The transformation of the molecular orbitals (MO’s) of open shell systems during a self-consistent field iteration step is compared for two different approaches : the usual procedure of iterative diagonalization of a Fock matrix and a simplified direct approach using the gradient and the one-electron Hessian matrix in a space of orbital rotation parameters. It is shown that the frequent failure of the former is due to inappropriate energy denominators given by the difference of MO energies. In contrast to closed shell systems, the elements of the one-electron Hessian matrix cannot be represented consistently by such differences in the case of open shell systems. The results of numerical calculations support this finding.