The ground state potential energy surface for HCP has been investigated theoretically, A large fraction of electron correlation is included by multireference internally contracted configuration interaction from CASSCF reference wave functions using large orbital expansions. The origin of the potential is then shifted and the force constants scaled to reproduce all spectroscopic data available for the four isotopically substituted species. Variational calculations of vibrational and rotational frequencies for transitions up to J = 7 <-- 6 have finally been performed, with accuracy which is typically +/- 5 cm(-1) for vibrations and +/- 10 MHz for most rotations. By comparison with the results of the perturbation treatment the importance of the nu(1):2 nu(3) Fermi interaction for vibrational frequencies and effective rotational constants has been determined. From computed and experimental ground state rotational constants, the molecular equilibrium geometry has also been estimated.