A method is presented to design order parameters that can be used as discriminator in two-phase crystal-liquid molecular dynamics simulations. The proposed methodology is an extension to molecular crystal structures of a previously introduced discriminator for the atomic fcc environment [Phys. Rev. Lett. 79, 5074 (1997)] and can be readily applied to any crystal structure with both translational and orientational order. As an example, the discriminator is applied to the molecular Pa3 environment and subsequently used to study crystal melting rates with a diatomic carbon dioxide potential. The system's melting temperature proves to be below the roughening transition which is exemplified by faceted growth. The dynamically corrected melting rates are easily fitted to a rate law for two-dimensional nucleation and growth from which the melting temperature is deduced. The feasibility of the method for the example system holds promise for more extensive microscopic investigations of molecular crystal growth and melting.