An ab initio SCF calculation of the dipole-moment derivatives and infrared-absorption intensities of the water-dimer molecule

The dipole-moment derivatives and infrared-absorption intensities of the water dimer including several deuterated species have been calculated using ab initio SCF techniques. The results are compared with the analogous, quantities for monomeric water. In addition to the highly enhanced intensity of the intramolecular OH stretch most intermolecular modes that occur in the 90–600 cm^?1 region are also found to be very intense. An electrostatic model for the water dimer has been explored with a view to developing a possible scheme for the calculation of infrared intensities of larger clusters. As a result of the significant exchange and charge-transfer effects such a model is found to be unreliable in describing the dipole-moment derivatives that directly involve the hydrogen bond.