Q-Chem allows the user to characterize the stationary point found by a
geometry optimization or transition state search without performing a full
analytical Hessian calculation, which is sometimes unavailable or
computationally unaffordable. This is achieved via a finite difference Davidson
procedure developed by Sharada *et al.*^{Sharada:2014} For a geometry
optimization, it solves for the lowest eigenvalue of the Hessian (${\lambda}_{1}$)
and checks if ${\lambda}_{1}>0$ (a negative ${\lambda}_{1}$ indicates a saddle
point); for a TS search, it solves for the lowest two eigenvalues, and
$$ and ${\lambda}_{2}>0$ indicate a transition state. The lowest
eigenvectors of the updated P-RFO (approximate) Hessian at convergence are used
as the initial guess for the Davidson solver.

The cost of this Hessian-free characterization method depends on the rate of convergence of the Davidson solver. For example, to characterize an energy minimum, it requires $2\times {N}_{\mathrm{iter}}$ total energy + gradient calculations, where ${N}_{\mathrm{iter}}$ is the number of iterations that the Davidson algorithm needs to converge, and “2" is for forward and backward displacements on each iteration. According to Ref. Sharada:2014, this method can be much more efficient than exact Hessian calculation for substantially large systems.

Note: At the moment, this method does not support QM/MM or systems with fixed atoms.

GEOM_OPT_CHARAC

Use the finite difference Davidson method to characterize the resulting energy
minimum/transition state.

TYPE:

BOOLEAN

DEFAULT:

FALSE

OPTIONS:

FALSE
do not characterize the resulting stationary point.
TRUE
perform a characterization of the stationary point.

RECOMMENDATION:

Set it to TRUE when the character of a stationary point needs to be
verified, especially for a transition structure.

GEOM_OPT_CHARAC_CONV

Overide the built-in convergence criterion for the Davidson solver.

TYPE:

INTEGER

DEFAULT:

0 (use the built-in default value 10${}^{-5}$)

OPTIONS:

$n$
Set the convergence criterion to 10${}^{-n}$.

RECOMMENDATION:

Use the default. If it fails to converge, consider loosening the criterion with caution.

$molecule -1 1 C 0.00000 -0.00078 0.98436 F -1.09414 -0.63166 1.47859 S 0.00000 0.00008 -0.94745 O 1.25831 -0.72597 -1.28972 O -1.25831 -0.72597 -1.28972 O 0.00000 1.45286 -1.28958 F 1.09414 -0.63166 1.47859 F 0.00000 1.26313 1.47663 $end $rem JOBTYPE opt METHOD BP86 GEOM_OPT_DMAX 50 BASIS 6-311+G* SCF_CONVERGENCE 8 THRESH 14 SYMMETRY FALSE SYM_IGNORE TRUE GEOM_OPT_TOL_DISPLACEMENT 10 GEOM_OPT_TOL_ENERGY 10 GEOM_OPT_TOL_GRADIENT 10 GEOM_OPT_CHARAC TRUE $end

$molecule 0 1 C 3.21659 -1.41022 -0.26053 C 2.16708 -0.35258 -0.59607 N 1.21359 -0.16703 0.41640 C 0.11616 0.82394 0.50964 C -1.19613 0.03585 0.74226 N -2.18193 -0.02502 -0.18081 C -3.43891 -0.74663 0.01614 O 2.19596 0.25708 -1.63440 C 0.11486 1.96253 -0.53088 O -1.29658 -0.59392 1.85462 H 3.25195 -2.14283 -1.08721 H 3.06369 -1.95423 0.67666 H 4.20892 -0.93714 -0.22851 H 1.24786 -0.78278 1.21013 H 0.25990 1.31404 1.47973 H -2.02230 0.38818 -1.10143 H -3.60706 -1.48647 -0.76756 H -4.29549 -0.06423 0.04327 H -3.36801 -1.25875 0.98106 H -0.68664 2.66864 -0.27269 H 0.01029 1.65112 -1.56461 H 1.06461 2.50818 -0.45885 $end $rem JOBTYPE freq EXCHANGE B3LYP BASIS 6-31G SCF_MAX_CYCLES 250 SYMMETRY false SYM_IGNORE true $end @@@ $molecule read $end $rem JOBTYPE ts SCF_GUESS read GEOM_OPT_DMAX 100 GEOM_OPT_MAX_CYCLES 1500 EXCHANGE B3LYP BASIS 6-31G MAX_SCF_CYCLES 250 GEOM_OPT_HESSIAN read SYMMETRY false SYM_IGNORE true GEOM_OPT_CHARAC true $end