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One-electron calculations of the optical excitation energy of the F centre in NaF are carried out to test the applicability of norm-conserving pseudopotentials and Kunz-Klein localising potentials to represent the electronic structure of nearby ions. The calculations include consistent staticdistortion and polarisation derived from an infinite embedding shellmodel lattice for the ground state. The basis set chosen for the wavefunction is optimised, and care is taken to avoid linear dependence. The results suggest that the electronic cores of nearby ions as determined in free-ion or perfect-lattice calculations are inadequate for the F centre.