However, there are formally some non-zero matrix elements between the boundary region and the neighboring B-spline functions. We thus enforce additional block diagonality at this point by requiring that also all other coupling elements between different basis blocks are set to zero in the Fock and overlap matrices, and S 2 sets since Eq. 63 is enforced and not exact. Often we prefer to use an orthonormal basis so if we want to use a localized VSCF basis in subsequent calculations it is desirable to orthonormalize it.