Can I always trust my results from a very sophisticated method such as CCSD(T) with a large basis set?
No! You should know in general terms, what kind of approximations are. For, example CCSD(T) is one of the most accurate methods that can be used for everyday calculations and it has proven to be consistently very good for many properties, however there are approximations in it. For example you may be dealing with a case where electron correlation from higher excitations is very important, or a with a case where the Born-Oppenheimer approximation fails. Its performance can also be affected by Hartree-Fock instabilities and near instabilities.
