What are the differences between the basis set of 6-311G** and cc-pVTZ? Why we call the basis set of cc-pVTZ is more accurate than 6-311G** basis set? When we obtain the transition structure by using MP2 and CCSD methods, in MP2 method gives TS state but CCSD does not give TS state (same input structure for TS calculations), why?
To compare the basis sets, see e.g. https://bse.pnl.gov/bse/portal and look at the elements and basis of interest.
To guide the TS search, Hessian is needed, where analytic second derivatives are available for MP2, but not for CCSD. At first, rule out technical problems.
And mainly, the geometries from MP2 are usually good enough for single point energy calculations, also at TS.
Thank you.
Why accuracy is high in the basis set of cc-pVTZ by comparing 6-311G** and what are the factors for this high accuracy(by points)?
Please have a look at http://en.wikipedia.org/wiki/Basis_set_%28chemistry%29 and the books referenced therein to get reasonable overview of the problem.
The cc-pvTZ basis set is a much larger basis set - more functions per atom - than 6-311G**. For example, for C, 6-311G** has 1 1s function, 3 2s functions, 9 2p functions (3 sets of 3), and 5 3d functions for a total of 18 basis functions. With cc-pvTZ, the specification is 4 1s functions, 3 sets (of 3 each) of 2p functions, 2 sets (of 5 each) 3d functions, and 1 set (of 7) 4f functions, for a total of 30 basis functions. The larger the basis set for a given type of calculation (HF, for example), the lower the variational energy. Also look at http://en.wikipedia.org/wiki/Basis_set_%28chemistry%29 as noted before for more help.
cc-pvtz is a dunning's correlation consistent basis set (triple zeta) which has redundant function removed and has been rotated in order to increase computational efficiency . in this basis set polarization function included in definition (in contrast 6-311g) .The correlation energy can be calculated better than the 6-311G** via cc-pvtz and the primitive Gaussian number for cc-pvtz is larger than 6-311g**.
Did you run any calculations with cc-pvtz, cc-pvDz, cc-pvqz and cc-pv5z with Casscf method ?
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