:term:`CFOUR_RELATIVISTIC <CFOUR_RELATIVISTIC (CFOUR)>`
"""""""""""""""""""""""""""""""""""""""""""""""""""""""

      Specifies the treatment of relativistic effects. The default is a non-relativistic treatment (OFF), while perturbational treatments are invoked via MVD1 (mass-velocity and 1-electron Darwin contribution), MVD2 (mass-velocity and 1- and 2-electron Darwin contribution), DPT2 (second-order direct perturbation theory approach), SF-DPT4 (scalar-relativistic part of fourth-order direct perturbation theory, DPT4 (full fourth-order DPT including spin-orbit corrections), SF-DPT6 (scalar-relativistic part of sixth-order direct perturbation theory), SFREE (spin-free treatment), X2C1E (spin-free X2C-1e treatment), or DPT (synonym with DPT2).

      * **Type**: string
      * **Possible Values**: OFF, MVD1, MVd2, DPT2, SF-DPT4, DPT4, SF-DPT6, SFREE, X2C1E, DPT
      * **Default**: OFF

