bluuebunny/crossref_metadata_2025_split huggingface dataset:
Quantum chemistry by quantum monte carlo: Beyond ground‐state energy calculations
Authors: Reynolds P. J. , Barnett R. N. , Hammond B. L. , Grimes R. M. , Lester W. A. 
Published: December 2004

AbstractWe present recent advances with the quantum Monte Carlo (QMC) method in its application to molecular systems. The QMC method is a procedure for solving the Schrödinger equation statistically, by the simulation of an appropriate random process. The formal similarity of the Schrödinger equation with a diffusion equation allows one to calculate quantum mechanical expectation values as Monte Carlo averages over an ensemble of random walks. We have previously obtained highly accurate correlation energies for a number of molecules, as well as the singlet‐triplet splitting in methylene and the barrier height for the H + H2 exchange reaction. Recently we have begun a program of extending the QMC approach to the calculation of analytic derivatives of the energy. A brief description of the approach is presented here, together with some preliminary results. In addition, we are now computing expectation values of properties other than the energy. We summarize how standard QMC must be modified, and present some results for H2 and N2. Finally, we describe preliminary work toward the goal of obtaining accurate molecular excited states through QMC.
Semiclassical time‐dependent theory of two‐photon spectroscopy. The effect of dephasing in the virtual level on the two‐photon excitation spectrum of isotachysterol
Authors: Birge Robert R. , Pierce Brian M. 
Published: December 2004

AbstractIt is commonly assumed that the nonlinear absorption of two photons is a process involving the simultaneous capture of two radiation quanta. The purpose of this paper is to evaluate semiclassically the spectroscopic consequences of relaxing this assumption by permitting dephasing in the virtual level. Semiclassical wave‐packet propagation theory is used to model the vibronic consequences of finite, virtual‐state population times. We demonstrate that extremely short virtual‐state lifetimes (1‒10 femtoseconds) can have a profound effect on two‐photon excitation line shapes and total vibronic envelopes. We provide experimental evidence suggesting that virtual state dephasing has an important influence on the two‐photon excitation spectrum of the polyene chromophore of isotachysterol. Our analysis suggests that dephasing in the virtual state is of poetntial importance in defining the vibronic development of two photon spectra of many polyatomic molecules in solution.
Basis‐Set dependence of exterior electron distributions of molecular orbitals
Authors: Ohno Koichi , Ishida Toshimasa 
Published: December 2004

AbstractInterior electron densities are divided from exterior electron densities (EED), when an effective boundary is introduced. The EED concept was used to study the quality of wavefunction tails; the basis‐set dependence of ab initio MO for NH3 was studied from EED. The EED for the nonbonding orbital (3a1) increases dramatically with the size of basis sets, whereas that for the bonding orbital (1e) shows saturation even at the double zeta class. Although inclusion of polarization functions always gives the better total energy the EED (3a1) may either increase or decrease or decrease depending on the choice of the smaller or the larger exponents. The conventional choice of the polarization functions for 6‐311G* and 6‐311G* was found to yield worse wavefunction tails than the case of 6‐311G. Much better results were obtained with the more diffuse polarization functions. The (11s7p)/[5s4p] set designed for Rydberg states as well as the 6‐311 + G set proposed for negative ions gave excellent results comparable to the case of the near Hartree‐Fock calculation with 56 CGTO. Considerable improvements were also achieved for 4‐31G and 6‐311G sets, when the exponent for the outermost s‐type function was modified. The largest EED (3a1) gave the best agreement with the experimental ratio of EED's derived from Penning ionization electron spectroscopy.
Structural deformations of the DNA double helix in the first stages of DNA transcription studied with a simple model
Authors: Bonaccorsi R. , Scrocco E. , Tomasi J. 
Published: December 2004

