Contributions of different degrees of freedom to thermal transport in the C60 molecular crystal

Three models of the ${\mathrm{C}}₆₀$ molecular crystal are studied using molecular dynamics simulations to resolve the roles played by intermolecular and intramolecular degrees of freedom (DOF) in its structural, mechanical, and thermal properties at temperatures between 35 and 400 K. In the full DOF model, all DOF are active. In the rigid body model, the intramolecular DOF are frozen, such that only center of mass (COM) translations and molecular rotations/librations are active. In the point mass model, the molecule is replaced by a point mass, such that only COM translations are active. The zero-pressure lattice constants and bulk moduli predicted from the three models fall within ranges of 0.15 and 20%. The thermal conductivity of the point mass model is the largest across the temperature range, showing a crystal-like temperature dependence (i.e., it decreases with increasing temperature) due to the presence of phonon modes associated with the COM translations. The rigid body model thermal conductivity is the smallest and follows two distinct regimes. It is crystal-like at low temperatures and becomes temperature invariant at high temperatures. The latter is typical of the behavior of an amorphous material. By calculating the rotational diffusion coefficient, the transition between the two regimes is found to occur at the temperature where the molecules begin to rotate freely. Above this temperature, phonons related to COM translations are scattered by the rotational DOF. The full DOF model thermal conductivity is larger than that of the rigid body model, indicating that intramolecular DOF contribute to thermal transport.