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

      Specifies treatment of anharmonic effects by calculating cubic and/or quartic force fields. VIBROT (=3) requests calculation of only those cubic constants of the form :math:`\phi_{nij}`, where n is a totally symmetric coordinate. These are sufficient to determine the vibration-rotation interaction constants needed to calculate vibrational corrections to rotational constants, but are *not* sufficient to generate the corresponding cubic constants of isotopologs that have a lower point-group symmetry (*i.e.* HOD isotopolog of water). VPT2 (=1, note that the old value CUBIC can be still used and is equivalent to VPT2) generates all cubic constants and all quartic constants apart from those of the form :math:`\phi_{ijkl}`, which is enough for: 1) generation of cubic constants of isotopologs (see manual entries associated with anharmonic calculations for an example); 2) calculation of vibrational energy levels with VPT2. This keyword also directs the program to analyze resonances and calculate intensities of one- and two-quantum transitions. FULLQUARTIC (=2) (not part of the public release) is largely self-explanatory; it directs the program to calculate all quartic constants. This is sufficient (but this has not been implemented) to generate the full quartic force field of all isotopologs.

      * **Type**: string
      * **Possible Values**: CUBIC, VPT2, FULLQUARTIC, VIBROT, OFF
      * **Default**: OFF

