Conical intersections in solution:: A QM/MM study using floating occupation semiempirical configuration interaction wave functions

We describe a hybrid quantum mechanical/molecular mechanical (QM/MM) method implemented in the context of floating-occupation molecular orbital semiempirical configuration interaction wave functions. The QM/MM approximation is compared to a fully QM treatment, emphasizing its accuracy with respect to features important to photochemical mechanisms such as conical intersection geometries and topography. The methodology we describe allows the location and characterization of solution-phase conical intersections for the first time. This is demonstrated explicitly by application to several biologically relevant chromophores solvated in clusters of up to 150 water molecules. The effect of solvation on the photochemical energy landscapes of these molecules is investigated, and we note a tendency for conical intersections to become absolute excited state minima upon hydration.