  We present a quantum Monte Carlo study of the hydrogen-benzene system where
binding is very weak. We demonstrate that the binding is well described at both
variational Monte Carlo (VMC) and diffusion Monte Carlo (DMC) levels by a
Jastrow correlated single determinant geminal wave function with an optimized
compact basis set that includes diffuse orbitals. Agreement between VMC and
fixed-node DMC binding energies is found to be within 0.18 mHa, suggesting the
calculations are well-converged with respect to the basis. Essentially the same
binding is also found in independent DMC calculations using a different trial
wave function of a more conventional Slater-Jastrow form, supporting our
conclusion that the binding energy is accurate and includes all effects of
correlation. We compare with empirical models and previous calculations, and we
discuss the physical mechanisms of the interaction, the role of diffuse basis
functions, and the charge redistribution in the bond.
