CCL: RESPONSE SUMMARY: Gaussian 03, Opt. Changing ''maximum allowed number of steps''.

 Sent to CCL by: "Niels Johan Christensen" <s991418 *
 student.dtu.dk>
 Dear All,
 Some weeks ago I asked for advice on how to change the maximum allowed
 number of optimisation steps in g03 -- that is to say, how to
 set the MaxCycles variable to any desired value.
 I got the following response from D. J. Fox (help * gaussian.com):
 >G03 imposes an absolute limit on the number of optimization cycles
 >based on the size of the molecule. The thinking is that if you are well
 >beyond the normal number of cycles there may be something more serious
 >wrong and the amount of numerical error built up in the force constant
 >matrix will become large enough to be a problem by itself.
 >
 >The first thing to do is to examine the last few structures at
 >this point. Is the structure still close to the local minimum you wanted
 >to explore? Is the gradient converged but the displacements are still
 >not? Are the approximate force constants of a reasonable size, say no
 >smaller than 0.01? Also is this the ultimate level of theory you plan to
 >report or is this preliminary to later calculations?
 >
 >If the structure is close then FREQ at this point may be sufficient
 >to confirm it really is a minimum or give you a good start for going on
 >to the next level of theory.
 >
 >If it is a good structure but clearly not the minimum OPT=CalcFC from
 >this geometry will likely get you there quickly, much more quickly than
 >trying to continue with gradients only and poor force constants.
 >
 >If it is not clearly the minimum you meant to explore look back along
 >the optimization, did it diverge at some point and you might want to
 >start from that structure. Perhaps you need to add a constraint and
 >reoptimize from the beginning to keep it from wandering on a flat potential
 >energy surface.
 >
 >In any case the code does not permit increasing the number of optimization
 >cycles without limit.
 Thank you Dr. Fox, OPT=CalcFC worked very well in my case.
 I found out, however, that g03 will actually permit the desired value of
 MaxCycles in a second run. One simply loads the checkpoint file of the failed
 optimisation. Thanks to Dr. Steen Hammerum for this hint.
 Best Regards,
 Niels Johan Christensen