From Many-Body <i>Ab Initio</i> to Effective\nExcitonic Models: A Versatile Mapping Approach Including Environmental\nEmbedding Effects

We present an original multistate projective diabatization\nscheme\nbased on Green’s function formalisms that allows the systematic\nmapping of many-body <i>ab initio</i> calculations onto\neffective excitonic models. This method inherits the ability of the\nBethe–Salpeter equation to describe Frenkel molecular excitons\nand intermolecular charge-transfer states equally well, as well as\nthe possibility for an effective description of environmental effects\nin a QM/MM framework. The latter is found to be a crucial element\nin order to obtain accurate model parameters for condensed phases\nand to ensure their transferability to excitonic models for extended\nsystems. The method is presented through a series of examples illustrating\nits quality, robustness, and internal consistency.