One remedy that has been proposed in the past, in the context of single-or multireference MP2, has been DCPT (degeneracy-corrected perturbation theory ). Another is regularization of the above expression to remove the singularity for ∆ $% &' → 0. Stück and Head-Gordon proposed a simple level-shift regularizer (δ) in the context of ROOMP2 (restricted orbital-optimized second-order perturbation theory). They found that this works well for single-bond breaking, but the required level shifts for multiple bonds were so large that they disrupted thermochemistry results. Lee and Head-Gordon have recently proposed two energy-gap-dependent regularizers: σ and κ. Although these forms were developed initially with OOMP2 in mind, Head-Gordon and coworkers have shown that even without orbital optimization, σ-and κ-MP2 can achieve better accuracy than ordinary (unregularized) MP2 for both main-group and transition-metal thermochemistry, barrier heights, and non-covalent interactions. They also found that both sets of regularizers performed comparably: hence we will focus here only on κ-regularized MP2 correlation, which has the following expression: