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Tatiana Alieva
vishnevoe00@list.ru

First, Thank you to all Firefly developers for Firefly package!
Now about the issue. I’ve 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 epsilon– 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 I’m planning to calculate much bigger complexes in water, I’m 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. I’ve 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
http://www.cup.uni-muenchen.de/oc/zipse/the-polarizable-continuum-model-pcm.html
is shown that these parameters can be of crucial importance for the geometry optimization efficiency.
Unfortunately, I haven’t 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.  
Tatiana

This message contains the 160 kb attachment [ gas_and_DPCM_opt.rar ] Input and output for gas and D-PCM optimizations

[ This message was edited on Wed Feb 19 '14 at 1:19am by the author ]