Diffusion Monte Carlo (DMC) calculations of the intermolecular vibrational ground states of CO(Ar)(n) clusters with n = 1-12, for CO upsilon = 0 are reported. The intermolecular degrees of freedom of the clusters are treated in full dimensionality and a pairwise additive potential surface is used in which the Ar-CO interaction is described by a recently developed scheme which combines density functional theory (DFT) with the long-range dispersion contributions obtained from the perturbative theory. The calculations yield intermolecular ground state energies, Ar density distributions, radial and angular density probability distributions. Optimal structures by Simplex minimization have been calculated to estimate zero point energy (ZPE) and quantum effects.