initio molecular orbital calculations. The basis sets 3‐21G and 6‐31G* have been used, and electron correlation has been included by Møller–Plesset calculations up to fourth order. Our calculations indicate that the reaction leading to acid and ethylene through a 6‐membered ring transition state is favored relative to a process involving a formyl hydrogen transfer via a 5‐membered ring to the alkyl unit leading to ethane, CO, and CO2. The predicted activation energies for these two reactions obtained at the highest level of calculation, MP4(SDTQ)/6–31G*, are 50.4 and 71.7 kcal/mol, respectively. The transition states have RHF wave functions that are stable relative to UHF solutions using the 3–21G basis. The geometry of the transition states and IRC following indicate that both reactions are strongly asynchronous: The CO bond rupture is virtually completed before hydrogen transfer occurs. For comparative purposes, analogous calculations have been performed for the ethylester of formic acid, where it is confirmed that a 6‐membered ring transition state is preferred relative to a 4‐membered one by around 42 kcal/mol at the highest level of calculation.
Analysis in terms of valence‐bond structures of environmental effects on the electronic structure of molecules
Authors: Solà Miquel , Lledòs Agustí , Duran Miquel , Bertràn Juan 
Published: January 2005

AbstractEnvironmental effects on the electronic structure of molecules caused by external perturbations are studied by means of an expansion of molecular orbital wave functions into valence‐bond (VB) wave functions. Applications are carried out to the π system of butadiene, hydroxyethylene, acrolein, and carbonyl oxide and also to the full electronic system of water. The external perturbations considered are uniform electric fields, nonuniform electric fields, and reaction fields created by solvents. A different behavior is found between nonpolar and polar species in the presence of solvents. Some consequences on the behavior of molecules under the influence of the different electric fields are discussed.
A time‐dependent molecular orbital approach to electron transfer in ion–metal surface collisions
Authors: Feng Eric Q. , Micha David A. , Runge Keith 
Published: January 2005

AbstractA time‐dependent molecular orbital method has been developed to study charge transfer in collisions of ions with metal surfaces at energies between 1 and 100 au. A set of localized basis functions consisting of generalized Wannier functions for the surface and s‐ and p‐atomic functions for the ion, is used to separate the system into primary and secondary regions. An effective Hamiltonian and time‐dependent equations for the electron density matrix are obtained in the primary region, where most charge transfer occurs. The equations for the electron density matrix are solved with a linearization scheme. The method is suitable to study atomic orbital orientation for collisions of ions and surfaces. A model calculation for Na+ + W(110) collisions with a prescribed trajectory is presented. The interaction potentials between the W(110) surface and Na+ 3s and 3p orbitals are calculated from Na+ pseudopotentials. Results show that the yield of neutralized atoms in 3p states changes as the collision energy is lowered.