Starting from the primitive basis functions, you can optimize the
radial shape variationally so that the accuracy can be increased.
See the details in the Section 'Orbital optimization'.
The isosurface map of the highest occupied state at the point
for NaCl with a Cl-site vacancy, which shows a F-center in NaCl with
a Cl vacancy. The isosurface map was drawn using XCrySDen
with the isovalue of 0.042 .
The calculation was done with the system charge of -1 using
a keyword 'scf.system.charge'. The watery and silver colors
correspond to sodium and chlorine atoms,respectively, and the yellow small ball
shows the position of empty atom.