To speed up the computation the innermost atomic shells are kept frozen. The frozen Core Orbitals (CO), which are solutions of a large-basis all-electron calculation on the isolated atom, are expressed in an auxiliary set of (Slater-type) basis functions cor-bas, distinct from the valence set. The core basis set and the expansion coefficients that give the COs expressed in them are stored in the database data files.
Orthogonality of the valence Molecular Orbitals (MO) to the COs is achieved with the help of so-called Core Functions (CF). These functions are included in the valence set but they are not additional degrees of freedom. Each of the normal valence functions is combined with a linear combination of all CFs in the molecule in such a way that the transformed function (cbas) is orthogonal to all frozen COs in the molecule. There are exactly as many CFs as COs so the orthogonality condition for all valence basis functions amounts to the solution of a linear system where the number of conditions equals the number of parameters.
This aspect once more increases the discrepancy between the number of expansion coefficients of an MO and the number of MOs: the expansion coefficients in the most elementary bas representation run over all bas functions, including the CFs among them. At some places there may, alternatively, be expansions in the core-orthogonalized BAS functions, CBAS, where the CFs do not count anymore: they are included implicitly in the cbas functions.
