The first descriptor σ, the molecular symmetry number, is the number of unique rotations that can be performed on the molecule and return an indistinguishable orientation, in addition to the identity. At minimum, σ must be 1. As σ increases, the number of microstates (orientations of the molecule) that produce the same crystal structure increases, thus increasing the overall entropy of the solid (according to the Boltzmann equation, S = k B lnΩ).