Electrostatics and exchange-repulsion appear at first order in VAB ÂAB , as E (10) elst and E (10) exch , respectively. The latter is identified with Pauli or steric repulsion, for reasons that are discussed in Section 3.1. Induction and dispersion do not appear until second order, which is why the second-order approximation is the simplest reasonable form of Eq. ( ). As indicated in Eq. ( ), we combine the second-order dispersion and induction terms in the polarization expansion with their exchange analogues to define the total dispersion and induction energies at second order, E