The first two terms are typically described by tailored surrogate models constructed from ab initio calculations carried out on an isolated molecule, while the remaining residual interactions in the crystal (U res ) are modelled using a transferable, empirical force-field. These residual interactions include repulsive/dispersive effects from Pauli exclusion and electron correlation respectively, inductive effects, many-body effects, as well as any systematic errors in the ab initio terms . Because the effects of Pauli exclusion and electron correlation tend to dominate the other residual terms, the residual interactions are often just referred to as the repulsion/dispersion interaction. To most effectively capture the combined effects of these various interactions, the force-field should be parameterized to suitable crystal data. The modular construction of HAIEFF models offers flexibility in handling the trade-off between accuracy and computational cost, which facilitates their use at multiple stages of a CSP investigation. For example, simple point-charge based electrostatics with limited molecular flexibility are initially used in the candidate generation stage, followed by a more detailed multipole description of the electrostatics and extensive molecular flexibility in subsequent refinement stages . This type of model and workflow were employed by several groups in the sixth blind test with good success in the identification of (meta)stable polymorphs .