where E0 is the energy of the unperturbed molecules, Fa is the field
at the origin, la, aab and babc are the components of dipole moment,
polarizability and the first order hyperpolarizabilities, respectively.
The total static dipole moment l, the mean polarizability a0, the
anisotropy of the polarizability Da and the mean first order hyperpolarizability
b0, using the x, y, z components they are defined as: