Intramolecular fundamentals of CO .. H2O are studied using a combination of diffusion Monte Carlo (DMC), and an adiabatic approximation based on frequency separation between inter-and intramolecular vibrations. Variational treatment in a Morse basis is employed for the stretch excitations. DMC is used to calculate the distribution of intermolecular configurations, and to evaluate the (substantial) influence of intermolecular zero-point motion on intramolecular frequency shifts. The dependence of the potential on intermolecular coordinates was derived with the help of ab initio Moller Plesset perturbation theory calculations. (C) 1997 American Institute of Physics.