In a recent paper (J. Phys. Cond. Matter vol.1, p.2199, 1989), M.C. Payne has analysed the method originally introduced by two of the authors to perform molecular dynamics simulations with interatomic forces derived directly from the electronic ground state (Phys. Rev. Lett. vol.55 p.2471, 1985). However the equations quoted in the paper by M.C. Payne do not correspond entirely to the original procedure. For this reason the authors have decided to write a joint comment in order to outline the differences and explain the consequences.