Classical Monte Carlo simulations have been carried out for liquid water in the NPT ensemble at 25 °C and 1 atm using six of the simpler intermolecular potential functions for the water dimer: Bernal–Fowler (BF), SPC, ST2, TIPS2, TIP3P, and TIP4P. Comparisons are made with experimental thermodynamic and structural data including the recent neutron diffraction results of Thiessen and Narten. The computed densities and potential energies are in reasonable accord with experiment except for the original BF model, which yields an 18% overestimate of the density and poor structural results. The TIPS2 and TIP4P potentials yield oxygen–oxygen partial structure functions in good agreement with the neutron diffraction results. The accord with the experimental OH and HH partial structure functions is poorer; however, the computed results for these functions are similar for all the potential functions. Consequently, the discrepancy may be due to the correction terms needed in processing the neutron data or to an effect uniformly neglected in the computations. Comparisons are also made for self‐diffusion coefficients obtained from molecular dynamics simulations. Overall, the SPC, ST2, TIPS2, and TIP4P models give reasonable structural and thermodynamic descriptions of liquid water and they should be useful in simulations of aqueous solutions. The simplicity of the SPC, TIPS2, and TIP4P functions is also attractive from a computational standpoint.

Comparison of simple potential functions for simulating liquid water

A description of the ab initio quantum chemistry package GAMESS is presented. Chemical systems containing atoms through radon can be treated with wave functions ranging from the simplest closed-shell case up to a general MCSCF case, permitting calculations at the necessary level of sophistication. Emphasis is given to novel features of the program. The parallelization strategy used in the RHF, ROHF, UHF, and GVB sections of the program is described, and detailed speecup results are given. Parallel calculations can be run on ordinary workstations as well as dedicated parallel machines. © John Wiley & Sons, Inc.

General atomic and molecular electronic structure system

CHARMM (Chemistry at HARvard Macromolecular Mechanics) is a highly flexible computer program which uses empirical energy functions to model macromolecular systems. The program can read or model build structures, energy minimize them by first- or second-derivative techniques, perform a normal mode or molecular dynamics simulation, and analyze the structural, equilibrium, and dynamic properties determined in these calculations. The operations that CHARMM can perform are described, and some implementation details are given. A set of parameters for the empirical energy function and a sample run are included.

CHARMM: A program for macromolecular energy, minimization, and dynamics calculations

Previous attempts to combine Hartree–Fock theory with local density‐functional theory have been unsuccessful in applications to molecular bonding. We derive a new coupling of these two theories that maintains their simplicity and computational efficiency, and yet greatly improves their predictive power. Very encouraging results of tests on atomization energies, ionization potentials, and proton affinities are reported, and the potential for future development is discussed.

A New Mixing of Hartree-Fock and Local Density-Functional Theories

Polarization functions are added in two steps to a split-valence extended gaussian basis set: d-type gaussians on the first row atoms C. N, O and F and p-type gaussians on hydrogen. The same d-exponent of 0.8 is found to be satisfactory for these four atoms and the hydrogen p-exponent of 1.1 is adequate in their hydrides. The energy lowering due to d functions is found to depend on the local symmetry around the heavy atom. For the particular basis used, the energy lowerings due to d functions for various environments around the heavy atom are tabulated. These bases are then applied to a set of molecules containing up to two heavy atoms to obtain their LCAO-MO-SCF energies. The mean absolute deviation between theory and experiment (where available) for heats of hydrogenation of closed shell species with two non-hydrogen atoms is 4 kcal/mole for the basis set with full polarization. Estimates of hydrogenation energy errors at the Hartree-Fock limit, based on available calculations, are given.

The influence of polarization functions on molecular orbital hydrogenation energies

The analytical fit to a large number of Hartree‐Fock computations for the water‐water interaction has been reanalyzed and used to study small clusters of water molecules. With the analytically fitted Hartree‐Fock potential, thousands of possible configurations for the dimers, trimers, tetramers, pentamers, hexamers, heptamers, and octamers of water have been compared in order to determine the configuration of lowest energy (maximal stabilization energy). For the dimer two possible stable configurations are found, corresponding to an open form and a cyclic form, with the open form being more stable. For the trimers and tetramers the cyclic forms are somewhat more stable than the open structures. For the larger clusters it is concluded that it is rather meaningless to consider a single structure, but what is physically relevant is the statistical distribution of different configurations, since many configurations with significantly different geometry have nearly the same energy. The comparison of the stabilization energy per molecule of the different clusters with the corresponding value for liquid water does not support the mixture‐model theories of the structure of liquid water.

