For orbital-optimized MP2, this form of regularization (using κ=1.4 E -1 H ) was found to yield accurate molecular thermochemistry while also restoring the Coulson-Fisher points in single-bond dissociation curves (the absence of the latter being a well know failure of canonical MP2). In this work, we also explore a different regularization approach. This is based on Brillouin-Wigner (BW) perturbation theory, where the second-order correlation energy reads: 26