A universal explicit electron correlation correction applied to Mukherjee's multi-reference perturbation theory

► We apply universal explicit electron correlation methods (F12) to Mk-MRPT2. ► The basis set incompleteness is reduced significantly by this F12 correction. ► One F12 formulation is additive with the conventional contribution. ► The Mk-MRPT2-F12 formulation influences the conventional contribution directly. ► Pilot applications are presented.