bluuebunny/crossref_metadata_2025_split huggingface dataset:
Accurate local‐space treatment of hydrogen bonding in large systems
Authors: Kirtman Bernard , Demelo Celso P. 
Published: December 2004

AbstractTests of the local‐space approximation for electronic structure are carried out on a variety of hydrogenbonded systems. Regardless of size they are all found to be well localized in the sense of this approximation. For a local space defined by the valence atomic orbitals on XH ··· Y, one obtains a bond energy within 6% of the “exact” full‐space value. The potential energy surface and the electronic charge distribution are also accurately determined. In contrast with cluster models the local‐space approximation satisfactorily accounts for long‐range electrostatic and polarization effects.
Intermolecular forces in van der waals dimers
Authors: Hurst G. J. B. , Fowler P. W. , Stone A. J. , Buckingham A. D. 
Published: December 2004

AbstractFor several van der Waals molecules, the results of ab initio intermolecular perturbation theory are compared with the simple Buckingham‐Fowler model. A selection of dimers is studied, with emphasis on cases where the experimental structure is not reliably predicted by the hard‐sphere electrostatic model. Cases are reported where the electrostatic term dominates, in agreement with the model: HF···HF, H3N···HF, N2O···HF, and H2CO··· HF; cases where the electrostatic term dominates but the molecular shapes are not well described by hard spheres: CO2···H2O and CIF···HF; and cases where there is a delicate balance between the electrostatic and other terms:H2O···HF, H2O···HCCH, CO2···HF, CO2···N, and CH3CN···HCN. It is concluded that, in general, the electrostatic interaction is central to the strength and orientation dependence of the forces between molecules and that the model provides a useful approximation to this interaction, while perturbation theory gives a more complete picture.
Two particle transition density and two particle bond order and their applications to electronic energy shifts caused by two particle interactions via geometric deformations
Authors: Hohlneicher Georg , Gutmann Michael 
Published: December 2004

AbstractThe concept of two particle transition density (TPTD) and two particle bond order (TPBO) is introduced and formally developed in order to extend the considerations of Callis, Scott, and Albrecht [1] to two particle interactions. These interactions may become important if the TPTD (TPBO) for one particle effects is small or the perturbation considered is essentially a two particle perturbation. As an example the geometric deformation of annulene perimeters is studied in detail. This deformation can be interpreted as a two particle perturbation because one particle effects do not contribute considerably. As a result of the perturbation characteristic energy shifts of the four lowest electronic singlet excitations depending on the type of deformation can be observed.
Research in the method of large molecular calculations utilizing transferability of LMO
Authors: Huai Sun , An‐min Tian , Guo‐Sen Yan 
Published: December 2004

AbstractThe localized molecular orbitals of some saturated hydrocarbons and their derivatives have been formed using ab initio method and M. P. [1–2] localization procedure.Two models, SLMO and ELMO, a set of parameters of LMO Fock matrix elements, and a technique called “Group Effect” are proposed. Based on these, we developed a procedure to simulate the ab initio calculations on large molecules. Some test calculations have been done. The results are compared with those of the ab initio method. In general, absolute errors of orbital energies are about 10−3 a.u., and the relative errors of total energies are about 10−4.For the original applications, we applied this procedure to some large systems of alkane and their derivatives as well as three Crown‐ether compounds. Satisfactory results are obtained.
Structure and properties of nonclassical polymers. III. Magnetic characteristics at finite temperatures
Authors: Tyutyulkov N. N. , Karabunarliev S. C. 
Published: December 2004

AbstractWithin the Ising model, the two‐dimensional alternant nonclassical polymers display magnetic order at finite temperatures and are characterized by a nonzero critical point Tc.It is shown that magnetic ordering at finite temperatures occurs when interchain interactions in a quasi‐ dimensional nonclassical altemant polymer are taken into account.
A theoretical study of hydrogen surface coverage over Ni(100)
Authors: Ruette Fernando , Ludeñta Eduardo V. , Hernǎandez Antonio 
Published: December 2004

