GNORM — Exit geometry optimizations when RMS gradient norm falls below a specified value.




This keyword allows manual replacement of RMS gradient norm termination criteria (n.n) for a geometry optimization. For example, the keyword GNORM=1.0 allows the geometry optimization to be concluded when the computed GNORM value falls below 1.0. The AMPAC default value is 0.1 kcal mol-1 Å-1 (or kcal mol-1 rad-1), and this is sufficient for most cases. However, some geometries require more extensive refinement and a smaller value such as GNORM=0.05 could be used. Also, in the case of very large molecules, a value larger than 0.1 may be required to allow reasonable termination. (See Chapter 4, Computational Procedures for an explanation of how the GNORM is computed.) Setting GNORM also may have an effect on the default SCF convergence criteria (SCFCRT). There are twi situations to be aware of when redefining the GNORM termination criteria. First, the optimization may halt if the calculation passes other tests but has not yet attained the proper gradient norm. This is overridden by use of GNORM=n.n. Second, a very small value of n.n can result in a calculation that will not reach completion in a reasonable amount of time.





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