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The Unitary Group Adapted State-Specific Multi-Reference Perturbation Theory(UGA-SSMRPT2) developed by Mukherjee et al [J. Comput. Chem. 2015, 36, 670] hassuccessfully realized the goal of studying bond dissociation in a numericallystable, spin-preserving and size-consistent manner. In this paper, we exploreand analyse the UGA-SSMRPT2 theory in the description of avoided crossings andinterlacing between a manifold of states belonging to the same space-spinsymmetry. In a state-specific formalism, since each state is an eigenstate ofits own effective operator, to include the information of the other statesrequires the theory to be sufficiently accurate. Three different aspects ofUGA-SSMRPT2 have been studied: (a) We introduce and develop the most rigorousversion of UGA-SSMRPT2 which emerges from the rigorous version of UGA-SSMRCCutilizing a linearly independent virtual manifold; we call this the'projection' version of UGA-SSMRPT2 denoted as UGA-SSMRPT2 Scheme P. We compareand contrast this approach with our earlier formulation that used extrasufficiency conditions via amplitude equations, which we will denote asUGA-SSMRPT2 Scheme A. (b) We present the results for a variety of electronicstates of a set of molecules which display the striking accuracy of both thetwo versions of UGA-SSMRPT2; with respect to three different situationsinvolving weakly avoided crossings, moderate/strongly avoided crossings andinterlacing in a manifold of PECs of same symmetry. Accuracy of our results hasbeen benchmarked against IC-MRCISD+Q. (c) For weakly avoided crossing betweenstates displaying differently charged sectors in the asymptotes, theinsufficient inclusion of state-specific orbital relaxation in a second orderperturbative theory might lead to an artefact of double crossing between thepair of PECs.

journal of chemical physics online/the journal of chemical physics/journal of chemical physics

Exploration of interlacing and avoided crossings in a manifold of  potential energy curves by a Unitary Group Adapted State Specific  Multi-Reference Perturbation Theory (UGA-SSMRPT)