Develop molecular gradients and non-adiabatic coupling vectors for CVX-SCC methods

Develop molecular energy gradients and non-adiabatic coupling vectors for convex similarity constrained coupled cluster (CVX-SCC) methods, using the Lagrangian formalism previously applied to related coupled cluster theories, to enable quantitative branching-plane comparisons and non-adiabatic dynamics simulations through ground-state conical intersections.

Background

The paper demonstrates that CVX-SCCSD and CVX-SCC2 can produce physically correct ground-state conical intersections for several molecular systems. However, the reported intersection geometries were obtained in standard CCSD branching planes rather than branching planes computed consistently at the CVX-SCC level. Such a quantitative treatment requires analytic molecular gradients and non-adiabatic coupling vectors for the CVX-SCC methods. These quantities are also necessary for the paper’s motivating application: adaptive coupled-cluster non-adiabatic dynamics simulations that can pass through ground-state conical intersections. The authors identify their derivation and implementation as a concrete subject for future research, pointing to a Lagrangian formalism used in earlier work.