AbstractA very simple model of DNA in biological media, consisting of electric charges related to the phosphate groups and to counterions immersed in structured media of constant permittivity, is introduced and tested with the study of a model for the first stages of the DNA transcription.This process is modeled into two steps involving three ‘states’: (1) the DNA system at the equilibrium, (2) the DNA with a small portion deprived of counterions, and (3) the DNA with a partial opening of the double helix in correspondence of the zone deprived of counterions.An extensive investigation involving as variable parameters the length of the DNA specimen (from 31 to 1511 base pairs), the amount of condensed counterion charge (from complete compensation to zero), and the geometrical parameters identifying the local opening has shown that the proposed transcription mechanism is reasonable, and that the DNA model considered here may fill a gap between accurate models including all the interactions—and employed at present for small fragments—and unstructured models addressed to inspect the behavior at the limit of infinite DNA length.
Semidiffuse states of diatomic molecules: Configuration interaction studies of the lowest 2Σ+ and 2Δ states of NS, SiF, and CCl
Authors: Karna S. P. , Grein F. 
Published: December 2004

AbstractIt has been shown by ab initio configuration‐interaction methods that the lowest 2Σ+ states of NS and SiF are ‘semidiffuse’ states, like the B 2Σ+ state of PO. The lowest 2Σ+ state of CCl also appears to be semidiffuse, although here the situation is not so clear. Semidiffuse states require diffuse orbitals in the wavefunction, but they are not Rydberg states. The second 2Σ+ state of each molecule is shown to be the lowest ns Rydberg state, whereas the third 2Σ+ state is a valence state for NS and CCl, and a 4po Rydberg state for SiF. The lowest 2δ state of each molecule derives from the 7σ → 3π valence configuration. Comparison with available experimental information shows, in general, good agreement.
Chemisorption of O and O2 on Ag(110): An LCGTO‐LSD cluster study
Authors: Selmani A. , Andzelm J. , Salahub D. R. 
Published: December 2004

AbstractAs part of a study aimed at better understanding of molecular and dissociative chemisorption of oxygen on Ag(110), linear combinations of Gaussian type orbitals‐local spin density (LCGTO‐LSD) calculations have been performed for O, O−, O2, O−2, O2−2 and a variety of silver clusters interacting with O or O2.For atomic O adsorption a very small cluster, Ag4, chosen to model the long‐bridge site already affords very good agreement with both recent EXAFS experiments and recent ab initio calculations. We calculate O to be 0.25 Å above the surface (exp. 0.2 Å). The Ag4O vibrational frequency is estimated to be 400 cm−1, in reasonable accord with the experimental EELS value of 325 cm−1.Determination of the geometry for O2 (ads.) and, ultimately, of the dissociation path are far more difficult tasks. An extensive search for local minima in the vicinity of the LB site is being carried out. Results to date for small, Ag2 and Ag4, clusters have furnished insight into the factors influencing the structure. Overlap between the π* orbital of the O2 moiety and Ag s functions is a key factor; that is, there is an important covalent component of the binding. For geometries with O2 parallel to the surface, this is achieved by twisting the O2 fragment with respect to the [11¯0] grooves (geometries either parallel or perpendicular to the grooves yield zero π‖*−s overlap by symmetry). The structure with O2 perpendicular to the surface also achieves reasonable overlap and lies close in energy to the most stable ‘parallel’ geometry.
On the minimization of the global variance in the 1‐Reduced local‐energy matrix
Authors: Thomas Gerald F. 
Published: December 2004

AbstractBy minimizing the global variance in the 1‐reduced local‐energy matrix E1(X1; X1′), subject to the normalization of the 1‐reduced density matrix ρ1(X1;X1′), one derives an integral matrix equation for E1(X1;X1′) as a functional of ρ1(X1;X1′) at the location (X1;X1′) of an arbitrary member of an N (≥ 2)‐particle system. The implications for the possible local improvement in the accuracy of approximate wave functions through the imposition of global constraints are briefly discussed.
Electronic structure calculations for the molecules Si2 and Ge2 using all electron ab initio HF‐CI methods
Authors: Shim Irene , Nagarathna‐Naik H. M. , Gingerich Karl A. 
Published: December 2004

