  Explicitly orbital-dependent approximations to the exchange-correlation
energy functional of density functional theory typically not only depend on the
single-particle Kohn-Sham orbitals but also on their occupation numbers in the
ground state Slater determinant. The variational calculation of the
corresponding exchange-correlation potentials with the Optimized Effective
Potential (OEP) method therefore also requires a variation of the occupation
numbers with respect to a variation in the effective single-particle potential,
which is usually not taken into account. Here it is shown under which
circumstances this procedure is justified.
