The accuracy of the fixed-node approximation and diffusion Monte Carlo method in computing the interaction energy of van der Waals systems was investigated. Tests were carried out by simulating the electronic structures of He-2 and He-LiH. These two systems were chosen as representative of two fundamentally different interactions, namely the weak dispersion forces in He-2 and the dipole/induced-dipole interaction in He-LiH. The results for both systems are in excellent agreement with "state of the art" calculations, thereby indicating a high accuracy for the fixed-node approximation. Also, our interaction energies for He-LiH indicate that the coupled cluster single double triple method [Taylor and Hinde, J. Chem. Phys. 111, 973 (1999)] gives an accurate prediction of the interaction potential for that system. (C) 2003 American Institute of Physics.