On the accuracy of pseudopotentials designed for QMC calculations on molecules of the first row atoms

► Atomization energies of diatomic molecules can be evaluated by the CCSD (T) method. ► The core correlation contribution can be extracted from all electron calculations. ► Computations with pseudo potentials should be compared with the frozen core results. ► This comparison must be done at all interatomic distances. ► The quality of a pseudo potential can be judged by looking at the discrepancies.