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

Effect of circular flow on single-molecule cross-mode correlations in liquid water.

Calculation of the third virial coefficients for water using ab initio two-body and three-body potentials

As is known, atomic and small molecular systems can be realistically simulated with quantum-mechanical models. In complex chemical systems, however, the natural parameters for a description are not only the electronic density and energy but also entropy, temperature, and time. We have considered as an example for a complex chemical system the structure of water surrounding DNA with counterions. No direct experimental determination of the solvent structure around a single DNA macromolecule is available from experimental data, despite continuous efforts during the last twenty years to obtain both single crystals of DNA and scattering data from solutions. A very detailed representation is now available concerning the structure and interaction energy of water molecules in the first solvation shell or in the “grooves” of the DNA. The data obtained by our computer simulation are in good agreement with indirect data from DNA fibers at different relative humidities and with other indirect evidence. In addition, our simulated results allow us to present a preliminary model for the solvent effects in transition processes between different DNA conformations. The model presented is also in agreement with available experimental data. Finally, our results report the first determination of the position of counterions in DNA at different relative humidities and at room temperature. This application demonstrates the flexibility of the computational approach. The method we have used is very general; namely, it is not limited to a biological system but is valid for any organic or inorganic problem requiring systematic simulation on a class of compounds interacting either as a few molecules or as a large ensemble of molecules.

Computer simulations of complex chemical systems: solvation of DNA and solvent effects in conformational transitions

Interaction energies from a CI calculation of NaCl and between the like ions Na+-Na+ and Cl--Cl-, previously reported, are accurately fitted to closed analytical expressions. Some qualitative comparisons are presented between the obtained Na+-Cl- potential and other frequently used empirical potentials. The non-empirical quantum mechanical pair-potential is used to calculate vibrational and rotational spectroscopic parameters for the diatomic sodium chloride. Phonon dispersion curves, obtained in inelastic neutron scattering measurements, are calculated within the rigid ion model. In addition we present our computations on the polymorphic transition pressure; several other crystal properties are extracted from the potential function. Also, results of a molecular dynamics study of fused NaCl are presented. The simulated values of the equilibrium and dynamical properties are shown to be in good agreement with the available experimental data. The calculated radial distribution functions g + +, g + - and g - ...

Theoretical study of some gas, liquid and crystal properties of sodium chloride using ab initio potentials

With the use of attached parallel processors as described by Clementi in this conference, we have carried out a program where starting from the Schrodinger equation, we can obtain useful information for quite complicated real macroscopic systems. As an example, we have employed configuration interaction method to calculate the potential function for the water-water interaction, which was then used in Monte Carlo and molecular dynamics simulations to study static and dynamic properties of liquid water. Various hydrodynamic constants, such as viscosity, thermal conductivity, and sound speed were obtained from Green-Kubo relations and the density and current correlations of the simulated water. Effects due to many-body interactions and intramolecular vibrations are reported. Correlation functions resulting from solving the fluctuating hydrodynamic equations for a dilute gas subjected to a temperature gradient are also presented.

Enrico Clementi

Papers

Effect of circular flow on single-molecule cross-mode correlations in liquid water.

Calculation of the third virial coefficients for water using ab initio two-body and three-body potentials

Computer simulations of complex chemical systems: solvation of DNA and solvent effects in conformational transitions

Theoretical study of some gas, liquid and crystal properties of sodium chloride using ab initio potentials

Parallel Computing from Fermion Systems to Hydrodynamics: Water as an Example