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 similar to 0.001 angstrom for systems composed of first row elements and 0.005-0.010 angstrom for systems composed of second row elements. The corresponding errors for harmonic vibrational frequencies are a few cm(-1) 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. (c) 2005 Elsevier B.V. All rights reserved.