How reliable are topographical characteristics of Hartree-Fock level molecular electron momentum densities?

The topology of molecular electron momentum densities (EMDs) of second and third row closed shell hydrides is investigated. A comparison of topographical features of these EMDs at the restricted Hartree-Fock (RHF) as well as at correlated (MP2 and MP4(SDQ)) levels is carried out. The EMD topology of second and third row hydrides exhibits similar critical structure in a given group. Further, as the electronegativity of the heteroatom increases in a period, there is a decrease in the EMD value at the origin and as the electronegativity decreases in a group there is a corresponding increase in the EMD value at the origin. The qualitative features of EMD distribution at the correlated level differ from the corresponding RHF ones if the basis set used is not sufficient. For a sufficiently large basis set (typically, 6-311 + + G(d,p) and better), the qualitative features of EMDs at RHF level agree well with those displayed by the correlated ones. The EMD topology of the HF molecule poses a challenge to theoretical techniques since the critical structure using various basis sets and levels of theory does not show consistent features. (C) 1997 Published by Elsevier Science B.V.