Constrained Variational Response to Fock‐Space Multi‐Reference Coupled‐Cluster Theory: Formulation for Excited‐State Electronic Structure Calculations and Some Pilot Applications

Abstract

Fock‐space (FS) multi‐reference (MR) coupled‐cluster (CC) method has emerged as compact tool to account for electronic structure of open‐shell systems and molecules in low‐lying excited states. Development of linear response (LR) has been one of the challenging problems in FSMRCC due to multiple‐root nature of effective Hamiltonian. The recently developed constrained variational approach (CVA) has opened up a promising tool for efficient evaluation of analytic response properties. In this article, we present formulation of the method for excited state calculations. We discuss the decoupling of equations as a result of spin‐adaptation and present some preliminary results for analytical dipole moments and polarizabilities of some molecules in low‐lying triplet excited states.