AbstractMolecular orbitals for Si2 and Ge2 have been optimized in hyper‐HF calculations and utilized in valenxe CI treatments to describe the low‐lying states of the molecules. The calculational results reveal pronounced similarities between the electronic structures of Si2 and Ge2. Thus, for both molecules the two lowest‐lying electronic states, 3Σ(σΠu3) and 3πu(∑g1πu3), have crossing potential energy curves, and the two lowestlying states of 1∑g+ symmetry exhibit crossing of configurations. The Sequence of the low‐lying electronic states can be rationalized on basis of a simple molecular‐orbital picture in which the σg and the πu valence orbitals are almost degenerate. The spectroscopic constants derived from the present work compare favorably with the results of more elaborate calculations. It appears that transition energies derived in valence CI calculations between states of identical configurations are improved in large CI calculations, whereas this is not the case for transition energies between states of different configurations. The valence CI calculations based on the molecular orbitals optimized in hyper‐HF calculations appear to effer reliable descriptions of the chemical bonds as well as of the electronic structures of the molecules Si2 and Ge2.
Ab initio calculations on the barrier height for the hydrogen addition to ethylene and formaldehyde. The importance of spin projection
Authors: Sosa Carlos , Bernhard Schlegel H. 
Published: December 2004

AbstractHeats of reaction and barrier heights have been computed for H + CH2CH2 → C2H5, H + CH2O → CH3O, and H + CH2O → CH2OH using unrestricted Hartree‐Fock and Møller–Plesset perturbation theory up to fourth order (with and without spin annihilation), using single‐reference configuration interaction, and using multiconfiguration self‐consistent field methods with 3‐21G, 6‐31G(d), 6‐31G(d,p), and 6‐311G(d,p) basis sets. The barrier height in all three reactions appears to be relatively insensitive to the basis sets, but the heats of reaction are affected by p‐type polarization functions on hydrogen. Computation of the harmonic vibrational frequencies and infrared intensities with two sets of polarization functions on heavy atoms [6‐31G(2d)] improves the agreement with experiment. The experimental barrier height for H + C2H4 (2.04 ± 0.08 kcal/mol) is overestimated by 7‒9 kcal/mol at the MP2, MP3, and MP4 levels. MCSCF and CISD calculations lower the barrier height by approximately 4 kcal/mol relative to the MP4 calculations but are still almost 4 kcal/mol too high compared to experiment. Annihilation of the largest spin contaminant lowers the MP4SDTQ computed barrier height by 8‒9 kcal/mol. For the hydrogen addition to formaldehyde, the same trends are observed. The overestimation of the barrier height with Møller‐Plesset perdicted barrier heights for H + C2H4 → C2H5, H + CH2O → CH3O, and H + CH2O → CH2OH at the MP4SDTQ/6‐31G(d) after spin annihilation are respectively 1.8, 4.6, and 10.5 kcal/mol.
Electron transport in condensed matter. A novel approach using quantum chemical methods
Authors: Larsson Sven 
Published: December 2004

AbstractThe theory of mixed valence hopping conductivity is reviewed and prepared in a way suitable for quantum chemical calculations. Metallic conductivity appears as a limit case, if the thermal barrier for electron transfer is approaching zero and the orbital contact between the electron exchanging subsystems is large enough for adiabatic transfer. The theory is exemplified by calculations on the molecular crystal model and a model for doped polyacetylene. In the latter case it is possible to explain the decreasing thermal barrier for increased concentration of doping molecules as due to increased interaction between these molecules through the polymer chain.
Ab initio theory of hydrogen bonding in vitamin B6
Authors: Greenwood H. H. , Plant J. S. 
Published: December 2004

AbstractAb initio MO computations have been carried out for the crystalline form of the B6 vitamin, pyridoxinium chloride. In the crystal, the pyridoxine is protonized to pyridoxinium, with the formation of a hydrogen bond NH+ ⃛ Cl−. For an isolated molecule, the calculations predict a single potential well, with H+ placed close to Cl− at the minimum. When neighboring molecules are included in the calculations, a double well is formed, and the lower minimum occurs for the proton placed near the experimentally observed position, in the vicinity of the N atom.Mulliken populations, which depend on the proton position, are described.
Geometry of density matrices. VI. Superoperators and unitary invariance
Authors: Casida Mark E. , Harriman John E. 
Published: December 2004

