where r i j is the distance between atoms i and j , and γ γ i is the angle between the vectors r i j and r i k . The coefficient E 0 is positive and of the order V α α 3 , where V is the ionization energy and α α is the mean atomic polarizability; the exact value of E 0 depends on the magnitudes of the dipole matrix elements and on the energies of the p orbitals.