Multi-State Pair-Density Functional Theory Provides Consistent Excited-State Energies for Strongly Interacting States

May 7, 2020

  

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

 

Multi-State Pair-Density Functional Theory Provides Consistent Excited-State Energies for Strongly Interacting States