excitation of CH2O · nH2O: An MRD-CI investigation using effective potentials for the representation of the water molecules Original Research Article

The n → π∗ electronic transition (1 1A1 → 1 1A1) of formaldehyde solvated by a varying number of water molecules is investigated using multi-reference CI calculations. The water molecules are considered as spectator groups and are represented by an effective potential. The orthogonality between the wavefunctions of the spectators and the active part of the total system is maintained by employing basis sets for the active part of the spectator which do not significantly overlap. The spectator potential has also been generated in a semiempirical NDDO calculation with negligible loss of accuracy compared to a full ab initio spectator treatment. The calculated solvent shift agrees very well with corresponding results of the reference calculations which consider all water molecules explicitly. The presented combination of sophisticated correlation methods for the region of interest, and semiempirical methods to include the effect of the surroundings allows for fast and reliable calculations of solvent effects in large systems.