The energy of a molecule in an external electrostatic field can be expanded as \[E=E^{\text{o}}- \unicode[Times]{x3BC}_{i}\ F_{i}- \frac{1}{2}\ \unicode[Times]{x3B1} _{ij}\ F_{i}\ F_{j}- \frac{1}{6}\ \unicode[Times]{x3B2} _{ijk}\ F_{i}\ F_{j}\ F_{k}- \frac{1}{24}\ \unicode[Times]{x3B3} _{ijkl}\ F_{i}\ F_{j}\ F_{k}\ F_{l}- \text{...}\] where \(E^{\text{o}}\) is the unperturbed energy, \(F_{i}\) is the component of the field in the i direction, \(\unicode[Times]{x3BC}_{i}\) is the permanent @D01761@, \(\unicode[Times]{x3B1}_{ij}\) is the @P04711@ tensor, and \(\unicode[Times]{x3B2}_{ijk}\) and \(\unicode[Times]{x3B3}_{ijkl}\) are the first and second hyperpolarizability tensors, respectively. \(\unicode[Times]{x3B2}\) is a third order symmetric tensor that measures the second order response of the molecular @E01929@ to the action of an external electric field and is thus often referred to as dipole hyperpolarizability.