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 a previous paper of this series (paper XIX) we reported an ab initio computation for the barrier to the internal rotation in the sugar‐phosphate‐sugar complex, C10H19O8P; the barrier corresponds to the configuration with ω″ = 45°, ω′ = Ψ′ = Ψ″ = φ′ = 0°, and φ″ varies from 0° to 360°. Here we repeat the same study with the C10H18O8P− complex (where one proton of the phosphate group has been removed). The new potential curve shows essentially a parallel displacement in total energy relative to the previously computed one. In addition we report two new computations of the barriers obtained by varying φ′ from 0° to 360°, while keeping ω″ = 45° and ω′ = Ψ′ = Ψ″ = φ″ = 0° (using Olson and Flory's notation). Again the Hartree‐Fock model predicts similar features for the rotational barriers of the two complexes C10H19O8P and C10H18O8P−. The calculated barrier heights are 4.0, 3.2, and 7.0 kcal/mole for the φ″ variation in the C10H19O8P complex, and 5.7, 3.2, and 8.2 kcal/mole in the C10H18O8P− complex. The barrier heights for the φ′ variation are 18.8 and 30.3 kcal/ mole for C10H19O8P, and 34.8 and 35.2 kcal/mole for C10H18O8P−. Finally, the total energy for the internal rotation of φ″ is decomposed into a sum of contributions, representing either the electronic rearrangement following the rotation for a number of fragments composing the C10H18O8P− complex, or the pairwise interaction of such fragments during the rotation. The location of the potential minima and maxima is predicted by the pairwise interaction of the fragments. The energy for the rearrangement within each fragment is large.

Study of the electronic structure of molecules XXII. Additional ab initio computations for the barrier to internal rotation in polynucleotide chains

Fully three-dimensional, SCF ground-state computations for the Hartree equation are carried out by a finite element approach that completely avoids forming or storing the Fock matrix. A combination of strategies is used to reduce storage and computational requirements by orders of magnitude over the traditional finite element approach, which makes three-dimensional molecular orbital computations feasible. Results using the three-dimensional formulation and computer program are shown for one-electron systems: He+ and H2+, and for two-electron systems: He and H2. The best results are within about 30–100 micro-Hartrees of the exact values of the total energies for the ground states of these systems, indicating that our three-dimensional approach has been correctly implemented in the computer code.

A three‐dimensional finite element approach towards molecular SCF computations

Nonlinear effects in molecular dynamics of water: large sample computer simulation of rise and fall transients

On the SCF‐LCAO‐MO and the SCF‐X α‐SW approximations: Computation of the barrier to internal rotation for ethane

The general theory of three-electron Hylleraas-Configuration-Interaction method using linear correlation factors of the form rij has been implemented for molecular systems using cartesian Gaussians as basis sets. A brief review of the theory and the form of the three-electron integrals is presented. Additionally, a table of numerical values of some selected three-electron integrals is given. Results from test calculations on H3 using the full form of the theory are presented for some simple basis sets. A discussion of the computational problems that need to be overcome before this approach is competitive with traditional methods is included.

Enrico Clementi

Papers

Study of the electronic structure of molecules XXII. Additional ab initio computations for the barrier to internal rotation in polynucleotide chains

A three‐dimensional finite element approach towards molecular SCF computations

Nonlinear effects in molecular dynamics of water: large sample computer simulation of rise and fall transients

On the SCF‐LCAO‐MO and the SCF‐X α‐SW approximations: Computation of the barrier to internal rotation for ethane

The Hylleraas-CI method in molecular calculations. III.: implementation and numerical verification of a three-electron many-center theory