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

The three-body system Li+(H2O)2 was analyzed to study that non-additive part of the interaction potential which can be obtained by the Hartree-Fock approximation.

Non-additivity in water-ion-water interactions

Study of the structure of molecular complexes. Coordination numbers for Li+, Na+, K+, F− and Cl− in water

We have reinvestigated the LiCN and LiNC molecules in the linear configurations. With very near to Hartree‐Fock functions we obtain the following equilibrium distances: R(Li–C)=3.65± 0.015 a.u., R(C–N)=2.14± 0.015 a.u. for LiCN; R(Li–N)=3.35± 0.015 a.u. and R(C–N)=2.18± 0.015 a.u. for LiNC. The Hartree‐Fock energy (at equilibrium) are −99.8176 a.u. for LiCN and −99.8275 a.u. for LiNC. These data are corrected using a statistical model to compute the correlation energy. The resulting atomization energy (total binding relative to dissociation into ground state atoms) is 13.48 eV for LiCN and 13.78 eV for LiNC. (These data are, therefore, confirming previous computation by Bak, Clementi, and Kortzeborn, done with smaller basis set.) With a constant value for the C–N distance, we have mapped the energy surface for a number of geometries in nonlinear conformations, following the lowest energy path leading from LiCN to LiNC: no energy barrier was found between the LiCN and the LiNC linear configuration or betwe...

Study of the electronic structure of molecules. XVIII. Interaction between a lithium atom and a cyano group as an example of a polytopic bond

In the present study the molecular atomization energies have been computed with some of the recent density functionals and with a semiempirical model developed in the IBM laboratory. In particular, the Lie–Clementi (LC), the Colle–Salvetti (CS), the Gunnarson–Lundqvist (GL), the Vosko, Wilk, and Nusair (VWN), the Perdew–Zunger (PZ), the Becke (B), and the Lee, Yang, and Parr (LYP) functionals have been considered. For GL and VWN the self‐interaction correction of Stoll, Pavlidou, and Preuss (SPP) has been included. For the PZ the gradient correction of Perdew has been included. For the LYP a new and simpler form as proposed by Michlich et al. has been implemented. More than 50 small to fairly large molecules have been chosen at test cases. The results obtained with the present semiempirical model are gratifying and indicate that further improvement and calibration is still possible. From the computed data it is concluded that the density functionals in most cases do provide a pragmatic solution to the det...

A comparative study of density functional models to estimate molecular atomization energies

The correlation energy for the ground‐state isoelectronic series of the second‐row atoms of the periodic table are estimated. In this estimate the relativistic energies for the closed and the open shells, and the Hartree—Fock energies are taken from calculations we have recently performed. As expected, the second‐row correlation energy parallels, in general, the characteristics of the first‐row correlation energy. The principal contribution to the correlation energy is due to pairing of electrons in a given orbital. The intershell correlation is smaller than the intrashell correlation (but not negligible) and the 3s—3s electron correlation is linear in Z.

Enrico Clementi

Papers

Non-additivity in water-ion-water interactions

Study of the structure of molecular complexes. Coordination numbers for Li+, Na+, K+, F− and Cl− in water

Study of the electronic structure of molecules. XVIII. Interaction between a lithium atom and a cyano group as an example of a polytopic bond

A comparative study of density functional models to estimate molecular atomization energies

Correlation Energy for Atomic Systems. II. Isoelectronic Series with 11 to 18 Electrons