Abstract: In a Van der Waals complex two or more monomers are loosely bound by weak forces. The standard harmonic approxi- mation which is effective for nearly rigid polyatomic molecules cannot be used to understand the spectrum of a Van der Waals complex. New ideas and numerical methods are required. I shall outline the difficulty of computing (ro-)vibrational spectra of Van der Waals molecules and present numerical methods for doing so. In particular, I’ll discuss how we use so-called contracted basis functions to compute many vibrational energy levels of water dimer (a 12-D problem). The water monomers are completely flexible. The key idea is to break the full problem into sub-problems that can be solved more easily. We have obtained the water dimer levels with stretch excitation and their shifts and a comparison with experimental results will be presented.