Journal Article Bifurcation CO release Mn complexes News QM/MM Research Support, Non-U.S. Gov't absorption spectroscopy density functional theory excited-state intramolecular proton transfer intrinsic reaction coordinate minimum energy path n-furcation nonadiabatic transition on-the-fly MD potential energy surface projected Hessian matrices quantum chemistry rhenium (I) complexes semiclassical spin-orbit coupling transition state tunneling pathway tunneling splitting valley-ridge inflection wave function