XMCQDPT (Extended Multi-Configuration Quasi-Degenerate Perturbation Theory) is the new approach to Multi-State Multi-Reference Petrurbation Theory developed by Dr. Alexander A. Granovsky of Firefly Proejct. ... XMCQDPT2 (XMCQDPT at second order of PT expansion, referred as MC-XQDPT2 in some older publications) is superior to MCQDPT2 and is aimed to completely replace MCQDPT2/DETPT2 calculations in the future. ... Fast Two-electron Integral Code . ... PC GAMESS/Firefly' NBO code . ...