Hello,
Please, can anyone assist in describing how to organize Gaussian fragment for counterpoise (CP) optimized transition state. Or is there any protocol to be followed when estimating basis set superimposition error in a transition structure. I have read a number of article, but I couldn't get it right.
Thanking you in advance for any useful directive.
Monsurat.
Well that's exactly the problematic thing about Counterpoise corrections, you can not really apply them to a (forming) bond, because what is an 'artifact' of having extra flexibility of basis functions on neighboring fragments halfway your reaction 'truly' starts to form a bond from overlapping orbitals. So your reaction profile / potential energy surface is discontinuous if you CP-correct it.
I would recommend trying to see if you can do a complete basis set extrapolation type of calculation, or just check if your activation energy converges with respect to your basis set.
If you are using DFT, usually fortunately there is not much sensitivity with respect to basis set size. So just go from double zeta to triple zeta to triple zeta + diffuse and see if it makes a huge difference. If you are using a post-HF method like MP2 or CCSD, basis set size makes a huge difference and I would recommend getting to the CBS limit. There are codes that can run gradients with CBS extrapolation and also use explicitly correlated methods (e.g. F12) for faster convergence to CBS.
Hope that helps,
Fedor
Some recent thought on CP: http://pubs.acs.org/doi/abs/10.1021/ct400990u
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