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

In this paper the energies of the low electronic states of the C2 molecule are calculated, using a semi‐empirical approach. In the calculation, the two‐center core integrals, and the one‐center Coulomb integral, are empirical parameters which are obtained by fitting of the Morse potential curves and from atomic data, respectively. The standard ASMO‐LCAO state wave functions are expanded in covalent, single ionic, and double ionic components, which are constructed so as to obey the Wigner‐Wittmer dissociation rules at infinite internuclear distances. The contributions for each component of the expanded state wave functions are obtained by the variational method. The states considered are 1Σg+ state of the πu(4) configuration (of 2p orbitals), the 3Πu and 1Πu states of the πu(3)σg(1) configuration, and the 3Σg−,1Δg,1Σg+ states of the πu(2)σg(2) configuration. The calculated energies of the 1Σg+ and 1Πu electronic states agree well with experiment. Finally, a critical analysis is given of the coefficients of the expansion in covalent and ionic structures.

Low Excited States in C2

SCF-LCAO-MO ab initio calculations were carried out for the interaction between a charged amino acid and a water molecule. The results obtained were fitted by an analytical potential function of the atom–atom type, and the corresponding potential surfaces were examined by means of orientationally optimized isoenergy contour maps. Monte Carlo simulations were also carried out on a few selected solute–water clusters at T = 300 K, in order to obtain insight into the solvation structure.

Lysinium, argininum, glutamate, and aspartate ions in water solution

A general program to compute two electron repulsion integrals

Carbazole and trinitrofluorenone: an ab initio investigation of ionization potentials, electron affinities and excited states

We discuss two experimental parallel computer systems 1CAP-1 and 1CAP-2 which can be applied to the entire spectrum of scientific and engineering applications. These systems achieve “supercomputer” levels of performance by spreading large scale computations across multiple cooperating processors — several with vector capabilities. We outline system hardware and software, and discuss our programming strategy for migrating codes from a conventional sequential system to parallel. The performance of a variety of applications programs is analyzed to demonstrate the merits of this approach. Finally, we discuss 1CAP-3 an extension to this computing system, which has been recently assembled.

Enrico Clementi

Papers

Low Excited States in C2

Lysinium, argininum, glutamate, and aspartate ions in water solution

A general program to compute two electron repulsion integrals

Carbazole and trinitrofluorenone: an ab initio investigation of ionization potentials, electron affinities and excited states

Large-Scale Computations on a Scalar, Vector and Parallel “Supercomputer”