:term:`CFOUR_ROT_EVEC <CFOUR_ROT_EVEC (CFOUR)>`
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      Specifies which eigenvector of the orbital rotation Hessian is to be used to rotate the original SCF orbitals. By default, it will use that associated with the lowest eigenvalue of the totally symmetric part of the block-factored Hessian, as this choice often leads to the lowest energy SCF solution. For RHF stability checks, only those instabilities which correspond to RHF solutions will be considered. It is important to understand that following non-symmetric eigenvectors lowers the symmetry of the wavefunction and that following RHF --> UHF stabilities leads to a UHF solution. To converge the SCF roots associated with such instabilities, one must run the calculation in reduced symmetry and as a closed-shell UHF case, respectively. Value *n* directs the program to follow the vector associated with the *n*\ th lowest eigenvalue having the proper symmetry (totally symmetric) and spin (RHF-->RHF or UHF-->UHF) properties. 0 means use the lowest eigenvalue.

      * **Type**: integer
      * **Default**: 0

