CCL: OPT on large molecules

 Sent to CCL by: Marcel Swart [m.swart * few.vu.nl]
On Saturday, January 28, 2006, at 09:55 PM, David M. Close closed~!~etsu.edu wrote: 
 
   I have a question about the necessity of optimizations.
I have uncovered an effect experimentally that involves changes in large molecules. By large I mean that even partial optimizations with modest basis sets take several days. The effect involves charge migration from one region of the molecule to another region. I would like to examine this effect with computations. After a weeks work I still don't have an answer because of the times taken to determine the optimized structures.
Of course it depends on the method you use (e.g. RHF, DFT or CCSD(T)) how long your optimization will take, and also on the program you use for it, but in the end, if you want to be able to understand the experimental effect, or be able to predict changes in it, you *will need* the optimized geometries, as well as the frequencies to show that 
 you didn't end up in some sort of n-th order saddle point.
Note that the convergence of the geometry depends largely on the type of coordinates you have chosen, where Cartesians are usually not such a good choice, and it is better to use natural internal coordinates, delocalized coordinates or our recently developed 
 adapted delocalized coordinates.
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 dr. Marcel Swart
 Theoretische Chemie (kamer R152)
 Vrije Universiteit Amsterdam
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 Starting May 1, 2006:
 ICREA researcher at
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