Multi-Reference Linear Response Methods for Valence Electron Spectroscopy

Our current focal point is combining multireference (MR) methods with linear response (LR) theory. For the LR methods, excitation energies and intensities of the UV/Vis and electronic circular dichroism spectra are obtained from the solution of eigenvalue equations that contain the electronic Hessian. With the LR ansatz, a diversity of excitations can be described without augmenting the active space with spectroscopically relevant orbitals and electrons. In contrast to more established approaches based on state averaging (SA), also spectra of organic transition-metal complexes can be simulated that often include plenty of pi orbitals and electrons and can hardly by calculated with the SA approach. In the past, we have worked on an efficient CASSCF-LR implementation [18,19] that is available in the ORCA quantum chemistry package of Prof. Frank Neese since 2019. Ongoing and future research projects cover the inclusion of dynamic correlation to increase the accuracy. For this purpose, density functional theory and second-order perturbation theory will be used.

B. Helmich-Paris; CASSCF linear response calculations for large open-shell molecules. J. Chem. Phys. 150 (7), 174121 (2019). https://doi.org/10.1063/1.5092613

B. Helmich-Paris; Benchmarks for Electronically Excited States with CASSCF Methods. J. Chem. Theory Comput., 15, 4170-4179 (2019). https://doi.org/10.1021/acs.jctc.9b00325