Electrostatic Interactions between Molecules

There is an electric field around a molecule which can be described in terms of its multipole moments. The energy of another nearby molecule in this field involves its multipole moments, and the electrostatic interaction between them can be expressed as a sum of terms, each involving a product of multipole moments for each molecule and an interaction tensor. These interaction terms can be expressed in either cartesian or spherical tensor form, most conveniently using multipole moments defined in local axes, as they do not change with the molecular orientation. The dependence of the multipole—multipole terms on the relative orientation of the molecules is then described using new functions (S functions). The dipole--dipole, dipole--quadrupole and quadrupole--quadrupole interactions are discussed in detail, with examples.