Study of the structure of molecular complexes. VI. Dimers and small clusters of water molecules in the Hartree‐Fock approximation

In order to obtain the heat of formation ΔH, for the ion‐water complexes previously studied in the Hartree‐Fock approximation in the first three papers of this series, we have computed the normal frequencies of the complexes, the zero‐point energy correction to ΔH, and the molecular extra correlation energy. The main contribution to ΔH is due to the Hartree‐Fock binding; the least important contribution results from the correlation effects. The Hartree‐Fock binding varies from about 35 kcal/mole (Li+–H2O) to about 12 kcal/mole (Cl−–H2O); the zero‐point correction is between 1 and 2 kcal/mole; and the molecular extra correlation correction is less than 1 kcal/mole. The computation of ΔH is analyzed in order to estimate upper and lower bounds. We conclude that the calculated ΔH values are accurate to about 2.0 kcal/mole. Experimental data support this conclusion. In the Appendix, the potentials for water‐ion complexes have been presented in the form of a simple analytical expansion. The expansion has been obtained by fitting the Hartree‐Fock computed energies for the water‐ion complexes.

Study of the structure of molecular complexes. V. Heat of formation for the Li+, Na+, K+, F−, and Cl− ion complexes with a single water molecule

Self-consistent field wavefunctions have been obtained for the ground states of the first row atoms and for the excited states belonging to the same configurations. They are the solution of the variational problem of finding the best orbitals for a given state, without any additional approximations except for those inherent in the expansion method.

Accurate analytical self-consistent field functions for atoms. ii. lowest configurations of the neutral first row atoms

Ab initio computations are presented for the reaction NH3+HCl→NH4Cl. The two reactants are studied at a large number of positions and for each point an SCF LCAO MO wavefunction and the corresponding total energy are obtained. These results are then collected in an energy surface diagram. All the electrons of the system are considered and a contracted Gaussian basis set has been used in this work. This study considers the two reactants as one system and both charge transfer as well as hydrogen‐bonding characteristics are analyzed within the molecular‐orbitals framework instead of the valence‐bond approximation as customary in the past. The reaction has been analyzed for the case where the HCl approaches the NH3 molecule in a path normal to the plane of the three hydrogens of NH3; the H atom of HCl is between the N atom and the Cl atom. The first stage of the reaction (NH3 and HCl at large distances) indicates mutual polarization of the two molecules. In this region the SCF LCAO MO approximation is rather p...

Study of the Electronic Structure of Molecules. II. Wavefunctions for the NH3+HCl→NH4Cl Reaction

The Matsuoka-Clementi-Yoshimine (MCY) configuration interaction potential for rigid water-water interactions has been extended to include the intramolecular vibrations The extended potential (MCYL), using no empirical parameters other than the atomic masses, electron charge, and Planck constant, is used in a molecular-dynamics simulation study of the static and dynamic properties of liquid water Among the properties studied are internal energy, heat capacity, pressure, radial distribution functions, dielectric constant, static structure factor, velocity autocorrelation functions, self-diffusion coefficients, dipole autocorrelation function, and density and current fluctuations Comparison with experiments is made whenever possible Most of these properties are found to improve slightly relative to the MCY model The simulated high-frequency sound mode seems to support the results and interpretation of a recent coherent inelastic neutron scattering experiment

Enrico Clementi

Papers

Study of the structure of molecular complexes. VI. Dimers and small clusters of water molecules in the Hartree‐Fock approximation

Study of the structure of molecular complexes. V. Heat of formation for the Li+, Na+, K+, F−, and Cl− ion complexes with a single water molecule

Accurate analytical self-consistent field functions for atoms. ii. lowest configurations of the neutral first row atoms

Study of the Electronic Structure of Molecules. II. Wavefunctions for the NH3+HCl→NH4Cl Reaction

Molecular-dynamics simulation of liquid water with an ab initio flexible water-water interaction potential