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

As a step towards the elucidation of the reaction mechanism of the enzyme carbonic anhydrase, we have considered the solvation of Zn++ and a linear Zn++⋅CO2 complex, using Monte Carlo techniques. The simulated temperature of the system is 300 K. The coordination numbers for the first and second solvation shell are computed as 6 and 16, respectively, for the case of Zn++. When placed in the first solvation shell the CO2 molecule displaces one water molecule, apparently without altering the remaining water structure in the first shell.

Monte Carlo simulations of water clusters around Zn++ and a linear Zn++⋅CO2 complex

Selected topics in ab initio computational chemistry in both very small and very large chemical systems

Intermolecular interaction energies of methane and methanol with a water molecule are computed in the SCF approximation with a minimal basis set and with the counterpoise correction; the value for the dispersion correction is estimated by a perturbation method. The computed interaction energies for a water molecule at a large number of different positions and orientations relative to either CH/sub 4/ or CH/sub 3/OH (the latter in three different conformations) are used to derive simple analytical atom-atom pair potentials. These potentials are used elsewhere in Monte Carlo studies of the hydration of CH/sub 4/ and CH/sub 3/OH.

Interaction of methane and methanol with water

We present results on the energetics of the hydrogen bridges for the Guanine–Cytosine pair obtained in a DNA fragment consisting of three stacked base pairs in the B-DNA conformation. The wave function computations on the 87 atom system are all-electron ab initio SCF-MO computations obtained with a basis set of 1032 primitive gaussian functions contracted to 315. Even if the results are only preliminary, one can tentatively advance conclusions relative to the molecular field effect of stacked base-pairs on the potential energy surface for a hydrogen bond. These computations have been performed with a modified version of our molecular program, which uses an IBM 4341 host and six to ten attached array processors (FPS-164) in parallel. The strategy to convert the program from sequential to parallel is briefly outlined and comparisons with our parallel system are made with a present-day “vector” super-computer. From these studies, we conclude that if one adopts the “parallel” approach presented and tested here, much larger chemical systems than previously are now amenable to all-electron ab initio computations.

Parallelism in quantum chemistry: Hydrogen bond study in DNA base pairs as an example

The effects of three body forces on the single molecule dynamical properties of liquid water are studied by molecular dynamics using ab initio two body and three body potential models. The molecule’s motion within the cage of neighbors becomes faster while the overall molecular diffusion and reorientation becomes slower. The latter brings the simulated NMR relaxation time into good agreement with experiment.

Enrico Clementi

Papers

Monte Carlo simulations of water clusters around Zn++ and a linear Zn++⋅CO2 complex

Selected topics in ab initio computational chemistry in both very small and very large chemical systems

Interaction of methane and methanol with water

Parallelism in quantum chemistry: Hydrogen bond study in DNA base pairs as an example

Single molecule dynamics of three body water