AbstractWe have used the MINDO/SR molecular orbital method in order to model the migration of hydrogen atoms over a Ni(100) surface. The present calculations indicate the existence of two different states for adsorbed hydrogen, a result which is in agreement with experimental thermal desorption data and LEED. A detailed analysis of the electronic factors involved in this process is presented.
Quantum theory of dielectric relaxation in polar solids
Authors: Georgiev M. , Gochev A. , Diaz‐Gongora A. 
Published: December 2004

AbstractA novel approach to the impurity dipole relaxation in polar solids is described in which the dipole reorientational sites are formed by an appropriate electron—phonon coupling interaction. The electronic states used for a two‐site formulation are defined as ones pertaining to the impurity and/or the associated intrinsic point defect. The theory takes into account the adiabaticity of the electron transfer in addition to the barrier‐controlled ionic tunneling. It compares favorably with available experimental relaxation‐time data on alkali halides.
Looking at orbitals in the laboratory: The experimental investigation of molecular wavefunctions and binding energies by electron momentum spectroscopy
Authors: Brion C. E. 
Published: December 2004

AbstractThe experimental technique of electron momentum spectroscopy (EMS) (i.e., binary (e, 2e) spectroscopy) is discussed together with typical examples of its applications over the past decade in the area of experimental quantum chemistry. Results interpreted within the framework of the plane wave impulse and the target Hartree—Fock approximations provide direct measurements of, spherically averaged, orbital electron momentum distributions. Results for a variety of atoms and small molecules are compared with calculations using a range of Fourier transformed SCF position space wavefunctions of varying sophistication. Measured momentum distributions (MD) provide a “direct” view of orbitals. In addition to offering a sensitive experimental diagnostic for semiempirical molecular wavefunctions, the MD's provide a chemically significant, additional experimental constraint to the usual variational optimization of wavefunctions. The measured MD's clearly reflect well known characteristics of various chemical and physical properties. It appears that EMS and momentum space chemistry offer the promise of supplementary perspectives and new vistas in quantum chemistry, as suggested by Coulson more than 40 years ago. Binding energy spectra in the inner valence region reveal, in many cases, a major breakdown of the simple MO model for ionization in accord with the predictions of many‐body calculations.Results are considered for atomic targets, including H and the noble gases. The measured momentum distribution for H2 is also compared with results from Compton scattering. Results for H2 and H are combined to provide a direct experimental assessment of the bond density in H2, which is compared with calculations. The behavior of the outer valence MD''s for small row two and row three hydride molecules such as H2O and H2S, NH3, HF, and HCl are consistent with well known differences in chemical and physical behavior such as ligand‐donor activity and hydrogen bonding. MD measurements for the outermost valence orbitals of HF, H2O and NH3 show significant differences from those calculated using even very high‐quality wavefunctions. Measurements of MD's for outer σg orbitals of small polyatomic molecules such as CO2, COS, CS2, and CF4 show clear evidence of mixed s and p character. It is apparent that EMS is a sensitive probe of details of electronic structure and electron motion in atoms and molecules.
Strong nonadiabatic effects in reactions of photoisomerization
Authors: Smedarchina Z. 
Published: December 2004

AbstractReactions of photoisomerization proceeding through a “funnel” are discussed (Fig. 1a, θ ≅ θ*, where θ is the angle of isomerization). Strong nonadiabatic interactions in the region of conical intersection of the multidimensional adiabatic potentials Us and Us0 are supposed to be responsible for the ultrafast nonradiative transition S → K2 S0 (K2 ≅ 1010‐1012 s−1). The K2 dependence on the solvent viscosity (isomerization of t‐stilbene in series ethanol‐‐octanol) and polarity (isomerization of cyanine dyes in polar solvents) was determined to be in a good agreement with experiments.
Thermal and state‐selected rate constant calculations for O(3p) + H2 → OH + H and isotopic analogs
Authors: Garrett Bruce C. , Truhlar Donald G. 
Published: December 2004

