9.1 Equilibrium Geometries and Transition-State Structures

9.1.2 Job Control

(June 30, 2021)

Obviously a level of theory, basis set, and starting molecular geometry must be specified to begin a geometry optimization or transition-structure search. These aspects are described elsewhere in this manual, and this section describes job-control variables specific to optimizations.

JOBTYPE
       Specifies the calculation.
TYPE:
       STRING
DEFAULT:
       Default is single-point, which should be changed to one of the following options.
OPTIONS:
       OPT Equilibrium structure optimization. TS Transition structure optimization. RPATH Intrinsic reaction path following.
RECOMMENDATION:
       Application-dependent.

GEOM_OPT_HESSIAN
       Determines the initial Hessian status.
TYPE:
       STRING
DEFAULT:
       DIAGONAL
OPTIONS:
       DIAGONAL Set up diagonal Hessian. READ Have exact or initial Hessian. Use as is if Cartesian, or transform if internals.
RECOMMENDATION:
       An accurate initial Hessian will improve the performance of the optimizer, but is expensive to compute.

GEOM_OPT_COORDS
       Controls the type of optimization coordinates.
TYPE:
       INTEGER
DEFAULT:
       -1
OPTIONS:
        0 Optimize in Cartesian coordinates.  1 Generate and optimize in internal coordinates, if this fails abort. -1 Generate and optimize in internal coordinates, if this fails at any stage of the optimization, switch to Cartesian and continue.  2 Optimize in Z-matrix coordinates, if this fails abort. -2 Optimize in Z-matrix coordinates, if this fails during any stage of the optimization switch to Cartesians and continue.
RECOMMENDATION:
       Use the default, as delocalized internals are more efficient. Note that optimization in Z-matrix coordinates requires that the input be specified in Z-matrix format.

GEOM_OPT_TOL_GRADIENT
       Convergence on maximum gradient component.
TYPE:
       INTEGER
DEFAULT:
       300 300×10-6 tolerance on maximum gradient component.
OPTIONS:
       n Integer value (tolerance = n×10-6).
RECOMMENDATION:
       Use the default. To converge GEOM_OPT_TOL_GRADIENT and one of GEOM_OPT_TOL_DISPLACEMENT and GEOM_OPT_TOL_ENERGY must be satisfied.

GEOM_OPT_TOL_DISPLACEMENT
       Convergence on maximum atomic displacement.
TYPE:
       INTEGER
DEFAULT:
       1200 1200×10-6 tolerance on maximum atomic displacement.
OPTIONS:
       n Integer value (tolerance = n×10-6).
RECOMMENDATION:
       Use the default. To converge GEOM_OPT_TOL_GRADIENT and one of GEOM_OPT_TOL_DISPLACEMENT and GEOM_OPT_TOL_ENERGY must be satisfied.

GEOM_OPT_TOL_ENERGY
       Convergence on energy change of successive optimization cycles.
TYPE:
       INTEGER
DEFAULT:
       100 100×10-8 tolerance on maximum (absolute) energy change.
OPTIONS:
       n Integer value (tolerance = value n×10-8).
RECOMMENDATION:
       Use the default. To converge GEOM_OPT_TOL_GRADIENT and one of GEOM_OPT_TOL_DISPLACEMENT and GEOM_OPT_TOL_ENERGY must be satisfied.

GEOM_OPT_MAX_CYCLES
       Maximum number of optimization cycles.
TYPE:
       INTEGER
DEFAULT:
       50
OPTIONS:
       n User defined positive integer.
RECOMMENDATION:
       The default should be sufficient for most cases. Increase if the initial guess geometry is poor, or for systems with shallow potential wells.

GEOM_OPT_PRINT
       Controls the amount of Optimize print output.
TYPE:
       INTEGER
DEFAULT:
       3 Error messages, summary, warning, standard information and gradient print out.
OPTIONS:
       0 Error messages only. 1 Level 0 plus summary and warning print out. 2 Level 1 plus standard information. 3 Level 2 plus gradient print out. 4 Level 3 plus Hessian print out. 5 Level 4 plus iterative print out. 6 Level 5 plus internal generation print out. 7 Debug print out.
RECOMMENDATION:
       Use the default.

