Energy partitioning schemes: a dilemma

Abstract

Two closely related energy partitioning schemes, in which the total energy is presented as a sum of atomic and diatomic contributions by using the “atomic decomposition of identity”, are compared on the example of N, N-dimethylformamide, a simple but chemically rich molecule. Both schemes account for different intramolecular interactions, for instance they identify the weak C–H⋯O intramolecular interactions, but give completely different numbers. (The energy decomposition scheme based on the virial theorem is also considered.) The comparison of the two schemes resulted in a dilemma which is especially striking when these schemes are applied for molecules distorted from their equilibrium structures: one either gets numbers which are “on the chemical scale” and have quite appealing values at the equilibrium molecular geometries, but exhibiting a counter-intuitive distance dependence (the two-center energy components increase in absolute value with the increase of the interatomic distances)—or numbers with too large absolute values but “correct” distance behaviour. The problem is connected with the quick decay of the diatomic kinetic energy components.