Scrutinizing the effects of polarization in QM/MM excited state calculations

In this paper we demonstrate the importance of including polarization-especially in a fully self-consistent-field manner-when calculating excited states within linear response QM/MM methods based on correlated electronic structure methods. We perform a systematic investigation of solvent polarization effects by identifying lower order polarization reaction fields as compared to the full treatment. In the process we highlight the different nature of static and dynamic reaction fields and demonstrate-by method of example on both solvated systems as well as on biomolecules-that inclusion of both of these is mandatory for an accurate description of excited states. Ultimately, these findings reflect the importance of the change in the solvent reaction field upon electronic excitations. In light of the recent increasing interest in excited state QM/MM methods incorporating mutual polarization between subsystems as a method for treating large molecular systems, the reported investigation constitutes an important step towards dissecting the accuracy of such calculations.