At a given temperature, pressure, and composition, stable and metastable polymorphs of one or more organic compounds are usually postulated to correspond to low-lying minima on the Gibbs free energy surface. However, evaluating the Gibbs free energy accurately within the extensive breadth of a CSP search is highly challenging, and in most cases computationally impractical. In practice, several approximations are applied. In particular, thermal contributions at low temperatures and the effects of pressure under atmospheric conditions are neglected initially. In effect, the Gibbs free energy is instead approximated by the lattice energy of the system at 0 K and 0 Pa. An important requirement in many CSP methodologies is thus to develop models of lattice energy that are sufficiently accurate to determine relative lattice energy differences between polymorphs, while remaining computationally efficient enough to allow a complete exploration of the landscape of polymorph candidates.