Mind the basis set superposition error

In an ab initio calculation, when the inclusion of core electron correlation is required, the best approach is to use a basis set developed for the electrons that are correlated. However, when a basis set is used that has not been developed for the number of the electrons which are correlated in a calculation, the quantities calculated (geometry, binding energies, frequencies, etc.) need to be corrected for the basis set superposition error. Several cases are discussed and the proper line of action is emphasized.