A general method is presented for constructing, from ab initio quantum chemistry calculations, the potential energy surface (PES) for H-2 absorbed in a porous crystalline material. The method is illustrated for the metalorganic framework material MOF-5. Rigid body quantum diffusion Monte Carlo simulations are used in the construction of the PES and to evaluate the quantum ground state of H-2 in MOF-5, the zero-point energy, and the enthalpy of adsorption at 0 K.