Values of the leading coefficients of the MacLaurin expansion of the electron momentum density Pi((p) over right arrow) are calculated by a fully numerical Hartree-Fock method for 78 diatomic molecules. Large basis set errors are found in previous computations of these quantities using self-consistent-field wave functions often considered to be of near-Hartree-Fock quality. The numerical Hartree-Fock values should be valuable in the development and calibration of basis sets for the calculation of electron momentum densities. (C) 2002 Elsevier Science B.V. All rights reserved.