Résumé : The use of single determinantal approximate molecular wavefunctions of the LCAO MO NDO type for the calculation of the momentum density ρ(p) and the radial momentum density distribution J(p) is discussed. In each case, these expressions should be orientationally invariant and the momentum density should be normalized. Combining these two requirements, it is shown that only two approximations are physically significant: (1) NDO wavefunctions are used and ρ(p) and I(p) are approximated respectively up to an INDO and a CNDO level; (2) Overlap integrals are explicitly taken into account when solving the Roothaan SCF equations or deorthogonalized NDO functions are employed, together with the unapproximated expressions. © 1976 Springer-Verlag.