Partial core correction

The contribution to electron density from core electrons is ignored in the evaluation of exchange-correlation energy in the pseudopotential method, although there is an non-linearity of exchange-correlation energy with respect to electron density. Thus, a partial core correction would be important in order to take account of the non-linearity. A partial core charge for the partial core correction can be constructed by the following keywords:

• charge.pcc.calc When a partial core charge is calculated, ON, otherwise OFF.
• pcc.ratio The keyword, pcc.ratio, is a parameter in the calculation of a partial core electron density. The core electron density is approximated using a fourth order polynomial below the cutoff radius $r_{\rm pcc}$ at which the ratio $\rho_{\rm c}/\rho_{\rm v}$ between the core electron density $\rho_{\rm c}$ and the valence electron density $\rho_{\rm v}$ becomes pcc.ratio.
• pcc.ratio.origin The keyword, pcc.ratio.origin, is a parameter in the calculation of a partial core electron density. The core electron density is approximated using a fourth order polynomial so that the core electron at the origin satisfies a relation, $\rho_{\rm c}(0)$=pcc.ratio.origin $\times\rho_{\rm c}(r_{\rm pcc})$.

Note that a precipitous partial core charge would cause numerical instabilities. Thus, a modest core charge is better from a numerical point of view.