AbstractWe examine and compare ways of dividing into subspaces the space whose elements are density matrices or other operators for the class of model problems defined by a finite one‐particle basis set. One method of decomposition makes the significance of the subspaces apparent. We show that this decomposition is also complete, in the group‐theoretic sense, for the group of unitary transformations of the set of one‐electron basis functions. The irreducible subspaces are labeled by particle number and by an additional integer we call the reduction index. For spaces of particle‐number‐conserving operators, all subspaces with the same reduction index are isomorphic, and an analogous isomorphism exists for non‐particle‐number‐conserving cases. The general linear group also plays a key role, and we introduce the term “canonical superoperators” to characterize those superoperators which commute with this group. When an appropriate basis set is chosen for the matrix spaces, the supermatrices corresponding to these superoperators have a particularly simple form: a block structure with the only nonzero blocks being multiples of unit matrices. The superoperators of interest can be constructed in terms of two operators, , and these two have been expressed simply in terms of creation and annihilation operators. When only real orthogonal transformations of the basis are considered, a further decomposition is possible. We have introduced superoperators associated with this decomposition.
Ru‐Oxo catalyzed epoxidations and the woodward–hoffmann rules
Authors: Cundari Thomas R. , Drago Russell S. 
Published: December 2004

AbstractAn analysis of the pathways and intermediates for the epoxidation of olefins using a Ru(IV)‐oxo model complex is presented. Simple bonding concepts, coupled with INDO/1 semiempirical MO calculations, provide a description of the metal‐oxo/olefin interaction that agrees well with experimental facts. Concerted [1 + 2] and [2 + 2] pathways are investigated and found to lead to unfavorable orbital interactions as with organic analogues. Nonconcerted process (those in which one bond is formed before the second), which connect the two concerted pathways, are preferred. A nonconcerted [1 + 2] pathway is preferred over a nonconcerted [2 + 2] pathway on the basis of less steric repulsions between the olefin and the metal ligands. Also, all open structures (those with one CO bond formed) investigated minimized to bound epoxides. In the most favorable pathway, a nonconcerted [1 + 2] pathway, radical cation character on the terminal carbon increases as the nonconcerted process is induced. Preference for the bound epoxide intermediate over the oxometallocycle is simply explained by partial charge considerations.
Ab initio calculations of many‐body energy expansion in LiF− clusters
Authors: Shalabi A. S. , Kamel M. A. , Eid Kh. M. 
Published: December 2004

AbstractThe effects of the basis‐set size on many‐body energy expansion in LiF− clusters are investigated and correlated with previously reported values on LiCl− analogs. Coulomb and non‐Coulomb energies in LiF− at different configurations are also examined. Although at the minimal STO‐3G basis Vna(3, 4) and Vna(4, 4) nonadditivity terms were the smallest in the D3h configuration, they were the largest at the extended 6‐311 ++G basis. V(m, n) terms where m = n ≥ 3 were found to be playing a small role in the chemistry and physics of LiF− clusters compared with V(3, n) terms in LiCl− clusters.
Evaluation of two‐center, two‐ and three‐electron integrals involving correlation factors over Slater‐type orbitals. I. Basic integrals
Authors: Budziński Jan 
Published: December 2004

AbstractThe modified and extended version of the Neumann expansion of the interrelectronic distance function r for u = −1, 0, 1, 2, using the set of orthogonal polynomials normalized to unity, is presented. This expansion has been utilized to obtain analytical expressions for evaluating two‐center two‐ and three‐electron integrals in the Slater orbital basis occurring if variational correlated functions are used.
On the redistribution of electrons for chemical reaction systems
Authors: Tachibana Akitomo , Parr Robert G. 
Published: December 2004

AbstractA regional density‐functional theory is formulated and applied to the study of ground‐state electron redistributions during the course of a chemical reaction. If for a given increment of the reaction process, accumulation of electrons occurs in a certain region of space, then it is called the dynamic acceptor region, denoted by P. The complement is called the dynamic donor region, denoted by Q. The regional energy itself is determined as a unique functional of the electron density of the total system. The regional transfer potentials are defined in such a way that they add to give the total chemical potential, and their values along the reaction coordinate are found to be different between P and Q. The difference between the regional transfer potentials is shown to provide the driving force for electron transfer from Q to P. A characteristic coordinate for following electron transfer and an associated excitation potential are introduced. The excitation potential is a measure of regional virtual excitation due to regional interactions. The regional transfer potential gives the local character of electron transferability, while the excitation potential gives the global character. The theory encompasses the concepts of regional hardness and softness and sheds light on the HSAB principle.
A global characterization and similarity analysis of two‐dimensional potential energy surfaces
Authors: Luo Xincai , Mezey Paul G. 
Published: December 2004