AbstractWe present a new parametrization (based on ab initio calculations) of the bending potentials for the two lowest potential energy surfaces of the reaction O(3P) + H2, and we use it for rate constant calculations by variational transition‐state theory with multidimensional semiclassical tunneling corrections. We present results for the temperature range 250–2400 K for both the rate constants and the intermolecular kinetic isotope effects for the reactions of O(3P) with D2 and HD. In general, the calculated rate constants for the thermal reactions are in excellent agreement with available experiments. We also calculate the enhancement effect for exciting H2 to the first excited vibrational state. The calculations also provide information on which aspects of the potential energy surfaces are important for determining the predicted rate constants.
On the pressure dependence of the muon spin polarization in mixtures of noble gases
Authors: Turner Ralph Eric , Senba Masayoshi , Arseneau Donald J. 
Published: December 2004

AbstractIn muon spin rotation (μSR) experiments, where spin‐polarized positive muons are stopped in condensed matter, three magnetically distinguishable chemical environments can be observed. That is, the Larmor frequencies associated with diamagnetic environments and two types of paramagnetic environments (muonium and radicals) can be resolved. The chemical identities of the latter two are distinct since their Larmor frequencies are distinct, whereas the chemical identities of the possible diamagnetic species are not determined by the Larmor frequency since chemical shifts can not be resolved in μSR experiments. However, two different diamagnetic species have been observed in experiments performed on mixtures of noble gases. Their distinction arises through different thermal rate constants that lead to “fast” and “slow” relaxing components of the diamagnetic signal. The pressure dependencies of the amplitudes associated with these components are related to the stopping dynamics of muons in noble gas targets. A set of coupled rate equations for muon spin dynamics, based upon quantal Boltzmann equations, have been developed to describe this process in single component gases. This theory is now extended to mixtures. In particular, the dynamics of the muon spin is generated by the muonium hyperfine interaction and by time dependent rate constants for the various chemical species that are assumed to be present, namely, muonium and three diamagnetic species. Radicals have not yet been observed in low pressure gases. The coupled quantal rate equations are solved for two models of the stopping dynamics wherein the rates are taken as square box functions of time.
Free energy simulations: Applications to the study of liquid water, hydrophobic interactions and solvent effects on conformational stability
Authors: Beveridge D. L. , Mezei M. , Ravishanker G. , Jayaram B. 
Published: December 2004

AbstractFree energy simulations using the Metropolis Monte Carlo method and the coupling parameter approach with umbrella sampling are described for several problems of interest in structural biochemistry: the liquid water, the hydrophobic interaction of alkyl and phenyl groups in water, and solvent effects on the conformational stability of the alanine dipeptide and the dimethyl phosphate anion in water. Proximity analysis of the results is employed to identify stabilizing factors. Implications of the result with respect to the structural chemistry of proteins and nucleic acids is considered.
The case of disordered two‐dimensional electron systems
Authors: Srivastava Vipin 
Published: December 2004

AbstractWe investigate the question of the existence or nonexistence of mobility edges that separate the localized and extended states in two‐dimensional disordered electron systems treated within the Anderson model. Evidence is produced to show that the mobility edges exist if the amount of disorder is less than a critical value. The following three different agruments are presented: (1) exact enumeration of self‐avoiding walks that contribute to the renormalized perturbation series of self‐energy whose convergence (divergence) indicates localization (delocalization); (2) “quantum percolation” in a strongly disordered binary alloy; and (3) influence of magnetic field on localized and extended states (the quantum Hall Effect problem).
From macroscopic irreversibility to microscopic reversibility via a nonlinear schrödinger‐type field equation
Authors: Schuch D. , Chung K. ‐M. 
Published: December 2004

