stands for the electronic energy, exclusively. In the absence of external fields other than generated by the nuclei in the system, the change in the external potential () v  r may be expressed with the displacements of the nuclei vectors ( ) A R , by introducing the external electric field strength vectors () A r ε for each nucleus (cf. the Appendix p. 9.1). Eq. 17 is transformed to a form with atoms clearly identified: