It is shown that the use of pseudo-potentials for replacing the core electrons leads to errors in relative energies for density functional calculations which are comparable to the errors from using double or triple zeta type basis sets for systems composed of first-and second-row elements, respectively. The pseudo-potential error is similar to 10 kJ/mol for relative energies, although errors in atomization energies are significantly larger. (c) 2005 Published by Elsevier B.V.