The zero threshold estimate of our dynamic configuration interaction procedure, is used to obtain the correlation energy correction for nonactive electron pairs in multireference configuration interaction treatments. For exact eigenvectors, the expression is shown to be equivalent to second order perturbation theory with the Epstein-Nesbet partitioning of the reference Hamiltonian. We describe our code which has been efficiently implemented on parallel computers. Few tests have been performed, to show the accuracy of our approach by comparison with the related multiconfigurational, Moller-Plesset-type treatment.