Electron pair densities and their moments associated with a set ofmulticonfigurational wavefunctions that systematically includenondynamic electron correlation have been constructed for theberyllium isoelectronic sequence. The obtained two-electronquantities are compared to the Hartree-Fock and nearly exactconfiguration interaction results. It is shown that theelectron-electron counterbalance density identifies thenon-dynamical (long-range) electron correlation. Hence, the wavefunction that maximizes the electron-electron counterbalancedensity contains all the non-dynamical electron correlation.This statement constitutes the proposed separation criterion.Explicitly calculated dynamical and non-dynamical Coulomb holesfor the isoelectronic series of beryllium illustrate thefeasibility of this approach.