Featured Researches

Chemical Physics

Comparison of electron density functional models

Presented here are calculations of the distortion of the density of an electron gas due to the electrostatic field of a proton. Several models based upon the local density approximation (LDA) of density functional theory [linear response theory, Kohn-Sham (KS), optimized Thomas-Fermi theory (OTF), and OTF plus perturbation corrections] are compared with one another. These models, in turn, are compared with available results of quantum Monte Carlo (QMC) calculations for the same system. Comparison of the KS results with the OTF results shows a very reasonable agreement that seems to be progressively improvable. This provides encouragement for the application of the OTF model to condensed phase systems. The QMC calculations of the density do not agree well with the density functional results. The reasons for this poor agreement are not clear. This particular system is expected to be an ideal one for application of the LDA and, thus, the poor agreement is of fundamental importance. The lack of detail presented in the available QMC results leads us to conclude that the QMC calculations should be attempted again. in print: Molec. Phys. 82, 245-261(1994)

Read more
Chemical Physics

Computational Study of the Structure and Thermodynamic Properties of Ammonium Chloride Clusters Using a Parallel J-Walking Approach

The thermodynamic and structural properties of (NH 4 Cl) n clusters, n=3-10 are studied. Using the method of simulated annealing, the geometries of several isomers for each cluster size are examined. Jump-walking Monte Carlo simulations are then used to compute the constant-volume heat capacity for each cluster size over a wide temperature range. To carry out these simulations a new parallel algorithm is developed using the Parallel Virtual Machine (PVM) software package. Features of the cluster potential energy surfaces, such as energy differences among isomers and rotational barriers of the ammonium ions, are found to play important roles in determining the shape of the heat capacity curves.

Read more
Chemical Physics

Correlation Energy Estimators based on Møller-Plesset Perturbation Theory

Some methods for the convergence acceleration of the Møller-Plesset perturbation series for the correlation energy are discussed. The order-by-order summation is less effective than the Feenberg series. The latter is obtained by renormalizing the unperturbed Hamilton operator by a constant factor that is optimized for the third order energy. In the fifth order case, the Feenberg series can be improved by order-dependent optimization of the parameter. Alternatively, one may use Pad{é} approximants or a further method based on effective characteristic polynomials to accelerate the convergence of the perturbation series. Numerical evidence is presented that, besides the Feenberg-type approaches, suitable Pad{é} approximants, and also the effective second order characteristic polynomial, are excellent tools for correlation energy estimation.

Read more
Chemical Physics

Critical Behavior of the Widom-Rowlinson Lattice Model

We report extensive Monte Carlo simulations of the Widom-Rowlinson lattice model in two and three dimensions. Our results yield precise values for the critical activities and densities, and clearly place the critical behavior in the Ising universality class.

Read more
Chemical Physics

Decisions, Decisions: Noise and its Effects on Integral Monte Carlo Algorithms

In the present paper we examine the effects of noise on Monte Carlo algorithms, a problem raised previously by Kennedy and Kuti (Phys. Rev. Lett. {\bf 54}, 2473 (1985)). We show that the effects of introducing unbiased noise into the acceptance/rejection phase of the conventional Metropolis approach are surprisingly modest, and, to a significant degree, largely controllable. We present model condensed phase numerical applications to support these conclusions.

Read more
Chemical Physics

Density Functional Calculations On First-Row Transition Metals

The excitation energies and ionization potentials of the atoms in the first transition series are notoriously difficult to compute accurately. Errors in calculated excitation energies can range from 1--4 eV at the Hartree-Fock level, and errors as high as 1.5eV are encountered for ionization energies. In the current work we present and discuss the results of a systematic study of the first transition series using a spin-restricted Kohn-Sham density-functional method with the gradient-corrected functionals of Becke and Lee, Yang and Parr. Ionization energies are observed to be in good agreement with experiment, with a mean absolute error of approximately 0.15eV; these results are comparable to the most accurate calculations to date, the Quadratic Configuration Interaction (QCISD(T)) calculations of Raghavachari and Trucks. Excitation energies are calculated with a mean error of approximately 0.5eV, compared with ∼1eV for the local density approximation and 0.1eV for QCISD(T). These gradient-corrected functionals appear to offer an attractive compromise between accuracy and computational effort.

