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Re: Slow convergence of geometry optimization using PCM

Alex Granovsky

Dear Tatiana,

Sorry for the delayed reply.

The current status of PCM gradient code in Firefly is as follows.
The PCM derivatives are partly analytical partly numerical.
In most cases, the semi-numeric nature of gradients does not
allow efficient geometry optimizations with tight gradient
convergence criteria. I'd recommend to relax OPTTOL to at least
3.0D-4 or above. In addition, one needs to increase the overall
precision of computations, e.g.:

 $contrl inttyp=hondo icut=11 itol=30 $END
 $scf nconv=6 $end

Hope this helps.

Kind regards,
Alex Granovsky

P.S. It is impossible to change the default number of number
of tesserae per sphere using input file.

On Wed Feb 19 '14 1:19am, Tatiana Alieva wrote
>Dear All,

>First, Thank you to all Firefly developers for Firefly package!
>Now about the issue. Iíve encountered a slow convergence of geometry optimization while using PCM. Water dimer (2 water molecules connected with an H-bond) were first optimized in gas (RHF 6-31G(d,p)). The stationary point was checked for minimum. Then the gas optimized dimer was optimized using PCM (either D-PCM or COSMO) and the same basis and level of theory. Reoptimization in PCM required 58 and over 200 iterations, correspondingly, (and in both cases ended in saddle points :( ). Usage HESS=CALC for D-PCM increased number of steps to >100. Attempts to optimize the complex first with a simpler basis (or smaller values of solvent epsilonĖ as an intermediate step between vacuum and water epsilon) and then do it in the target basis, level of theory, and epsilon showed to be also inefficient.

>Since water dimer is the simplest system in my study, and Iím planning to calculate much bigger complexes in water, Iím getting worried about PCM geometry optimization for them.

>Slow convergence of the optimization process and oscillations using PCM as far as I see is a well-known phenomenon. Iíve learned that Gaussian 98 enables users to specify the number of surface elements (tesserea) for each sphere and the area of the surface elements. Here
>is shown that these parameters can be of crucial importance for the geometry optimization efficiency.
>Unfortunately, I havenít found corresponding keywords in the Firefly documentation. Are there such tools in Firefly? If not, is it difficult to provide them to make Firefly more flexible?
>Are there other suggestions how to speed up convergence in PCM? †I will be grateful for any piece of advice. †

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