Ab initio study of the π*â†n electronic transition in formic acid?(water)n (n = l, 2) hydrogen bonded complexes

Hydrogen bonded complexes of formic acid with one and two molecules of water have been studied using multiconfigurational SCF and second-order perturbation theory (CASSCF/CASPT2). Equilibrium geometries in the ground electronic state, SO, and vertical S1�S0 transition energies have been calculated for the three 1 : 1 complexes, and four 1 : 2 conformers found. The most stable conformers are the six-membered ring structures for the 1 : 1 and 1 : 2 complexes in agreement with previous calculations. All vertical transitions are blue-shifted with respect to the corresponding monomer transition.