We have adapted our established protocol for modelling molecular spin dynamics to take into account the phonon modes in the crystalline phase. We first take each unique molecule in the optimised unit cell (expanded to contain complete molecules) and perform a gas-phase DFT calculation using Gaussian 09d, with Dy substituted by Y to avoid a multiconfigurational ground state, and determine the atomic charges required to reproduce the external molecular electrostatic potential using the CHELPG method (PBE functional, Stuttgart RSC 1997 effective core potential (ECP) and associated valence basis set for Y and the cc-pVDZ basis sets for all other atoms ).