NAME

       stable - MO Hessian stability analysis program

DESCRIPTION

       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 FORMAT

       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)