When two electronic states of the same symmetry are close in energy, they have strong interactions, and they cannot be treated separately – one must use multi-state methods. To incorporate state interactions in multiconfiguration pair-density functional theory (MC-PDFT), we have developed two new multi-state (MS) methods, namely extended multi-state PDFT (XMS-PDFT) and variational multi-state PDFT (VMS-PDFT. The new methods have been tested successfully for eight systems exhibiting locally avoided crossings among two to six states. Since both XMS-PDFT and VMS-PDFT are much less expensive than XMS-CASPT2, they will allow well-correlated calculations on large systems for which perturbation theory is undoable. The figure shows schematically how nonphysical results are obtained for potential curves when they are treated separately – on the left, and how this is remedied when they are treated together by a multi-state method – on the right.
“Multi-state pair-density functional theory,” J. J. Bao, C. Zhou, Z. Varga, S. Kanchanakungwankul, L. Gagliardi and D. G. Truhlar, Faraday Discussions 224, 348-372 (2020). doi.org/10.1039/D0FD00037J
May 7, 2020