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

Electronic States in the C4 Molecule1a

The molecular dynamics of liquid water have been simulated over a wide range of thermodynamic conditions using a five by five site–site model for the intermolecular potential recently developed by Evans. The model reproduces the pressure satisfactorily for temperatures up to 1273 K at constant density of 1 g/cm3. Within the uncertainty, the experimental pressure is also reproduced satisfactorily at the critical point. Two simulations at 773 and 1043 K have also been carried out at constant molar volume of 8.5 cm3/mol. The molecular dynamics of the sample were investigated at each state point with a range of auto‐ and cross‐correlation functions. Some of these have been computed at constant molar volume over a 15 kbar range of pressure and 1000 K range of temperature. They suggest that diffusional dynamics in liquid water are largely determined by density. Some results at 10 (1 bar) and 77 K (1 bar) were obtained by ‘‘splat quenching’’ at constant molar volume. The oxygen–oxygen pair distribution functions from these simulations have been compared with the results available from amorphous solid water at these temperatures.

Molecular dynamics simulation of water from 10 to 1273 K

Kinetically balanced geometric Gaussian basis set calculations for relativistic many-electron atoms with finite nuclear size

Correlation Energy in the CH4 Molecule

We have simulated the perturbation involved by a solvent (or by other periodic perturbants) on a polymer (polyethylene) and we have analyzed (1) the effect of the perturbation on the energy bands of the polymer and (2) the internal charge transfer occurring on the polymer as a response to the external perturbation. The band structure is computed in the tight binding approximation using a self‐consistent field model originally proposed by Ladik and subsequently improved and programmed by Andre. The model is an ab initio model, aimed at Hartree–Fock solutions for the band structure of periodic systems. All the many center integrals of coulomb and exchange type are accurately computed (the computation is an all‐electron computation, and with no approximation for the atomic potential of the atoms in the unit cell). We have found that by surrounding the polymer with positive or negative point charges, the entire band system raises or lowers its entire energy spectrum. The band structure (and the energy gap) wa...

Enrico Clementi

Papers

Electronic States in the C4 Molecule1a

Molecular dynamics simulation of water from 10 to 1273 K

Kinetically balanced geometric Gaussian basis set calculations for relativistic many-electron atoms with finite nuclear size

Correlation Energy in the CH4 Molecule

Study of the Electronic Structure of Molecules. XIV. Point Charge Perturbation on Periodic Systems