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 way the MOTECC-89 project started in the early sixties at the IBM Research Laboratory in San Jose, California. The six years of post-doctoral research, first with Giulio Natta on conductive polymers, with Michael Kasha on spin-orbit effects, with Kenneth S. Pitzer on high temperature molecules and thermo dynamics and with R. S. Mulliken in the quantum chemistry of small molecules had demonstrated pragmatically the importance of a broad-based research and also let me taste some of the excitement to be derived from interdisciplinarity. Thus when I started to gather a department in the newly opened IBM Research Laboratory in San Jose, California, I purposely named it "Large Scale Scientific Computation Department," avoiding reference to chemistry, physics, statistical mechanics or fluid dynamics, which were our main tasks. In the sixties interdisciplinarity was more and more recognized as a most important if not nec essary avenue to cope with the technical needs of our society. However, at that time interdisciplinarity was synonymous with "team work," and true interdisciplinarity was a formidably difficult objective. Although I headed an excellent group of scientists with different backgrounds and there was much progress and creativity, still each one of us was more or less conducting his own research in his own field with occasional cross-field partnerships and with some of the computational techniques as our common base. Later, in 1974, I made a second attempt, this time starting a new department at the Donegani Institute, Montedison, in Novara, Italy."

Modern Techniques in Computational Chemistry: MOTECC™ -89

The molecular orbital theory is used in appropriate semi-empirical forms to predict the properties of carbon vapor. The results indicate that linear polyatomic molecules: C = C = C - - - C = C: are the important species. Experimental results from the literature for C/sub 3/ are combined with the calculated conjugation or resonance energies and with the heats of formation of allene and ethylene to predict heats of formation for all larger carbon molecules. It is found that the odd species have closed shell structures and lower energies than the even species but that the even species should show greater electron affinity. Both of these results are consistent with the mass spectrometric results of Honig and of Chupka and Inghram. Molecular spectroscopic data on C/sub 3/O/sub 2/ are used to estimate the free energy function increments for the species above C/sub 3/. The calculated partial vapor pressures predict C/sub 5/ to be the most abundant species in the saturated vapor even at 2000 deg K with C/sub 7/T becoming comparably abundant in the 2500 to 3000 deg K range. At higher temperatures even larger molecules should become important. The results are shown to be generally consistent with allmore » reliable vaporization data provided the evaporation coefficients decrease rapidly for increasing molecular size and vary for different crystal surfaces of graphite. The calculated electronic energy levels for C/sub 2/ and C/sub 3/ agree satisfactorily with the observed spectra and trends are predicted for both even and odd larger species. It is proposed that liquid carbon consists of essentially infinite linear chains of this type. Both entropy and energy considerations lead to predicted heats of fusion of about 10 kcal/gm atom at at least the absence of any serious inconsistency. (auth)« less

https://escholarship.org/content/qt7t5822s0/qt7t5822s0.pdf?t=p0wtlz

Large molecules in carbon vapor

Electric-field induced intramolecular electron transfer in spiro .pi.-electron systems and their suitability as molecular electronic devices. A theoretical study

The Hartree‐Fock energy of the water dimer has been computed for 216 different nuclear configurations using the basis set given in the first paper of this series. Near the equilibrium configuration, a calculation was carried out using a large Gaussian basis set with optimized orbital exponents for the oxygen 3d ‐ and 4f ‐type and hydrogen 2p ‐ and 3d ‐type functions in order to get results close to the Hartree‐Fock limit. In the vicinity of the equilibrium configuration the mechanism for binding of the dimer is analyzed with the help of the bond energy analysis formalism. The importance of polarization (internal charge transfer), as pointed out previously by a number of authors, is clearly evident. The computed energies have been used to derive a simple analytical expression that reproduces the Hartree‐Fock potential energy surface to a high degree of accuracy. This analytical potential is compared with the empirical effective pair potentials proposed by Rowlinson and Ben‐Naim and Stillinger for the descr...

Enrico Clementi

Papers

Modern Techniques in Computational Chemistry: MOTECC™ -89

Large molecules in carbon vapor

Electric-field induced intramolecular electron transfer in spiro .pi.-electron systems and their suitability as molecular electronic devices. A theoretical study

Study of the structure of molecular complexes. IV. The Hartree‐Fock potential for the water dimer and its application to the liquid state

Revisiting small clusters of water molecules