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Iris Theophilou edited RDMFT1.tex
over 9 years ago
Commit id: 4f60b003479eff72250326b4c027c962e62d9b5f
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E=\sum_{i=1}^\infty\int d\mathbf{r} n_{i}\phi^{*}_{i}(\mathbf{r})\left(-\frac{\nabla^2}{2}\right) \phi_{i}(\mathbf{r})+\sum_{i=1}^\infty \int d\mathbf{r} V_{\mathrm{ext}}(\mathbf{r})n_{i}|\phi_{i}(\mathbf{r})|^{2}\nonumber\\
+\frac{1}{2}\sum_{i,j=1}^\infty n_{i} n_{j}\int d\mathbf{r} d\mathbf{r'} \frac{|\phi_{i}(\mathbf{r})|^{2} |\phi_{j}(\mathbf{r})|^{2}}{|\mathbf{r}-\mathbf{r'}|} + E_{xc}\left[\{n_{j}\},\{\phi_{j}\}\right]\label{eqenergy}
\end{eqnarray}
the part that needs to be approximated $E_{xc}[\gamma]$ comes only from the interaction term (contrary to KS-DFT), as the interacting kinetic energy can be explicitely expressed in terms of $\gamma$. A common approximation for $E_{xc}$ is the
Mueller, M\"uller, which has the following form
\begin{eqnarray}
E_{xc}(\{n_j\},\{\phi_j\})=\frac{1}{2}\sum_{i,j=1}^\infty E_{xc}(\{n_j\},\{\phi_j\})=-\frac{1}{2}\sum_{i,j=1}^\infty \sqrt{n_{i} n_{j}}\int d\mathbf{r} d\mathbf{r'} \frac{\phi_{i}^{*}(\mathbf{r})\phi_i(\mathbf{r'}) \phi_{j}^{*}(\mathbf{r'})\phi_j(\mathbf{r})}{|\mathbf{r}-\mathbf{r'}|}
\end{eqnarray}
and is the only $E_{xc}$ implemented in octopus for the
momment. moment.
\par
For closed-shell systems the necessary and sufficient conditions for the 1-RDM to be $N$-representable, i.e.\ to correspond to a $N$-electron wavefunction is that $ 0 \leq n_{i} \leq 2$ and
\begin{eqnarray}
\sum_{i=1}^{\infty}n_{i}=N.\label{eqsumocc}
\end{eqnarray}
Note that within the RDMFT implementation in octopus only closed-shell systems are treated at the
momment. moment. Minimization of the energy functional of Eq. \ref{eqenergy} is performed under the $N$-representability constraints and the orthonormality requierements of the natural orbitals,
\begin{eqnarray}
\langle \phi_{i} | \phi_{j}\rangle = \delta_{ij}. \label{eqorth}
\end{eqnarray}