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 self‐consistent field functions for the ground state of the first‐ and second‐row atoms (from He to Ar) are computed with a basis set in which two Slater‐type orbitals (STO's) are chosen for each atomic orbital. The reported STO's have carefully optimized orbital exponents. The total energy is not far from the accurate Hartree—Fock energy given by Clementi, Roothaan, and Yoshimine for the first‐row atoms and unpublished data for the second‐row atoms. The obtained basis sets have sufficient flexibility to be a most useful starting set for molecular computations, as noted by Richardson. With the addition of 3d and 4f functions, the reported atomic basis sets provide a molecular basis set which duplicate quantitatively most of the chemical information derivable by the more extended basis set needed to obtain accurate Hartree—Fock molecular functions.

Simple Basis Set for Molecular Wavefunctions Containing First‐ and Second‐Row Atoms

Ab initio molecular orbital wavefunctions have been constructed for the planar and pyramidal conformations of ammonia in its ground electronic state. These solutions are very close to the Hartree–Fock limit and exhibit a lower total energy (ETequil = − 56.22191 hartree) than any other such calculations which have been carried out. The computed inversion barrier is 5.08 kcal/mole (exptl = 5.8 kcal/mole) and results almost solely from the differential mixing of d‐type polarization functions in the planar and pyramidal geometries. In contrast to the conclusions of much earlier work, the inversion barrier may be quantitatively obtained within the framework of the molecular orbital (Hartree–Fock) approximation. Other properties of ammonia that have been determined and discussed are: binding energy, heat of formation, correlation energy, dipole moment, population and energy component analysis, force constants, and the Walsh diagram.

Electronic Structure and Inversion Barrier of Ammonia

A semiempirical functional of the Hartree‐Fock type density, previously obtained for the Hartree‐Fock atomic functions and applied with good success to the second row hydrides (in paper XXI of this series), is now applied to the molecules H2(X 1Σg+), Li2(X 1Σg+), Be2(1Σg+), B2(X 3Σg−), C2 (x1 Σg+), N2(X 1Σg+), O2(X 3Σg−), and F2(X 1Σg+). For each molecule the potential energy curve is computed from the repulsive region to large internuclear distances. In order to follow the dissociation to the correct limits, we have added to the Hartree‐Fock configuration of each molecule those configurations which are needed for describing proper dissociation (HFPD functions). It is found that for certain molecules (Li2, B2, C2, and O2) there are configurations which contribute quite significantly in the bonding regions and must be added to the HFPD function to reach a proper reference state function as previously defined (see paper XXI). With our functional of the density applied to the HFPD functions, the calculated d...

Study of the electronic structure of molecules. XXII. Correlation energy corrections as a functional of the Hartree‐Fock type density and its application to the homonuclear diatomic molecules of the second row atoms

Results of Hartree-Fock calculations on the ions of ${\mathrm{Li}}^{\ensuremath{-}}$, ${\mathrm{B}}^{\ensuremath{-}}$, ${\mathrm{C}}^{\ensuremath{-}}$, ${\mathrm{N}}^{\ensuremath{-}}$, ${\mathrm{O}}^{\ensuremath{-}}$, ${\mathrm{F}}^{\ensuremath{-}}$, ${\mathrm{N}}^{\ensuremath{-}\ensuremath{-}}$, ${\mathrm{O}}^{\ensuremath{-}\ensuremath{-}}$ in states of the lowest electronic configuration, along with correlation and relativistic corrections, are given. These data are used to compute electron affinities for the atoms of the first row of the periodic table, and the stabilities of the excited states of the ions relative to the neutral atoms. Computed electron affinities for atoms on which experimental determinations are not available are 0.58\ifmmode\pm\else\textpm\fi{}0.05 eV for lithium, 0.3\ifmmode\pm\else\textpm\fi{}0.05 eV for boron, and -0.27\ifmmode\pm\else\textpm\fi{}0.11 eV for nitrogen. Computed electron affinities for other first row atoms are in agreement with experiment with the exception of oxygen, where the computed value of 1.22\ifmmode\pm\else\textpm\fi{}0.14 eV is significantly less than the experimental determination of 1.47 eV. For the first row atoms, no excited states of the ground-state electronic configuration are lower in energy than the neutral atom, although ${\mathrm{C}}^{\ensuremath{-}}(^{2}D)$ is only 0.08\ifmmode\pm\else\textpm\fi{}0.05 eV above.

Atomic Negative Ions

The two‐body Hartree‐Fock potential for the ion‐water interaction and the two‐body Hartree‐Fock potential for the water‐water interaction have been used in the pairwise additivity approximation to study the Li+(H2O)n, the Na+(H2O)n, the K+(H2O)n, the F−(H2O)n, and the Cl−(H2O)n complexes, where n =2,3,4, ⋯ ,10. The complex configurations have been constrained to have either symmetrical geometries around the central ion or to be free to assume the lowest energy configuration. For n smaller than 5 (depending on the specific ion in consideration), the symmetrical configuration is the lowest energy configuration. For higher values of n, some of the water molecules tend to form a second shell of solvated water around the ion. The configurational optimalization was carried out only at T =0°K; but for a small cluster containing only four molecules of water, calculations have been performed at T =298°K. From the study at 298°K we have computed the correlation functions gI–O, gI–H, gO–O, gO–H, and gH–H (where the ...

Enrico Clementi

Papers

Simple Basis Set for Molecular Wavefunctions Containing First‐ and Second‐Row Atoms

Electronic Structure and Inversion Barrier of Ammonia

Study of the electronic structure of molecules. XXII. Correlation energy corrections as a functional of the Hartree‐Fock type density and its application to the homonuclear diatomic molecules of the second row atoms

Atomic Negative Ions

Study of the structure of molecular complexes. VIII. Small clusters of water molecules surrounding Li+, Na+, K+, F−, and Cl− ions