THEORY OF THE DEVIATION FROM THE RUEDENBERG ENERGY RELATION FOR MOLECULES

Ruedenberg has proposed that for self-consistent field (SCF) methods, E=3/2Es, where E is the total energy and Es the eigenvalue sum. Here the deviation Δ=E-3/2Es is studied. Deviations of both signs are, in principle, possible. The most transparent form for Δ is that derived from a low order gradient expansion in the electron density. Writing the result of this expansion in terms of the chemical potential μ, it can be seen that when μ is large and negative, Δ is also large and negative. If the relation between μ and electronegativity proposed by Parr et al is used, the strongly electronegative elements will result in Δ large and negative. This prediction is confirmed from available numerical SCF calculations. © 1979 American Institute of Physics.