GEOM_OPT_SYMFLAG
       Controls the use of symmetry in Optimize.
TYPE:
       LOGICAL
DEFAULT:
       TRUE
OPTIONS:
       TRUE Make use of point group symmetry. FALSE Do not make use of point group symmetry.
RECOMMENDATION:
       Use the default.

GEOM_OPT_MODE
       Determines Hessian mode followed during a transition state search.
TYPE:
       INTEGER
DEFAULT:
       0
OPTIONS:
       0 Mode following off. n Maximize along mode n.
RECOMMENDATION:
       Use the default, for geometry optimizations.

GEOM_OPT_MAX_DIIS
       Controls maximum size of subspace for GDIIS.
TYPE:
       INTEGER
DEFAULT:
       0
OPTIONS:
       0 Do not use GDIIS. -1 Default size = min(NDEG, NATOMS, 4) NDEG = number of molecular degrees of freedom. n Size specified by user.
RECOMMENDATION:
       Use the default or do not set n too large.

GEOM_OPT_DMAX
       Maximum allowed step size. Value supplied is multiplied by 10-3.
TYPE:
       INTEGER
DEFAULT:
       300 = 0.3
OPTIONS:
       n User-defined cutoff.
RECOMMENDATION:
       Use the default.

GEOM_OPT_UPDATE
       Controls the Hessian update algorithm.
TYPE:
       INTEGER
DEFAULT:
       -1
OPTIONS:
       -1 Use the default update algorithm.  0 Do not update the Hessian (not recommended).  1 Murtagh-Sargent update.  2 Powell update.  3 Powell/Murtagh-Sargent update (TS default).  4 BFGS update (OPT default).  5 BFGS with safeguards to ensure retention of positive definiteness (GDIIS default).
RECOMMENDATION:
       Use the default.

GEOM_OPT_LINEAR_ANGLE
       Threshold for near linear bond angles (degrees).
TYPE:
       INTEGER
DEFAULT:
       165 degrees.
OPTIONS:
       n User-defined level.
RECOMMENDATION:
       Use the default.

FDIFF_STEPSIZE
       Displacement used for calculating derivatives by finite difference.
TYPE:
       INTEGER
DEFAULT:
       100 Corresponding to 0.001 Å. For calculating second derivatives.
OPTIONS:
       n Use a step size of n×10-5.
RECOMMENDATION:
       Use the default except in cases where the potential surface is very flat, in which case a larger value should be used. See FDIFF_STEPSIZE_QFF for third and fourth derivatives.

Example 9.1  As outlined, the rate of convergence of the iterative optimization process is dependent on a number of factors, one of which is the use of an initial analytic Hessian. This is easily achieved by instructing Q-Chem to calculate an analytic Hessian and proceed then to determine the required critical point

$molecule
   0  1
   O
   H  1  oh
   H  1  oh 2 hoh

   oh  = 1.1
   hoh = 104
$end

$rem
   JOBTYPE    freq   Calculate an analytic Hessian
   METHOD     hf
   BASIS      6-31g(d)
$end

$comment
Now proceed with the optimization making sure to read in the analytic
Hessian (use other available information too).
$end

@@@

$molecule
   read
$end

$rem
   JOBTYPE            opt
   METHOD             hf
   BASIS              6-31g(d)
   SCF_GUESS          read
   GEOM_OPT_HESSIAN   read   Have the initial Hessian
$end

View output

Example 9.2  Optimization in Z-matrix coordinates. The input must be specified in Z-matrix format with coordinates specified. In the example below there are two coordinates representing the bond length and bond angle of a water molecule.

$molecule
   0 1
   O
   H 1 r
   H 1 r 2 ang

   r   0.95
   ang 104.5
$end

$rem
   JOBTYPE           OPT
   METHOD            HF
   BASIS             STO-3G
   GEOM_OPT_COORDS   2
$end

View output