The dispersion component of the EFP energy is expressed in terms of C 6 coefficients that represent the leadingorder dipole-dipole contribution to dispersion. The C 6 coefficients are obtained from the well-known expression involving frequency-dependent, isotropic atomic polarizabilities (α p ) evaluated at imaginary frequencies. That expression, which is usually attributed to Casimir and Polder, is given by 44,47-51