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

Computational chemistry encompasses a growing number of fields and corresponding techniques. The traditional techniques for stationary-state quantum chemistry and equilibrium Monte Carlo' and Molecular Dynamics* are being expanded with Quantum Monte Carlo: Quantum Molecular Dynamics,' Micmdynamics6 but also by research and computer programs on data base, interactive animation, artificial intelligence, and chemical knowledge processing! This very vigorous expansion should, however, not be taken as indication that the problems characteristic in the traditional areas have been overcome. Indeed, some new avenues have been introduced to complement, if not bypass, some of the previous ones. Presently, we are a t the end of the IVth computer generation,'* where we have witnessed not only increased MIPS performance, but especially MFLOPS performance, either because of vector or parallel architectures. From personal computers, to workstations, to mainframes and supercomputers, the advances have been on a very broad front. The next computer generation" is the VIst and, eventually, we shall make use of the products of the much heralded Vth generation. In the VIst generation we have already announcements of supercomputers with peak performance of about 20-30 GFLOPS; thus it is expected we shall grow between 1W-500 GFLOPS, partly by decreasing the clock speed, mainly because of multiprocessing. We already see workstations with the same speed as the old CRAY XMP (single processor) and the disk storage is being addressed by channels working in parallel, with optical fibers of one or more miles in interconnected distances. Slowly, also the Vth computer generation will appear, since artificial

Selected Topics in ab initio Computational Chemistry in Both Very Small and Very Large Chemical Systems

The Coulomb-Hole-Hartree-Fock method introduced by E. Clementi in the early 1960s and reparametrized more recently by S. Chakraworty and E, Clementi to compute the correlated electronic energy in atomic systems, is here extended to compute molecules. The new parametrization is obtained empirically by fitting first and second atomic ionization potentials from He to Ca and a few diatomic molecules. The present formulation makes use of either one or more determinants in order to ensure proper dissociation products, following the early proposal of G.C. Lie and E.Clementi in the context of density functional computations for molecular systems. The new formulation is tested against the dissociation energies of a large number of molecules and it is found satisfactory. 22 refs., 5 figs., 7 tabs.

Extension of the Coulomb-Hole-Hartree-Fock theory to molecules

Translational anisotropy induced by a circularly polarised laser field applied to liquid water

Despite the availability of fast computers for the last ten years, computations of Hartree-Fock quality wavefunctions (wf) for systems with more than 30 to 40 atoms are nonexistent, and very few examples of computations with 50 or more atoms are available at the minimal basis set level.

Distribution of Matrix Elements for Wavefunction Computations of Large Molecular Systems

The photoionization spectra of B-DNA and Z-DNA are predicted and interpreted from ab initio computations of B-DNA and Z-DNA fragments. Each peak of the spectra of the two conformers is assigned in terms of unambiguous atomic components not only for the well-separated inner shell electron peaks but also for the more complex valence electrons region.

Enrico Clementi

Papers

Selected Topics in ab initio Computational Chemistry in Both Very Small and Very Large Chemical Systems

Extension of the Coulomb-Hole-Hartree-Fock theory to molecules

Translational anisotropy induced by a circularly polarised laser field applied to liquid water

Distribution of Matrix Elements for Wavefunction Computations of Large Molecular Systems

Photoionization spectra of B‐DNA and Z‐DNA