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:
When a partial core charge is calculated, ON, otherwise OFF.
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 at which the ratio between the core electron density and the valence electron density becomes pcc.ratio.
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, =pcc.ratio.origin .
Note that a precipitous partial core charge would cause numerical instabilities. Thus, a modest core charge is better from a numerical point of view.