MOLCAS/OpenMolcas is an ab initio electronic structure program providing a large set of computational methods from Hartree-Fock and density functional theory to various implementations of multiconfigurational theory. This article provides a comprehensive overview of the main features of the code, specifically reviewing the use of the code in previously reported chemical applications as well as more recent applications including the calculation of magnetic properties from optimized density matrix renormalization group wave functions.

Ultrafast non-adiabatic dynamics of the small heteroaromatic compound 2-thiopyridone has been studied with surface hopping simulations based on multi-configurational quantum chemistry. Initial excitation of the bright S₂ (π,π^∗) state is found to promptly relax to S₁ (n,π^∗) through in-plane motion. The subsequent dynamics are oppositely driven by out-of-plane motion, which results both in complex population transfers among all of the available states and intersystem crossing predominantly through the ‘El-Sayed forbidden ’S₁ (n,π^∗) to T₂ (n,π^∗) channel, through significant mixing of electronic excitation characters. Despite this complexity, the femto- to picosecond triplet population, expected from several spectroscopic measurements, is well described as a simple exponential decay of the singlet state manifold. No proton transfer is found in the reported trajectories, but two mechanisms for its possible mediation in previously reported experiments are proposed based on the observed structural dynamics: (i) ultrafast intra-molecular transfer driven by the initially coherent in-plane motion and (ii) inter-molecular solvent-mediated transfer driven by the out-of-plane modes that dominate the later motion.