Dear Dr Case,
The problem of threshold in geometry optimization & charge
reproducibility was defined in R.E.D. version I in 2002/2003 and
reported at Anaheim in 2004 -
http://oasys2.confex.com/acs/227nm/techprogram/P705829.HTM - but
related to the Gaussian & GAMESS programs.
In short, geometry optimization always use Opt=Tight in Gaussian (4
criteria of convergence) and OPTTOL=1.0E-06 in GAMESS (2 criteria of
convergence) - SCF convergence 10-8, while SCF convergence is set to
10-6 for bio-organic molecules in MEP computation. In these conditions
the reproducibility of charge value achieved is ~1.10-4 e.
See also the section "Limitations of the strategy presented" at:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2918240/
regards, Francois
>> At the end of the Ambertools 1.5 it states that for AM1-BCC charges for
>> 585 drug molecules, MOPAC and sqm give essentially similar charges for all
>> the cases and the average charge difference is 0.005. So what are the
>> advantages to sqm over MOPAC for AM1-BCC, since there appears to be tight
>> convergence issues with sqm?
>
> The were a number of reasons we chose sqm over mopac, the most important one
> being the continuing difficulties we faced in getting mopac to
> compile and run
> correctly on a variety of architectures and compilers. We found it to be
> quite difficult to get mopac to give the same charges (say to 0.001 electron)
> on different compilers, which made it frustrating to set up automated tests.
>
> It was only after sqm was run "in the wild" on many more compounds than
> we ever initially tested that we became aware of occasional convergence
> issues.
>
> By default, sqm demands tight convergence for both SCF and geometry
> optimizations, whereas MOPAC has much less stringent criteria. We may not
> have the most appropriate defaults, especially for am1-bcc calcs. This is
> discussed in the antechamber chapter of the AmberTools Users' Manual.
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Received on Sun Jan 15 2012 - 03:00:02 PST
