ON THE EVALUATION OF MOLECULAR ELECTRON-AFFINITIES BY APPROXIMATE DENSITY FUNCTIONAL THEORY

The ability of approximate Density Functional Theory to calculate molecular electron affinities has been. probed by a series of calculations on the hydrides CH3, NH2, OH, and HC2 as well as the multibonded species CN, BO, N3, OCN, and NO2. The simple Hartree-Fock Slater scheme lacks dynamic correlations and underestimates on the average the adiabatic electron affinities (EA(ad)) by 0.7 eV. A considerable improvement is obtained by the Local Density Approximation (LDA) in which dynamic correlation is included. Values from LDA calculation underestimate, on the average, the adiabatic electron affinities by 0.4 eV. The best agreement with experiment is obtained by the LDA/NL scheme in which a nonlocal correction recently proposed by Becke is added to the LDA energy expression. The LDA/NL method underestimates EA(ad) by 0.2 eV. It is concluded that the LDA/NL method affords EA(ad)'s in as good agreement with experiment as ab initio techniques in which electron correlation is taken into account by extensive configuration interaction. A full geometry optimization has been carried out on the nine neutral sample molecules as well as the corresponding anions.