Note that n c = n v so the system becomes charged in the fractional-occupation SCF calculation. This can be a problem for DFT calculations under periodic boundary conditions, and therefore some charge-neutral alternatives have been explored. These are discussed below. A summary of different approximations is provided in Table , given in the form ∆E = F v -F c where F v and F c are simple functions of the virtual and core eigenvalues, respectively, computed from one or more SCF calculations with non-Aufbau occupancies that are allowed to be fractional. The list includes several variants of STM, as this has been tried with different fractions of an electron, as well as some alternatives such as the FCHM, 18,58-60 which is the n c = 0 analogue of the TPM/HCH method in eq 6. In FCHM, a full electron is removed from the core but nothing is placed in the virtual space: