摘要:

We present a comprehensive study of the lattice restricted primitive model, i.e., a lattice gas consisting of an equal number of positively and negatively charged particles interacting via on-site exclusion and a1/rpotential. On the cubic lattice, Monte Carlo simulations show a line of Ne´el points separating a disordered, high-temperature phase from a phase with global antiferromagnetic order. At low temperatures the (high-density) ordered phase coexists with the (low-density) disordered phase. The Ne´el line meets the coexistence curve at a tricritical point,Tt≃0.14,&rgr;t≃0.4.A simple mean-field analysis is in qualitative agreement with simulations. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1301530

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

There has been longstanding interest in computer simulation of finite-wavevector(k)dielectric properties of fluids. Wavevector dependence can be used to obtain measurable dielectric properties by extrapolation tok=0of the finite-ktransverse&khgr;T0(k,&ohgr;)and longitudinal&khgr;L0(k,&ohgr;)dielectric susceptibility components. These components, especially&khgr;L0(k,&ohgr;),are of interest in their own right since they serve as input to nonlocal dielectric theories of electrostatic solvation. The formulation of&khgr;L0(k,&ohgr;)in terms of charge instead of longitudinal dipole density fluctuations makes it possible to extend finite-kdielectric theory to nondipolar fluids. Using the results obtained in our group for thek-depended dielectric susceptibilities of several nonpolarizable model liquids, we illustrate how they vary with the properties of molecules and their interactions, such as charge distribution, polarity, and hydrogen bonding. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1301531

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

Charged polymers and, in particular, polyelectrolytes are of great interest, but our understanding has been much poorer in comparison with neutral polymers. The interest in polyelectrolytes is strong, because many biopolymers such as DNA are polyelectrolytes, and also synthetic polyelectrolytes have important commercial uses. Part of the difficulty in treating polylectrolytes resides in the Coulomb interactions. Recently, molecular dynamics simulations have proven to be a valuable tool in over-coming some of these challenges and have fundamentally enhanced our understanding of polyelectrolytes. These simulations are based on the bead-spring model that has been successively applied to neutral polymers. To model polyelectrolytes, monomers may now be charged, and solvent ions are introduced and treated explicitly. The long ranged Coulomb interaction is treated by a particle-mesh Ewald method. Results of these simulations will be discussed. The structure of a polyelectrolyte chain as a function of concentration, added salt and charge distribution has been calculated. A variety of phenomena have been encountered in these simulations and will be discussed. These include counterion condensation, ion capture and bundle binding. No aggregation occurs for monovalent counterions. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1301532

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

A review is given of a recent theoretical approach to the electronic structure variations in solution via the truncated adiabatic basis-set (TAB) method. Compared to past efforts, novel aspects of the TAB approach include incorporation of both the linear and nonlinear electronic polarizabilities and account of electronic transitions to excited states. Its implementation with the self-consistent field method and application to study structure, dynamics and spectroscopy of liquid water via the Molecular Dynamics computer simulation technique are described. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1301533

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

While wavefunction-based treatments of electron correlation have been very successful for the study of small molecules, they cannot be readily applied to large molecules because their computational cost rises too steeply with molecular size. For example, second order Møller-Plesset perturbation theory (MP2), the simplest such method, involves computational costs that asymptotically increase with the 5th power of molecular size. In this article we discuss the development of new local electron correlation models that ameliorate this problem, by truncating the number of substituted determinants that are included in the correlation treatment. Using atom-centered functions to span the occupied and virtual subspaces permits the truncations to be made by an atomic criterion, that satisfies all of the requirements of a well-defined theoretical model chemistry. The double substitutions that arise in MP2 theory generally involve promoting electrons from occupied orbitals on two atoms to unoccupied (virtual) orbitals on two other atoms, or tetra-atomics in molecules. The simplest restriction is to require one occupied and one virtual orbital to be on a common atom, leading to a triatomics in molecules (TRIM) model. A stronger approximation is to model double substitutions by the direct product of two such atomic excitations, which is a diatomics in molecules (DIM) model of electron correlation. The still more drastic approximation of forcing all double substitutions to be centered on single atoms, cannot describe dispersion interactions, and is not considered here. The theory of the DIM and TRIM models is outlined, and methods for obtaining the atom-centered functions spanning the occupied and virtual subspaces are discussed. Some numerical results are provided to compare the performance of the DIM and TRIM models against untruncated MP2 theory. Finally the outlook for the application of these methods to large molecules is discussed. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1301534

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

Some recent advances in dielectric continuum models for static and dynamic aspects of molecular solvation are discussed, and connections with molecular-level solvent models are noted. The traditional Born-Onsager-Kirkwood (BKO) model is compared to a more flexible model (the so-called frequency-resolved cavity model (FRCM)) which assigns distinct inner and outer solute cavities in accommodating, respectively, the inertialess (optical) and inertial solvent response. Sample calculations of solvent reorganization energy(&lgr;s)are presented for various thermal and optical electron transfer (ET) processes, based on self-consistent reaction field models using molecular orbital (MO) or configuration interaction (CI) solvent wave functions. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1301535

