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)

Dibyajyoti Chakravarti; Koustav Hazra; Riya Kayal; Sudip Sasmal; Debashis Mukherjee

doi:10.1063/5.0054731

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)

J Chem Phys. 2021 Jul 7;155(1):014101. doi: 10.1063/5.0054731.

Authors

Dibyajyoti Chakravarti¹, Koustav Hazra¹, Riya Kayal¹, Sudip Sasmal², Debashis Mukherjee³

Affiliations

¹ School of Chemical Sciences, Indian Association for the Cultivation of Science, Kolkata, India.
² Physikalisch-Chemisches Institut, Universität Heidelberg, Heidelberg, Germany.
³ Centre for Quantum Engineering, Research, and Education (CQuERE), TCG-CREST, Kolkata, India.

PMID: 34241385
DOI: 10.1063/5.0054731

Abstract

The Unitary Group Adapted State-Specific Multi-Reference Perturbation Theory (UGA-SSMRPT2) developed by Mukherjee et al. [J. Comput. Chem. 36, 670 (2015)] has successfully realized the goal of studying bond dissociation in a numerically stable, spin-preserving, and size-consistent manner. We explore and analyze here the efficacy of the UGA-SSMRPT2 theory in the description of the avoided crossings and interlacings between a manifold of potential energy curves for states belonging to the same space-spin symmetry. Three different aspects of UGA-SSMRPT2 have been studied: (a) We introduce and develop the most rigorous version of UGA-SSMRPT2 that emerges from the rigorous version of UGA-SSMRCC utilizing a linearly independent virtual manifold; we call this the "projection" version of UGA-SSMRPT2 (UGA-SSMRPT2 scheme P). We compare and contrast this approach with our earlier formulation that used extra sufficiency conditions via amplitude equations (UGA-SSMRPT2 scheme A). (b) We present the results for a variety of electronic states of a set of molecules, which display the striking accuracy of both the two versions of UGA-SSMRPT2 with respect to three different situations involving weakly avoided crossings, moderate/strongly avoided crossings, and interlacing in a manifold of potential energy curves (PECs) of the same symmetry. Accuracy of our results has been benchmarked against IC-MRCISD + Q.

(c) For weakly avoided crossing between states displaying differently charged sectors around the crossing region, the insufficient inclusion of state-specific orbital relaxation and the absence of dynamic correlation induced by orbital relaxation in the first order wavefunction for a second order perturbative theory lead to an artifact of double crossing between the pair of PECs.