(2 -D tt )B shift for Fock matrix elements in the active orbital when electrons are excited in or out, respectively. Thus, the zeroth-order Hamiltonian will have a positive shift (with B shift > 0) for open-shell character configurations, and the denominator part of the amplitude equation (the second term in Eq. ( )) shifts to a larger value. Thus, the overestimation of the second-order perturbative contribution of these configurations is reduced. As a side effect, the positive shift often alleviates the intruder state problem.