Electronically excited states of molecular systems are challenging to study using ab initio theory, especially when one is interested in photophysical and photochemical processes, which require accurate description of the excited states away from the Franck-Condon region, as well as adequate description of conical intersections. Multireference methods provide a natural way to deal with these problems. They are however very expensive, so they are limited to small systems. We  have developed an approach to improve the scaling and efficiency of multireference configuration interaction calculations. This approach substitutes the molecular orbitals with  natural orbitals derived from a correlated calculation on a  high multiplicity  ground state. Using natural orbitals allows truncation of virtual orbitals without any significant error introduced into the calculation. This method works well to reproduce excited state energies and smooth potential energy surfaces, as well as conical intersections.  These developments and applications for excited states will be presented.

Theoretical developments for modeling excited states and conical intersections

244th National Meeting (2012)

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