Analytical SA-HCISCF Nuclear Gradients from Spin-Adapted Heat-Bath Configuration Interaction.

This work reports an implementation of the analytical nuclear gradients and nonadiabatic couplings with state-averaged SCF wave functions from a spin-pure selected configuration interaction (SCI) method. At the core of the implementation lies the evaluation of the Lagrange multipliers required for the variational calculation of the nuclear gradient. Using the same code infrastructure, we developed a fully CI-coupled second-order orbital optimization method. Both the calculation of the nuclear gradient and the second-order orbital optimization make use of density fitting in order to accelerate the calculation of the two-electron integrals. We demonstrate the use of analytical nuclear gradients in excited-state geometry optimizations for conjugated molecules. In addition, the first triplet excited-state geometry of a transition-metal catalyst, Fe(PDI), was optimized with up to 30 orbitals in the active space. Our results outline the capabilities of the implemented methods as well as directions for future work.