In this paper we present a theoretical study of the coupling between intermolecular and intramolecular vibrational motion in large van der Waals complexes, which rest on the expansion of the total vibrational wave function in terms of the "free" molecule wave function, resulting in a set of coupled differential equations for the expansion coefficients. The application of Van Vleck perturbation theory results in explicit first-order and second-order kinetic energy and potential energy corrections. These corrections lead to a shift of the intramolecular vibrational frequencies and to intermolecular vibrational energies which depend on the vibrational quantum numbers for the intramolecular modes. Consideration of the intermode coupling in a model system consisting of one harmonic, intramolecular mode, and one intermolecular mode, predicts a linear dependence of the intermolecular vibrational energy on the vibrational quantum number for the intramolecular motion. This prediction is in accord with the recent experimental results for the S0→S1 transition of the trans-stilbene · Ar complex.