Near-degeneracy corrections for second-order perturbation theory: comparison of two approaches

We compare two approximate perturbation schemes which were developed recent ly to deal with the (quasi)degeneracy problem in many-body perturbation the ory. We conclude that although the two methods were introduced on quite dif ferent theoretical grounds, their performances are quite similar, and prese nt an improvement over traditional perturbation theory. Both methods are ch eap in computation time, but cannot compete in accuracy with more sophistic ated schemes such as complete-active-space perturbation theory or dressed p article theories.