  We present a six-dimensional potential energy surface for the H2-H2 dimer
based on ab initio electronic structure calculations. The surface is intended
to describe accurately the bound and quasibound states of the dimers H2-H2,
D2-D2, and H2-D2 that correlate with H2 or D2 monomers in the rovibrational
levels (v, j) = (0, 0), (0, 2), (1, 0), and (1, 2). We use four experimentally
measured transition energies for these dimers to make two empirical adjustments
to the ab initio surface; the adjusted surface gives computed transition
energies for 56 experimentally observed transitions that agree with experiment
to within 0.036 cm^{-1}. For 29 of the 56 transitions, the agreement between
the computed and measured transition energies is within the quoted experimental
uncertainty. We use our potential energy surface to predict the energies of
another 34 not-yet-observed infrared and Raman transitions for the three
dimers.
