A correlation-energy formula due to Colle and Salvetti [Theor. Chim. Acta 3'7, 329 (1975)], in which the correlation energy density is expressed in terms of the electron density and a Laplacian of the second-order Hartree-Fock density matrix, is restated as a formula involving the density and local kinetic-energy density. On insertion of gradient expansions for the 1ocal kinetic-energy density, density-functional formulas for the correlation energy and correlation potential are then obtained. Through numerical calculations on a number of atoms, positive ions, and molecules, of both openand closed-shell type, it is demonstrated that these formulas, like the original Colle-Salvetti formulas, give correlation energies within a few percent.

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Prompted by a recent paper by Maynard and co-workers (Maynard, A. T.; Huang, M.; Rice, W. G.; Covel, D. G. Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 11578), we propose that a specific property of a chemical species, the square of its electronegativity divided by its chemical hardness, be taken as defining its electrophilicity index. We tabulate this quantity for a number of atomic and molecular species, for two different models of the energy−electron number relationships, and we show that it measures the second-order energy change of an electrophile as it is saturated with electrons.

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