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       stable - MO Hessian stability analysis program


       The   program   stable  performs  a  stability  analysis  on  the  molecular  orbitals  by
       diagonalizing the molecular orbital Hessian.   The  analysis  includes  orbital  rotations
       which  may break spin or spatial symmetry.  If an instability is detected (i.e., there are
       negative eigenvalues of the MO Hessian), the previous Hartree-Fock self  consistent  field
       (SCF)  computation  has  landed  on  a  local  minimum, not the global minimum, in orbital
       rotation space.  For spatial- or symmetry-breaking rotations, this merely means that there
       is another orbital occupation or a UHF wavefunction with a lower energy.  For instabilties
       which preserve the reference type (RHF or UHF) and are totally  symmetric,  this  suggests
       that  the SCF computation has simply found the wrong solution and should be re-run using a
       new initial guess.

       Instabilities corresponding to spatial symmetry breaking rotatios can be resolved  by  re-
       running  the  calculation in a lower computational point group.  (It may also be necessary
       to  employ  some  trick  to  break  the  symmetry  of  the  initial  guess  wavefunction).
       Instabilities corresponding to spin symmetry breaking rotations (RHF->UHF) can be resolved
       by re-running the SCF using a UHF reference.

       For UHF wavefunctions with totally-symmetric instabilities, it is possible to  follow  the
       instability automatically toward the global minimum by setting FOLLOW=TRUE.


       Input for this program is read from the file The following keywords are valid:

       CACHELEV = integer
              This  is  the  cache level used for the DPD files.  It functions the same way as in
              the coupled-cluster codes.  Currently, this is hardwired to 0.

       FOLLOW = boolean
              If TRUE, then the eigenvector corresponding to the most negative eigenvalue of  the
              MO  Hessian  will  be followed to try to find a lower energy Hartree-Fock solution.
              The orbitals will be rotated, and the cscf module should be re-run using these  new
              guess orbitals.

       NUM_EVECS_PRINT = integer
              Gives the number of MO Hessian eigenvectors to print.

       PRINT = integer
              The  print level determines how much information is printed by the program.  Values
              typically range from 1 (minimal printing) to 5 (very verbose printing for debugging
              only).  The default is 1.

       REFERENCE = string
              This is the reference type, RHF, ROHF, or UHF.  The default is RHF.

       SCALE = real
              This  is  the  scale factor used for taking steps in eigenvector following.  Normal
              values would be between 0 and 1.  The default is 0.5.

                                           23 Aug, 2003                                 stable(1)