AbstractThe aim of our work was to find an unambiguous connection between irreversible macroscopic dynamics and reversible microdynamics that makes it possible to manifest irreversibility on a submacroscopic level without the use of coarse graining arguments. On this level the state of a physical system can be described by a field amplitude Ψ and the time evolution of this system is determined by a field equation for Ψ. For conservative systems, this field equation is formally identical with the linear Schrödinger equation, which can be constructed with the help of the classic Hamiltonian function for the corresponding problem. Regarding irreversible phenomena like damping due to a frictional force, for those dissipative systems no classic Hamiltonian function exists. Therefore the corresponding field equation cannot be obtained in the usual way. Nevertheless, also for dissipative systems it is possible to obtain a field equation in an unambiguous way using only Newton's form of classic mechanics. The result of our method is a nonlinear Schrödinger‐type field equation with a logarithmic nonlinearity. We discuss in more detail the properties of our logarithmic nonlinearity that corresponds to a macroscopic frictional force in a unique way. A figurative interpretation in terms of environment and interaction can be given. From a more formal point of view, the compatibility of our nonlinear operator with principles known from the theory of linear operators is investigated. One of the surprising results is the fact that although our nonlinear Hamiltonian HNL is “Hermitean” in the usual sense, in contrast to the linear theory an operator exp(i · HNL) is not unitary. Furthermore, in our theory the time‐derivative of the mean value of an operator is not only essentially determined by (the mean value of) its commutator with the Hamiltonian. There also occurs an additional term that causes irreversibility of the evolution and is connected with the feature of our theory that (in general) time derivative and construction of the mean value are no longer commuting operations. This fact shows some similarity with coarse graining theories, but in our theory the reason can be traced back unambiguously to an irreversible physical phenomenon.
Etude MNDO des composes conjugues enneagonaux
Authors: Ouamerali Ourida , Gayoso Jose 
Published: December 2004

AbstractWe present MNDO calculations of a series of nine‐membered conjugated compounds. The geometries have been fully optimized. Cyclononatetraenyl and benzocyclononatetraenyl anions, nonapyrrol, and nonathiophene appear to be planar aromatic compounds. Following MNDO, nonafulvene has a planar prefered conformation, but a polyenic structure, while nonafuran and cyclononatetraene are gauche molecules. The Universal Huckel Method (UHM), a simple all valence electron empirical technique which describes σ systems using localized MO, gives, qualitatively, the same results as the MNDO method. The quantum calculation carried out with MNDO and UHM have been used to describe the physicochemical properties of ninememberedring conjugated compounds under consideration. To our knowledge, we present here the first theoretical study dealing with nine‐membered conjugated molecules.
Electronic charge density and mean kinetic energy of lithium orbitals in LiH, LiH+, Li2, Li2+, LiH2+, and Li2H+
Authors: Cardelino B. H. , Eberhardt W. H. , Borkman R. F. 
Published: December 2004

AbstractThe electron density near the lithium nucleus in the species LiH, LiH+, Li2, Li2+, LiH2+, and Li2H+ was analyzed by transforming the SCF molecular orbitals into a sum of atomic contribnutions, for both core and valence orbitals. These “hybrid‐atomic” orbitals were used to compare: electron densities, orbital polarizations, and orbital mean kinetic energies with the corresponding lithium atom quantities. Core‐orbital electron densities at the lithium nucleus were observed to increase by up to 0.5% relative to the lithium atom 1s orbital. Lithium cores also exhibited polarization but, surprisingly, in the direction away from the internuclear region. Similar dramatic changes were seen in the electron densities of the valence orbitals of lithium: The electron density at the nucleus for these orbitals increased two‐fold for homonuclear species and twenty‐fold for heteronuclear triatomic species relative to the electron density at the nucleus in lithium atom. The polarization of the valence orbital electronic charge, in the vicinity of the lithium nucleus, was also away from the internuclear region. The mean “hybrid‐atomic” orbital kinetic energies associated with the lithium atom in the molecules also showed changes relative to the free lithium atom. Such changes, accompanying bond formation, were relatively small for the lithium core orbitals (within 0.2% of the value for lithium atom). The orbital kinetic energies for the lithium valence electrons, however, increased considerably relative to the lithium atom: By a factor of about 2 in homonuclear diatomics, by a factor of 7 in heteronuclear diatomics, and by a factor of 11 in the triatomic species. In summary, the total electronic density (core plus valence) at the lithium nucleus remained remarkably constant for all of the species studied, regardless of the effective charge on lithium. Thus, the drastic changes noted in the individual lithium orbitals occurred in a cooperative fashion so as to preserve a constant total electron density in the vicinity of the lithium nucleus. In all cases, bond formation was accompanied by an increase in the orbital kinetic energy of the lithium valence orbital. We suggest that these two observations represent important and significant features of chemical bonding which have not previously been emphasized.
On the state of the art of quantum chemistry
Authors: Löwdin Per‐Olov 
Published: December 2004

