Subject: Optimization in Gamess and in Gaussian

From: FyD <fyd(at)u-picardie.fr>
Date: Thu, 29 May 2003 17:49:14 +0200 (CEST)

Dear All,

I am interested in better understanding the optimization procedure implemented 
in Gamess & Gaussian. In Gaussian, the optimization accuracy is controlled by 
the OPT keyword. Gaussian checks four parameters: the "Maximum Force", "RMS 
Force", "Maximum Displacement" and "RMS Displacement". From the numerous tests 
I ran using Gaussian, the "Maximum Displacement" and "RMS Displacement" 
thresholds are far MORE difficult to reach than the "Maximum Force" and "RMS 
Force" ones. Thus, after some optimization steps, the "Maximum Force" and "RMS 
Force" thresholds are usually satisfied. On the contrary, to satisfy the 2 
other ones (related to the "Displacement"), this demands a certain numbers of 
additional optimization steps...

- In GAMESS, the optimization is controlled by the OPTTOL keyword. The 
optimization is stopped when the "Maximum gradient" and the "RMS gradient" are 
below the demanded threshold value. It means (to me) that the "Maximum 
Displacement" and the "RMS Displacement" are NOT checked by GAMESS. Am I right ?

- Since the two "Displacement" thresholds are the MOST difficult criterions to 
be satisfied in Gaussian and as they are NOT checked by GAMESS (from what I 
understand), what are the implications of such differences in the optimization 
accuracy in GAMESS ?

Thank you very much, best regards, Francois