$DERIV group          (optional for numerical derivatives)
 
    This group sets options for numerical gradients. Do
not confuse with $FORCE ($FORCE is for second derivatives).
Note that numerical gradients for FMO cannot use GDDI
(the only option is to use DDI) whereas non-FMO runs can
use GDDI).
 
VIBSIZ =    Displacement size (in bohr). If negative, do
            not transform coordinates projecting out
            translations+rotations; use Cartesian
            coordiantes and no symmetry for offsets.
            Otherwise, if positive, project out T+R and
            use symmetry. For PBC, use a negative VIBSIZ.
            Default=0.01
 
NVIB   =    1 or 2, the number of offsets per coordinate,
            similar to NVIB in $FORCE.
            Default=2
 
STRESS =    a logical flag; if set, then the numerical
            gradient is computed with respect to cell
            parameters (defining a stress tensor) in PBC.
            Currently, only DFTB/PBC uses STRESS.
            Default=false