It is shown that the use of effective core potentials for replacing the core electrons in connection with density functional calculations leads to errors in equilibrium bond lengths of ∼0.001 Å for systems composed of first row elements and 0.005-0.010 Å for systems composed of second row elements. The corresponding errors for harmonic vibrational frequencies are a few cm in both cases. These errors are slightly larger than the errors from all-electron calculations using a triple zeta type basis sets. There is little difference in the effective core potential error for the two different exchange-correlation potentials tested.
