Temperature-Accelerated Method for Exploring Polymorphism in Molecular Crystals Based on Free Energy

The ability of certain organic molecules to form multiple crystal structures, known as polymorphism, has important ramifications for pharmaceuticals and high energy materials. Here, we introduce an efficient molecular dynamics method for rapidly identifying and thermodynamically ranking polymorphs. The new method employs high temperature and adiabatic decoupling to the simulation cell parameters in order to sample the Gibbs free energy of the polymorphs. Polymorphism in solid benzene is revisited, and a resolution to a long-standing controversy concerning the benzene II structure is proposed.

Figure 1

(a) Crystal-AFED trajectories (with $T_{\mathbf{h}}=31000\mathrm{K}$) for benzene at 100 K; (b) crystal-AFED trajectories for benzene at 300 K; (c) a smooth phase transition from a crystal-AFED trajectory of benzene at 300 K.

Figure 2

Probability distribution, unit cell structures, and corresponding free energies and lattice energies of the stable polymorphs of benzene at 100 K and 2 GPa obtained via crystal-AFED. The unit cell for the mixed stacking structure [III(d)] is one representative structure among all those generated in the simulation. The lattice energy is the sum of the intermolecular energy and the $PV$ contribution at 0 K and 2 GPa, which, for III(d), is averaged over several different mixed forms. Both the lattice energy and the free energy of benzene V are set to zero as reference values.