Despite this lack of theory for SAPT derivatives, finite-field numerical differentiation has enabled the study of some essential properties of van der Waals complexes. For example, Heijmen et al. demonstrated that the interaction-induced dipole moment of a simple complex such as He•••H 2 can be derived from the derivative of SAPT components with respect to an external electric field. In addition, several studies have focused on the electric polarizabilities and hyperpolarizabilities of complexes using SAPT or energy decomposition schemes. More recently, Iglesias-Reguant et al. presented an innovative approach to understand the infrared spectroscopy of molecular complexes through the decomposition of interaction energy and derivatives of its components with respect to normal modes.