Read more
Chemical Physics

Density functional theory using an optimized exchange-correlation potential

We have performed self-consistent calculations for first and second row atoms using a variant of density-functional theory, the optimized effective potential method, with an approximation due to Krieger, Li and Iafrate and a correlation-energy functional developed by Colle and Salvetti. The mean absolute deviation of first-row atomic ground-state energies from the exact non-relativistic values is 4.7 mH in our scheme, as compared to 4.5 mH in a recent configuration-interaction calculation. The proposed scheme is significantly more accurate than the conventional Kohn-Sham method while the numerical effort involved is about the same as for an ordinary Hartree-Fock calculation.

Read more
Chemical Physics

Dimensional Perturbation Theory on the Connection Machine

A recently developed linear algebraic method for the computation of perturbation expansion coefficients to large order is applied to the problem of a hydrogenic atom in a magnetic field. We take as the zeroth order approximation the D→∞ limit, where D is the number of spatial dimensions. In this pseudoclassical limit, the wavefunction is localized at the minimum of an effective potential surface. A perturbation expansion, corresponding to harmonic oscillations about this minimum and higher order anharmonic correction terms, is then developed in inverse powers of (D−1) about this limit, to 30th order. To demonstrate the implicit parallelism of this method, which is crucial if it is to be successfully applied to problems with many degrees of freedom, we describe and analyze a particular implementation on massively parallel Connection Machine systems (CM-2 and CM-5). After presenting performance results, we conclude with a discussion of the prospects for extending this method to larger systems.

Read more
Chemical Physics

Direct subsurface absorption of hydrogen on Pd(111)

We summarize and discuss some of the available experimental and theoretical data important for understanding the role played by subsurface sites in dissociative chemisorption calculations for the H 2 /Pd(111) system. Then we use a semi-empirical potential energy surface (PES) to model the interaction of a H 2 molecule impinging on a Pd(111) surface. The London-Eyring-Polanyi-Sato (LEPS) construction has been extended to make direct subsurface absorption possible. A 2-dimensional wave packet calculation is used to find qualitative trends in the direct subsurface absorption and to reveal the time scales involved. We suggest that a partial in-plane relaxation occurs for the slowest incoming particles, thus resulting in a higher direct subsurface absorption probability for low energies.

Read more
Chemical Physics

Dynamical Fluctuating Charge Force Fields: Application to Liquid Water

A new molecular dynamics model in which the point charges on atomic sites are allowed to fluctuate in response to the environment is developed and applied to water. The idea for treating charges as variables is based on the concept of electronegativity equalization according to which: (a) The electronegativity of an atomic site is dependent on the atom's type and charge and is perturbed by the electrostatic potential it experiences from its neighbors and (b) Charge is transferred between atomic sites in such a way that electronegativities are equalized. The charges are treated as dynamical variables using an extended Lagrangian method in which the charges are given a fictitious mass, velocities and kinetic energy and then propagated according to Newtonian mechanics along with the atomic degrees of freedom. Models for water with fluctuating charges are developed using the geometries of two common fixed-charge water potentials: the simple point charge (SPC) and the 4-point transferable intermolecular potential (TIP4P). Both fluctuating charge models give accurate predictions for gas-phase and liquid state properties, including radial distribution functions, the dielectric constant, and the diffusion constant. The method does not introduce any new intermolecular interactions beyond those already present in the fixed charge models and increases the computer time by only a factor of 1.1, making this method tractable for large systems.

Read more

Ready to get started?

Join us today