*key* defines the optimization method.

For *minimization* the following options are valid for *key*:

`RF`- Rational Function method (default).
`AH`- Augmented Hessian method. This is similar to
`RF`algorithm but uses a more sophisticated step restriction algorithm. `DIIS`- Pulay's Geometry DIIS method. As an an additional option
you may add the number of geometries to be used in GDIIS interpolation (default 5)
and the interpolation type (i.e. the subspace in which the GDIIS interpolation
is made.
`METHOD,DIIS,`*number*,*type**type*may be`GRAD`interpolation using the gradients (default), working good for rigid molecules,`STEP`interpolation using Quasi-Newton steps which could be advantageous in dealing with very floppy molecules,`ENER`interpolation using energies, which is an intermediate between the above two. `QSD`- Quadratic steepest descent method of Sun and Ruedenberg.
`SRMIN`- Old version of
`QSD`.

For *transition state* searches
(invoked with the `ROOT` option, see section 46.2.11) *key* can be

`RF`- Rational Function method (default).
`DIIS`- Pulay's Geometry DIIS method (see above).
`QSD`- Quadratic Steepest Descent Transition State search using
the image Hessian method
(see J. Sun and K. Ruedenberg,
*J. Chem. Phys.***101**, 2157 (1994)) The use of this option is recommended for transition state searches - especially in complicated cases. The optimization step is checked and the Hessian is recalculated when approaching a troublesome region of the PES. Thus**this method is somewhat safer (and often faster) in reaching convergence than the RF or DIIS method**. The Hessian recalculation safeguard may be turned off using the`METHOD,QSD,NOHESS`input card. `SRTRANS`- Old version of
`QSD`.

For *reaction path following* the input *key* is

`QSDPATH`- Quadratic Steepest Descent reaction path following.
This methods determines reaction paths (intrinsic reaction coordinates, IRCs)
by following the exact steepest descent lines of subsequent quadratic
approximations to the potential energy surface. The Hessian matrix is
calculated numerically at the first optimization step and subsequently
updated by Powell or BFGS update. If a given arc length of the steepest
descent lines is exceeded, the Hessian is recalculated numerically
(see
`OPTION`section 46.2.16). For details see J. Sun and K. Ruedenberg,*J. Chem. Phys.***99**, 5269 (1993) It is also possible to recalculate the Hessian after each*m*steps using the`NUMHES`,*m*command (see section 46.2.7). If the Hessian matrix is recalculated in every optimization step (`NUMHES`,) a algorithm different to the one with updated Hessians is used, which is very accurate. Using the`PRINT,OPT`card, this algorithm prints in every optimization step a*reaction path point r*which is different from the point where the energy and the gradient is calculated but closer to the real reaction path (for further details of the algorithm see J. Sun and K. Ruedenberg,*J. Chem. Phys.***99**, 5257 (1993)). For further input options of the QSD reaction path following see`OPTION`section 46.2.16. `SRSTEEP`- Old Version of
`QSDPATH`.

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