Documentation on Firefly v. 8.0.0 numerical gradient features.

               The numerical gradient code is activated as follows:

$contrl numder=.t. $end

               The primary purpose of this code is to allow computations normally requiring analytic energy gradients, to be performed using those QC methods for which analytic gradients are not yet programmed. The idea behing numerical gradients code is very simple, i.e. gradients are obtained using various finite defference formulas applied to the energy itself so the approach is based on multiple reevaluations of energy values at a set of slightly displaced geometries. The numerical gradients code is tightly integrated into Firefly and can be used anywhere where the analytical energy gradients are required by default. It operates in both standard parallel, XP, and extended XP modes, allowing parallel computations of gradients for those QC methods for which parallel  implementation is not available yet and improving parallel scalability of methods that are already well parallelized. The numerical gradient code handles molecular symmetry by the very efficient way and always uses the minimal required number of single-point energy evaluations. The code supports both static and dynamic load balancing.

               As with any finite differencing, numerical gradients require extra high precision in computed energy values. It is therefore the user's responsibility to increase the precision of all the stages involved into computation of target energies.

               The relevant input group is called $numgrd and contains the following keywords:

DELTA                  Step size (in atomic units) for finite differencing. The default is 0.01 bohr.

ORDER                 The order of finite difference formulas to use. Use one of 1 (less precise, less expensive), 2, 4, or 6 (most precise, most expensive). The default is 2.

NGRADS             For methods computing energies of several states at once (e.g. CI, MCSCF, TDHF, TDDFT, MCQDPT2, XMCQDPT2) this option can be used to request computation of gradients of NGRADS lowest states of interest.  The default is NGRADS=1 i.e. gradient will only be computed for the lowest energy state. Use of this option does not introduce any additional computational overhead. Note, the value of NGRADS should not be larger than the actual number of states available!

ISTATE                 This option selects the target state of interest i.e. the state which gradient is to be used by other parts of the program, e.g. during geometry optimization involving numerical gradients, etc... The default is ISTATE=1. This option is intended for use together with the NGRADS option.  For instance, to optimize the second root of TDDFT or XMCQDPT, set NGRADS to be equal or greater than 2, set ISTATE to 2 and properly set other relevant parameters controlling the number of computed roots during TDDFT or XMCQDPT procedure to capture at least two states.

JSTATE                Selects the second state of interest. The default is JSTATE=0 i.e. no second state. One must provide nonzero JSTATE together with valid NSTATE and ISTATE for numerical location of interstate crossings or conical intersections.

DELROT               Logical variable affecting the behavior of numerical differencing code.  If set to .true., it triggers on  approximate elimination of rigid body rotations during computations of numerical gradients thus reducing the number of required reevaluations of energy. If set to .false., rotations will not be eliminated. Normally, DELROT does not affect optimized geometries but does affect double-numerical Hessians.  The default value is DELROT=.true. This option must be manually disabled for any RUNTYP other than optimize or sadpoint and for any optimization runs involving HSSEND option.

CHEBSH               Logical variable. Selects the use of Chebyshev grids for finite differencing. The default is to use the equidistant central finite differences i.e. CHEBSH=.false.

SPLINE                Integer  variable. Selects the use of spline differentiation of order SPLINE over the selected grid (either the default equidistant or Chebyshev).  It has effect only for ORDER=4 or ORDER=6 finite differences. Valid values of SPLINE are 1, 2, 3, and 4.  SPLINE=0 (default) disables spline differentiation.

CONFIT                Integer variable selecting one of the dedicated finite differencing algorithms which are the most appropriate and most precise in the vicinity of conical intersections. Available for finite differences of ORDER=4 or 6 only. Default is CONFIT=0 i.e. not to use these methods. Available options are CONFIT=1, 2, or 3.

CFCOND              Parameter controlling activation of CONFIT procedure. With CONFIT enabled, it will not be in action unless the condition number of fit obtained with CONFIT procedure is below CFCOND. The default is CFCOND=6.0. Smaller values effectively enable CONFIT to handle more displacements. Larger values effectively disable CONFIT except the very vicinity of conical intersection.  

PRAXES                              Logical variable. When .true., requests the use of principal axes coordinate frame for finite differencing. Default is not to use principal axes i.e. PRAXES=.false. This option should not be used for symmetric molecules.

FNOISE                The threshold for automatic numerical noise detection. The automatic noise detection is available only for ORDER=4 or 6 differencing. The default value is 1.0*10-7. Larger values makes noise detection algorithm less sensitive to numerical noise.

TOL                      The threshold used by the numerical gradient code during elimination of redundant geometry displacements. The default value is 1.0*10-6. It is generally not recommended to change the default value.


               Finally, the new runtyp=NUMGRAD is a slightly cheaper alternative to runtyp=GRADIENT when using numerical gradients. In addition,  runtyp=NUMGRAD always prints  gradients for all NGRADS states while runtyp=GRADIENT will only print gradient for ISTATE state.


See also:

Last updated: July 29, 2012