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A semiempirical functional of the Hartree-Fock density is presented for estimates of the correlation energy correction. The functional is similar to the one proposed by Gombas and is (a) parametrized with reference to few atomic systems, and (b) is modified as to reproduce the atomic correlation correction not only for closed but also for open shell systems. The functional is then applied to the ground state function of the hydrides LiH(1Σ+), BeH(2Σ+), BH(1Σ+), CH(2Π), NH(3Σ−), OH(2Π), and HF(1Σ+). Several internuclear distances have been considered for each hydride, scanning the potential energy curve from the repulsive region to the dissociation products (∼ 10 a.u.). For these points a simple multiconfigurational function (consisting of no more than three configurations) was computed to obtain Hartree-Fock functions with proper dissociation behavior (H-F-P-D functions). The semiempirical functional was applied both to the traditional H-F functions and the H-F-P-D functions in order to study how to select a proper reference wavefunction. The computed binding and a number of spectroscopic constants from vibrational and rotational analysis are discussed. The new semiempirical functional of the electronic density appears to be a rather promising one for reliable estimates of the correlation energy correction.

The analytical fit to a large number of Hartree-Fock computations for the water-water interaction has been reanalyzed and used to study small clusters of water molecules. With the analytically fitted Hartree-Fock potential, thousands of possible configurations for the dimers, trimers, tetramers, pentamers, hexamers, heptamers, and octamers of water have been compared in order to determine the configuration of lowest energy (maximal stabilization energy). For the dimer two possible stable configurations are found, corresponding to an open form and a cyclic form, with the open form being more stable. For the trimers and tetramers the cyclic forms are somewhat more stable than the open structures. For the larger clusters it is concluded that it is rather meaningless to consider a single structure, but what is physically relevant is the statistical distribution of different configurations, since many configurations with significantly different geometry have nearly the same energy. The comparison of the stabilization energy per molecule of the different clusters with the corresponding value for liquid water does not support the mixture-model theories of the structure of liquid water.

Articles you may be interested inThe liquid structure of tetrachloroethene: Molecular dynamics simulations and reverse Monte Carlo modeling with interatomic potentials Study of the structure of molecular complexes. XII. Structure of liquid water obtained by Monte Carlo simulation with the Hartree-Fock potential corrected by inclusion of dispersion forces J. Chem. Phys. 62, 2195 (1975); 10.1063/1.430787 Study of the structure of molecular complexes. VII. Effect of correlation energy corrections to the HartreeFock waterwater potential on Monte Carlo simulations of liquid water J. Chem. Phys. 60, 4455 (1974); 10.1063/1.1680923Study of the structure of molecular complexes. IV. The HartreeFock potential for the water dimer and its application to the liquid state A water-water interaction potential obtained from configuration interaction calculations has been used to simulate liquid water. at 25'C. by a Monte Carlo technique. The resulting radial distribution functions and x-ray and neutron scattering intensities are compared with experiment and found to be in satisfactory agreement. Some thermodynamic properties are also computed and discussed. The overall agreement seems to indicate that many-body effects contribute little in determining the structure of liquid water. although they seem to be important for an accurate simulation of internal energy and related quantities.