We recently introduced a Dynamically Weighted Complete Active Space Self-Consistent Field (DW-CASSCF) electronic structure for excited-state dynamics. In this Communication, we reformulate analytical gradients at this level of theory using a Lagrangian approach, thereby reducing the required number of coupled-perturbed CASSCF calculations to one per state gradient. In addition, we derive and implement derivative couplings at the DW-CASSCF level for the first time. We demonstrate the new formulation of DW-CASSCF gradients by optimizing a conical intersection for the p-hydroxybenzylidene-imidazolinone anion, the green fluorescent protein chromophore, to shed light on its observed radiationless decay dynamics in the ultraviolet region.

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The X-ray absorption spectrum (XAS) of the hydrated electron (e (aq) − ) has been simulated using time-dependent density functional theory with a quantum mechanics/molecular mechanics description. A unique XAS peak at 533 eV is observed with an energy and intensity in quantitative agreement with recent time-resolved experiments, allowing its assignment as arising from water O1s transitions to the singly occupied molecular orbital (SOMO) in which the excess electron resides. The transitions acquire oscillator strength due to the SOMO comprising an admixture of a cavity-localized orbital and water 4a 1 and 2b 2 antibonding orbitals. The mixing of antibonding orbitals has implications for the strength of couplings between e (aq)− and intramolecular modes of water.