Photochemistry in real space: batho‐ and hypsochromism in the water dimer

The development of chemical intuition in photochemistry faces several difficulties which result from the inadequacy of the one-particle picture, the Born-Oppenheimer approximation, and other basic notions used to build models. We show herein how real space approaches can be efficiently used to get valuables insights in photochemistry via a simple example of red and blue shift effects: the double hypso- and bathochromic shift in the low-lying valence excited states of (H2O) 2 . We demonstrate that (i) the use of these techniques allows to maintain the perturbative language used in the theory of intermolecular interactions even in the strongly interacting short-range regime; (ii) that it is one and only one molecule that gets photoexcited in each of the addressed excited states and (iii) that the electrostatic interaction between the in-the-cluster molecular dipoles provides a fairly intuitive rationalization of the observed batho- and hypsochromism. The methods exploited and illustrated in this paper are able to maintain the individuality as well as the properties of the interacting entities in a molecular aggregate, and thereby they allow to keep and build chemical intuition in general states, at any geometry and using a broad variety of electronic structure methods.