The accuracy of the pseudopotential approximation within density-functional theory

We have investigated the accuracy of pseudopotential (PSP) density-functional calculations with respect to the corresponding all-electron (AE) results for a variety of atoms and small molecules. It is found that most of the deviations between ab-initio PSP and AE calculations are due to the linearization of the exchange-correlation functional within the PSP approach. This problem can be eliminated by applying nonlinear core corrections (NLCC). We find that a correct description of spin-polarized states requires the NLCC, even for first-row atoms. This is essential for simulations of magnetic systems and reaction processes which involve radicals. The NLCC is also essential for a realistic description of elements with more long-range core states such as alkali atoms. A further improvement of pseudopotential accuracy may be achieved by explicitly including semi-core states in the calculation.