where the sum goes over all excited degenerate states | e i ⟩ ⟩ , and E e , i ( 0 ) , E g ( 0 ) are the energies of the excited states and the ground state, respectively, the sum excludes the state i = 0 , where | e 0 ⟩ ⟩ = | g ⟩ ⟩ . Historically, J.H. Van Vleck called this term the "high frequency matrix elements".