AbstractPotential energy surfaces can be classified into combinatiorial equivalence classes, based on their partitioning into catchment regions. Two classification theorems are proven: one for reaction spheres and another for reaction tori. A method for constructing all possible equivalence classes of reaction spheres and reaction tori is presented. As illustration of the general results, it is shown that not all the two‐dimensional reaction spheres are combinatorially equivalent to polyhedra in the three‐dimensional Euclidean space. As examples, several reaction spheres are calculated by using the RHF method at the 3‐21G* level, describing the interactions between a series of polyatomic ions and H+. The calculations show that the potential energy surface of the CO …︁H+ interaction, combinatorially equivalent to that of the NO−3 …︁H+ interaction, is not combinatorially equivalent to any polyhedron in 3‐space; however, the combinatorially different potential energy surface of the PO …︁ H+ interaction is equivalent to a polyhedron in 3‐space. The topological classification scheme is proposed for the study of similarities between various families of chemical reactions.
Electronic‐phase transition associated with instability of the Hartree–Fock solution of infinite one‐dimensional system
Authors: Tanaka Kazuyoshi , Kobayashi Hiromi , Okada Mayumi , Kobashi Masahiro , Yamabe Tokio 
Published: December 2004

AbstractElectronic‐phase transition associated with the singlet instability problem of the Hartree‐Fock solution of the extended system is studied employing the metallic trans‐polyacetylene. Concerning the eigenstates of the stability matrix set up for the crystal orbitals at the Fermi level, classification of the electronic phases is attempted and the origin of their emergence in relation to the interelectronic interaction is discussed.
Behavior of coupled cluster energy in the strongly correlated limit of the cyclic polyene model. Comparison with the exact results
Authors: Piecuch Piotr , Čížek Jiří , Paldus Josef 
Published: December 2004

AbstractPerturbation expansion for the ground‐state energy in the strongly correlated limit of cyclic polyenes, as described by the Pariser–Parr‐Pople (PPP) and Hubbard Hamiltonians, is examined. Focus is placed on the first‐ and second‐order contributions in terms of the resonance integral, and the performance of the approximate coupled pair theory corrected for the connected quadruple excitations (ACPQ), as well as ACPTQ and ACPQ + T(ACPQ) methods, which are two different versions of ACPQ approximately accounting for the triexcited clusters, is studied. The second‐order contribution obtained with the ACPQ, ACPTQ, and ACPQ + T(ACPQ) approaches is then compared with the exact result for the fully correlated limit as provided by the diagonalization of the nearest‐neighbor Heisenberg Hamiltonian. It is shown that all three coupled pair theories yield vanishing first‐order contribution, in agreement with the exact result. The best result for the second‐order contribution for the cyclic polyenes considered in this article is provided by the ACPQ + T(ACPQ) method. In this respect, all three coupled pair theories seem to deteriorate as the polyene size approaches infinity, in spite of yielding excellent energies. Comparison is also made with the results of alternant molecular orbital method.
A theoretical study of atomic oxygen chemisorption on the Ni(100) and Ni(111) surfaces
Authors: Siegbahn Per E. M. , Wahlgren Ulf 
Published: December 2004

AbstractAtomic oxygen chemisorption has been studied for the fourfold hollow site of the Ni(100) surface and for the threefold hollow site of the Ni(111) surface. To model the Ni(100) surface, 10 different clusters in the range Ni5 to Ni41 were used, and for the Ni(111) surface, 11 different clusters in the range Ni13 to Ni43 were used. A detailed analysis of the orbital occupations of the cluster with and without oxygen for the different clusters shows that there are three different possible bonding mechanisms. In two of these, the basic feature is that a1 electrons of the cluster are kicked out by the oxygen 2pz orbital and moved to holes in the 2px, y orbitals. A picture where the oxygen electrons fit into the electronic structure of the cluster is emphasized. The third mechanism, which is applicable for only one cluster, can be described as the formation of two covalent bonds of E symmetry. The final best estimate of the oxygen chemisorption energy for the Ni(100) surface is about 130 kcal/mol, and for the Ni(111) surface, about 115 kcal/mol. In particular for the Ni(111) surface, an excited oxygen state with radical character is identified, which might be a catalytically important species. The excitation energy to reach this state should be on the order of 10 kcal/mol for the Ni(111) surface.