Using effective polarizabilities ᾱeff a obtained from Eq. ( ), Becke and Johnson proposed a model for C 6,ab that is expressed in terms of the dipole moment of the exchange hole, d x (r). That quantity depends on position, because the hole is a function of the position of a reference electron, but the exchange-hole dipole moment enters the Becke-Johnson model as its spherically averaged second moment, d 2