I am working on a reaction in which there is a step involving the reaction between ch3 and ch2 . I have done this with wb97xd/6-311+g(d,p). now I want to repeat the same reaction which involves mostly open-shell species with a higher level of theory like MP2 with large basis set. i am doughtful that it is acuurate inouph for open shell species also. so should go for more accurate dft functional like b2plyp or should stick with mp2 ? Also I cannot afford CCSD(T) calculations.
I also want to ask that if the system is small like having nearly 20 atoms. is it necessary to include results from 2 levels of theory