the structures of acl and mcp in the gas phase were optimized at the b3lyp/6 - 31 g * level . the structures of the water clusters were determined with classical molecular dynamics trajectories ( see details in the si ) . the cluster models include solvents that have at least one of the atoms within 3 and 4  from acl and mcp , respectively . the geometry of the acl and mcp in the water clusters was further optimized at the b3lyp / cc - pvdz and cam - b3lyp / cc - pvdz levels , respectively . in the optimization , the water molecules were fixed at the md structure and replaced by point - charges ( tip3p charges ) . we also fixed the c atom next to the o atom of acl . in mcp , the atomic coordinates of the central c atom were fixed . the hf orbitals were transformed into mos localized within each fragment ( solute and solvent ) . our transformation uses reference orbitals ( rmos ) obtained with external calculations for isolated molecules . , we show the populations at the fragments . in the perturbation - selection step of the sac - ci calculations , a set of threshold ,  levelfour  , as a service to our authors and readers , this journal provides supporting information supplied by the authors . such materials are peer reviewed and may be re - organized for online delivery , but are not copy - edited or typeset . technical support issues arising from supporting information ( other than missing files ) should be addressed to the authors
intermolecular interactions regulate the molecular properties in proteins and solutions such as solvatochromic systems . some of the interactions have to be described at an electronic - structure level . in this study , 
 a commutator for calculating the excitation energy is used for deriving a first - order interacting space ( fois ) to describe the environmental response to solute excitation . the fois wave function for a solute - in - solvent cluster 
 is solved by second - order perturbation theory . 
 the contributions to the excitation energy are decomposed into each interaction and for each solvent .