An extension of the virial theorem in the Born-Oppenheimer framework is presented, which is valid for arbitrary (not necessarily variational) wave functions built up by using finite local one-electron basis sets. It permits to define explicitly the terms causing deviations from the 'ideal' virial ratio; the standard formulations of the virial theorem can also be obtained from it as special cases. (c) 2007 Elsevier B.V. All rights reserved.