Study of the electronic structure of molecules. XXI. Correlation energy corrections as a functional of the Hartree-Fock density and its application to the hydrides of the second row atoms

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.