  The gas equation of state may be one of the critical factors for the disk
instability theory of gas giant planet formation. This letter addresses the
treatment of H$_2$ in hydrodynamical simulations of gravitationally unstable
disks. In our discussion, we point out possible consequences of erroneous
specific internal energy relations, approximate specific internal energy
relations with discontinuities, and assumptions of constant $\Gamma_1$. In
addition, we consider whether the ortho/para ratio for H$_2$ in protoplanetary
disks should be treated dynamically as if the species are in equilibrium.
Preliminary simulations indicate that the correct treatment is particularly
critical for the study of gravitational instability when $T = 30$-50 K.