AbstractA brief general survey of the current state of the art of quantum chemistry is given with some aspects also towards the future. It is emphasized that, if one wants to incorporate such concepts as temperature, entropy, and free energy into quantum chemistry, it is necessary to make a transition from pure quantum mechanics based on wavefunctions to the more general “quantum statistics” based on density matrices and system operators. In addition to the Schrödinger equations, one obtains the Liouville equations, and it is shown that both the time‐dependent and the time‐independent equations may be solved in both cases by using analogous Hilbert‐space methods. Some of the methods for solving the time‐independent eigenvalue problems are reviewed, and the need for giant “number crunchers” in this connection are discussed. It is shown that the resolvent methods combined with the “inner projection” technique for the calculations provide a powerful tool for handling the eigenvalue problems in the future in both the Hamiltonian and Liouvillian formalisms. It is stressed that, by going over to supercomputers, one may gain a factor of 100, and that one may gain another factor of 100 by going over to more powerful theoretical methods; however, for programming reasons, it will take a long time before one can reach the combined efficiency factor 100 ✗ 100.
Density functional calculation of quinone electrode potentials
Authors: Reynolds Christopher A. 
Published: December 2004

AbstractHere, we have applied density functional methods, in combination with free energy hydration calculations, to calculate two‐electron electrode potentials for quinones and naphthoquinones. While we find that the free‐energy perturbation method, implemented within a molecular dynamics framework, is superior to the PM3—SM3 continuum method for determining free energies of hydration, the computationally less expensive PM3—SM3 method does perform well when there is not an internal hydrogen bond. Generally, all the density functional approaches investigated gave good energetics when applied to this problem, but the Beck '88—Vosko—Wilk—Nusair combination of functionals for the exchange‐correlation energy gave the best results. The density functional results are marginally better than the Møller‐Plesset second‐order perturbation results. Moreover, because the results are obtained using a thermodynamic cycle which involves taking differences in total energies, the results are not too dependent on the quadrature scheme used to calculate the exchange‐correlation energy. By using semiempirically optimized geometries and the PM3—SM3 method for determining free energies of hydration, it has been possible to calculate electrode potentials for a series of large molecules (naphthoquinones) to within about 30 mV of experiment. This result is extremely encouraging and shows that density functional methods offer great promise in the design of redox‐active molecules such as bioreductive anticancer agents. © 1995 John Wiley & Sons, Inc.
Density functional study of the intershell interaction energy in carbon onions
Authors: Diez R. Pis , Iñtiguez M. P. 
Published: December 2004

AbstractThe intershell interaction energy in carbon onions is self‐consistently obtained by using the local density approximation for the π electrons in a spherically averaged pseudopotential. Onions formed by two, three, and four shells with up to 1800 atoms are considered. Our results agree well with recent large‐scale ab initio calculations and with experimental evidence that onions are more stable than are single fullerenes for cluster sizes larger than 100 atoms. The small electronic redistribution that takes place when onions are formed supports the picture of weak van der Waals forces between single fullerenes. © 1995 John Wiley & Sons, Inc.