The accuracy of the pseudopotential approximation within density-functional theory

We have investigated the accuracy of pseudopotential (PSP) density-function al 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 d eviations between ab-initio PSP and AE calculations are due to the lineariz ation 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 s ystems and reaction processes which involve radicals. The NLCC is also esse ntial for a realistic description of elements with more long-range core sta tes such as alkali atoms. A further improvement of pseudopotential accuracy may be achieved by explicitly including semi-core stales in the calculatio n.