Weak intermolecular interactions calculated with diffusion Monte Carlo -: art. no. 184106

The performance of fixed node diffusion Monte Carlo (FNDMC) for weakly interacting molecules is investigated. The effect of Gaussian basis sets on the asymptotic description of the molecular orbitals which is crucial for a successful importance sampling is analyzed for the example of the hydrogen atom. We find that accurate reference binding energies of the water, the ammonia, and the T-shaped as well as the parallel-displaced benzene dimer are correctly reproduced by FNDMC. The binding energies for the benzene dimers are -3.00(0.38) and -3.58(0.38) kcal/mol, respectively. The description of the methane dimer which has the smallest binding energy and a quite large intermolecular distance requires a more flexible basis set of diffuse quadruple-zeta quality in order to prevent sampling errors. (c) 2005 American Institute of Physics.