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17.2.9 Asymptotic correction for xc-potentials(ASYMP)

ASYMP,shift,$\alpha$,$\beta$,b

Activates the gradient-regulated asymptotic correction (GRAC) approach for exchange-correlation potentials of Grüning et al. (J. Chem. Phys. 114, 652 (2001)). The user has to supply a shift parameter ( $\Delta_\mathrm{xc}$) for the bulk potential which should approximate the difference between the HOMO energy ( $\varepsilon_\mathrm{HOMO}$) obtained from the respective standard Kohn-Sham calculation and the (negative) ionisation potential of the monomer ($\mathrm{IP}$):

\begin{displaymath}
\Delta_\mathrm{xc}=\varepsilon_\mathrm{HOMO}-(-\mathrm{IP})
\end{displaymath} (1)

This method accounts for the derivative discontinuity of the exact xc-potential and that is missing in approximate ones. The parameters $\alpha$ and $\beta$ determine the interpolation function (see Eq. (2.3) in the above reference) and are set to $\alpha=0.5$ and $\beta=40$ by default, respectively. The parameter b is the parameter of the asymptotic xc-potential from van Leeuwen and Baerends (Phys. Rev. A 49, 2421 (1994), Eqns. (54,55)) and is set to b=0.05 by default.

In case of gradient or laplacian functionals the modified GRAC scheme of Bast et al. (Chem. Phys. Chem. 9, 445 (2008)) is used.

If shift is set to zero in the input the program will estimate the ionisation energy from the HOMO energy during the SCF (as soon as the HOMO energy is converged to a given threshold) and then sets the bulk shift automatically. This is done by using a linear fit of DFT HOMO energies to ionisation energies calculated with the $\Delta$SCF method for a range of molecules (see also S. Hirata et al., J. Phys. Chem. A, 107, 10154 (2003)).



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