Importance of nonlinear core corrections for density-functional based pseudopotential calculations

We have investigated the importance of nonlinear core corrections (NLCC) for accurate density-functional based pseudopotential (PSP) calculations. The quality of the PSP approach is assessed by comparing it to accurate all-electron calculations. it is found that a correct description of spin-polarized states requires the NLCC, even for first-row atoms. This is essential for simulations on magnetic systems and reaction processes that involve radicals. The NLCC is also essential for a realistic description of elements with mon long-range core states such as alkali atoms. We propose a functional form for the partial NLCC, which is feasible in plane-wave calculations and suggest a scheme for choosing the default cutoff radius. [S0163-1829(99)02043-3].