X-Git-Url: https://hackdaworld.org/gitweb/?a=blobdiff_plain;f=posic%2Fthesis%2Fbasics.tex;h=f6a8eda37e41458fa71099fe61ac63fe811dcbe8;hb=e9b904cd12213f534e64f904e48071337f1f6cb6;hp=cba8c5626362e9512c93717412d7a88c16e6a85e;hpb=e3a0b64c27ef578fffd44e26b43953917b858e86;p=lectures%2Flatex.git diff --git a/posic/thesis/basics.tex b/posic/thesis/basics.tex index cba8c56..f6a8eda 100644 --- a/posic/thesis/basics.tex +++ b/posic/thesis/basics.tex @@ -421,7 +421,7 @@ for the exchange-correlation energy, where $\epsilon_{\text{xc}}(\vec{r};[n(\til Expressing $n(\tilde{\vec{r}})$ in a Taylor series, $\epsilon_{\text{xc}}$ can be thought of as a function of coefficients, which correspond to the respective terms of the expansion. Neglecting all terms of order $\mathcal{O}(\nabla n(\vec{r}))$ results in the functional equal to LDA, which requires the function of variable $n$. Including the next element of the Taylor series introduces the gradient correction to the functional, which requires the function of variables $n$ and $|\nabla n|$. -This is called the generalized gradient approximation (GGA), which expresses the exchange-correlation energy density as a function of the local density and the local gradient of the density +This is called the generalized-gradient approximation (GGA), which expresses the exchange-correlation energy density as a function of the local density and the local gradient of the density \begin{equation} E^{\text{GGA}}_{\text{xc}}[n(\vec{r})]=\int\epsilon_{\text{xc}}(n(\vec{r}),|\nabla n(\vec{r})|)n(\vec{r}) d\vec{r} \text{ .} @@ -532,6 +532,7 @@ Using PPs the rapid oscillations of the wave functions near the core of the atom More importantly, less accuracy is required compared to all-electron calculations to determine energy differences among ionic configurations, which almost totally appear in the energy of the valence electrons that are typically a factor $10^3$ smaller than the energy of the core electrons. \subsection{Brillouin zone sampling} +\label{subsection:basics:bzs} Following Bloch's theorem only a finite number of electronic wave functions need to be calculated for a periodic system. However, to calculate quantities like the total energy or charge density, these have to be evaluated in a sum over an infinite number of $\vec{k}$ points. @@ -631,10 +632,8 @@ The path exhibiting the minimal energy difference determines the diffusion path \begin{figure}[t] \begin{center} -\subfigure[]{\label{fig:basics:crto} -\includegraphics[width=0.5\textwidth]{crt_orig.eps}} -\subfigure[]{\label{fig:basics:crtm} -\includegraphics[width=0.5\textwidth]{crt_mod.eps}} +\subfigure[]{\label{fig:basics:crto}\includegraphics[width=0.45\textwidth]{crt_orig.eps}} +\subfigure[]{\label{fig:basics:crtm}\includegraphics[width=0.45\textwidth]{crt_mod.eps}} \end{center} \caption{Schematic of the constrained relaxation technique (a) and of a modified version (b) used to obtain migration pathways and corresponding configurational energies.} \label{fig:basics:crt} @@ -655,10 +654,9 @@ In the modified version respective energies could be higher than the real ones d Structures of maximum configurational energy do not necessarily constitute saddle point configurations, i .e. the method does not guarantee to find the true minimum energy path. Whether a saddle point configuration and, thus, the minimum energy path is obtained by the CRT method, needs to be verified by caculating the respective vibrational modes. - Modifications used to add the CRT feature to the VASP code and a short instruction on how to use it can be found in appendix \ref{app:patch_vasp}. -Due to these constraints obtained activation energies can effectively be higher. % todo % advantages of pw basis with respect to hellmann feynman forces / pulay forces +% crt sketch needs increased text