Full density-functional theory

Add the remaining terms in Eqs. (18,20), those involving $\epsilon_{xc}$ and $\epsilon_{xc}'$, to get() and H(). You may use the functions excVWN() and excpVWN() in Appendices E and F, which implement the operators $\epsilon_{xc}(\vec n)$ and $\epsilon_{xc}'(\vec n)$, respectively, using the Vosko-Wilk-Nusair parameterization.