CASSCF CASPT2 STUDIES OF THE LOWEST STATES OF H(5)O2(+)/

CASSCF and CASPT2 calculations have been performed on the ground and t wo lowest singlet and triplet excited states of the protonated dihydra te. Two subsystems can be identified in the excited states: an excited water molecule and a ground state oxonium ion. The electric propertie s have been computed (dipole moment and dipole polarizabilities). The dipole polarizabilities are very large in the excited states of the co mplex (up to 500 au for the largest component). This can be due to the presence of nearly degenerate higher excited states or to an easy pro ton/hydrogen transfer process. The excitation energies, around 8 eV fo r both states, show that the photodissociation of H5O2+ is probably po ssible in the atmosphere, The stabilization energy of the excited stat es with respect to the isolated subsystems is about 5-6 kcal/mol, incl uding the relaxation contribution.