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

We have recently proposed a new quantum reaction field model that does not require the empirical specification of cavity size for spherical ions and molecules. When the solute wavefunction is optimized under the boundary condition that it is fully contained within the cavity, reaction field stabilization and repulsive kinetic energy compression are the competing forces that determine the optimal cavity size. When this model was investigated with the simplest reaction field method, the Born theory, we found that Born theory with wavefunction containment gives good agreement with experimental enthalpies of solvation for cations, but is highly imperfect for anions. Furthermore, that model was not extensible to molecules that are not spherically symmetric. Our new model relaxes the strict separation of solute and solvent electron densities to allow leakage of the solute wavefunction into the surrounding dielectric with a density functional penalty that is a function of solvent and solute electron density. In the limit of infinite solvent density we recover the original electronic reaction field cavity optimization model, thereby allowing us to predict energies and cavity sizes based solely on theoretical grounds for molecules of arbitrary symmetry. By interpreting the solvent density parameter in molecular terms, we show significant improvement for anion energies and radii for low solvent densities, while correct free energies for cations require high local densities like that of full containment. Future prospects for our parameter free model are discussed. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1301536

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

Our previously published, grid-based compressible electrostatic continuum model [1] includes explicit treatment of electrostriction, a nonlinear dielectric effect. An improved version of this method, which more accurately treats the solvent-solute cavity dielectric boundary, is presented and tested against the model of Wood, Quint and Grolier [2] for the case of a spherical ion in supercritical water atT=653&hthinsp;K.This application illustrates the importance of electrostriction—and thus also of models which explicitly include this effect—for solvation in supercritical water. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1301537

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

It is shown that an understanding of some key biological processes such as the thermodynamics of enzyme-ligand binding or the selectivity of ion-channels is ultimately dependent on an understanding of the details of ion hydration. Therefore, a model for calculating the hydration free energy of ions in aqueous solvent is presented. The model is used to first calculate the proton hydration free energy,&Dgr;Ghyd(H+),in an effort to resolve the uncertainty concerning its exact value. In the model we define&Dgr;Ghyd(H+)as the free energy change associated with the following process:&Dgr;G{H+(gas)+[H2O]n(aq)→H+[H2O]n)(aq)},where the solvent is represented by a neutral n-water cluster embedded in a dielectric continuum and the solvated proton is represented by a protonated n-water cluster also in the continuum. All solvated species are treated as quantum mechanical solutes (B3LYP, MP2, MP4, CCSD(T)) coupled to a dielectric continuum using a self consistent reaction field (SCRF) cycle. An investigation of the behavior of&Dgr;Ghyd(H+)as the number of explicit waters of hydration is increased reveals convergence byn=4.The converged value is −262.23 kcal/mol and is independent of theab initiomethod used. These results indicate that the first hydration shell of the proton is composed of at least 4 water molecules. The result strongly suggests that the proton hydration free energy is at the far lower end of the range of values obtained from the literature. The methodology is then used to calculate the hydration free energies of other ions relative to that of the proton. These include cationic forms of the alkali earth elements Li, Na, and K, and anionic forms of the halogens F, Cl, and Br. The relative ion hydration free energy is defined as&Dgr;[&Dgr;Ghyd(Z±)]=G(Z±[H2O]n(aq))−G(H+[H2O]n(aq))−G(Z±(gas))−G(H+(gas))),where the solvated ions are represented by ion-water clusters coupled to a dielectric continuum using a self-consistent reaction field (SCRF) cycle. An investigation of the behavior of&Dgr;[&Dgr;Ghyd(Z±)]as the number of explicit waters of hydration is increased reveals convergence byn=4.This convergence indicates that the free energy change for addition of water to a solvated proton-water complex is the same as the free energy change associated with the addition of water to a solvatedZ±-water complex. This is true as long as there are four explicitly solvating waters associated with the ion. This convergence is independent of the type of monatomic ion studied and it occurs before the first hydration shell of the ions (typically ⩾ 6) is satisfied. Structural analysis of the ion-water clusters reveals that waters within the cluster are more likely to form hydrogen bonds with themselves when clustering around anions, than when clustering around cations. This suggests that for small ion-water clusters, anions are more likely to be externally solvated than cations. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1301538

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

The electrostatic potentials from molecular dynamics (MD) trajectories and Poisson-Boltzmann calculations on a tetra peptide are compared to understand the validity of the resulting free energy surface. The Tuftsin peptide with sequence, Thr-Lys-Pro-Arg, in water is used for the comparison. The results obtained from the analysis of the MD trajectories for the total electrostatic potential at points on a grid using the Ewald technique are compared with the solution to the Poisson-Boltzmann (PB) equation averaged over the same set of configurations. The latter was solved using an optimal set of dielectric constant parameters. Structural averaging of the field over the MD simulation was examined in the context of the PB results. The detailed spatial variation of the electrostatic potential on the molecular surface are not qualitatively reproducible from MD to PB. Implications of using such field calculations and the implied free energies are discussed. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1301539

出版商:AIP    

年代:1999

数据来源: AIP