We present an algorithm for the automatic determination of an orbital active space guess to help preserve active space consistency for a photoactive molecule in different nuclear configurations and embedded in different chemical environments. The algorithm relies on the singular value decomposition (SVD) of the overlap matrix between a subsystem's reference active space orbitals and the full system's molecular orbitals, yielding orbitals with maximum overlap with the reference space. Our approach avoids the need to manually choose orbitals for an active space guess over every geometry in an ensemble, allowing for high-throughput electronic structure calculations over many geometries and different solvent environments with CASSCF. We apply this technology, which we call the Automatic Orbital Selection for Multiconfigurational Self-Consistent Field (AOS-MCSCF), to constructing a linear absorption spectrum for the solvated GFP chromophore, with a particular focus on the states relevant to GFP photoexcitation from UV radiation, and find excellent agreement with recent experiments. The technology presented herein provides systematically improvable approaches to obtaining highly accurate absorption and emission spectra with the inclusion of dynamic correlation through CASPT2.

Automatic orbital selection for multiconfigurational self-consistent field

ACS Fall 2021

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