The geometry optimization termination criteria (see Criteria) in both gradient minimization and energy minimization can be over-ridden by specifying a gradient norm requirement. For example, GNORM=20 would allow the geometry optimization to exit as soon as the gradient norm dropped below 20.0 kcal/mol/Ångstrom, the default being 1.0.
For proteins and solids, i.e., large systems, GNORM=10 should be used. Lower values might work, but there is an increased risk of the heat of formation not decreasing, and all that happens is that CPU time is wasted.
For small system high-precision work, GNORM=0.0 is recommended. Unless LET is also used, the GNORM will be set to the larger of 0.01 and the specified GNORM. Results from GNORM=0.01 are easily good enough for all high-precision work.
N.b.: Do not confuse GNORM, the keyword, with GNORM, the value of the scalar of the calculated gradient. The keyword GNORM defines the criterion for an optimized geometry, GNORM is the value calculated during a geometry optimization, and is printed in the output at the end of each cycle. When GNORM, drops below the level set by GNORM=n.nn, the geometry optimization will terminate.