this approach is efficient enough,
so the <TT>"xc basis"</TT> basis set is not generally required.

<P>
For the DFT module, the input options for defining the basis sets in a given
calculation can be summarized as follows;

<UL>
<LI><TT>"ao basis"</TT> - Kohn-Sham molecular orbitals; required for all 
calculations
</LI>
<LI><TT>"cd basis"</TT> - charge density fitting basis set; optional, but
recommended for evaluation of the Coulomb potential
</LI>
<LI><TT>"xc basis"</TT> - exchange-correlation (XC) fittin