The lattice energy of a (meta)stable crystal structure on the energy surface defined by the HAIEFF model is computed by local minimisation of the energy function over the variables Ω and θ. With the inclusion of intramolecular energy contributions and molecular flexibility, changes in molecular conformation become a major factor in model complexity. This is not only due to the need to create detailed models for intramolecular energy (∆U intra (θ)) for each system, but also due to the additional dependencies of the functions r 0Ii,lJj and Q on θ. For the parameter estimation, we avoid this added complexity by fixing the molecule to the crystalline conformation determined in the DFT-D reference data (θ ref ), and only minimise the U inter component over the remaining rigid-body geometric variables (Ω). In effect, we assume that the DFT-D crystalline conformation and its corresponding ∆U intra would be accurately predicted by our HAIEFF model. This is not unreasonable given that in a flexible minimisation, these intramolecular terms would be determined by similar DFT calculations (albeit with different levels of theory and basis sets). Moreover, given that the DFT-D reference energies can be quite easily decomposed into U inter and ∆U intra , this approach is prudent. The values of the geometric variables at the optimal crystal geometry are denoted Ω * and are determined as the arguments of a local minimisation (arg min) of the crystal structure at fixed conformation as shown in Problem (LM).