0

I have this equation

\begin{equation}
\hat{H} = -\displaystyle\frac{\hslash^{2}}{2}\sum\limits_{\alpha=1}^{n}\frac{\nabla_{\alpha}^{2}}{M_{\alpha}} -\displaystyle\frac{\hslash^{2}}{2m_{e}}\sum\limits_{i=1}^{N}\nabla_{i}^{2} - \frac{e^{2}}{4\pi\epsilon_{0}}\sum\limits_{\alpha=1}^{n}\sum\limits_{i=1}^{N}\frac{Z_{\alpha}}{\vert\vec{r_{i}} - \vec{R_{\alpha}}\vert}  + \frac{e^{2}}{4\pi\epsilon_{0}}\sum\limits_{\alpha\leq\beta}^{n}\frac{Z_{\alpha}Z_{\beta}}{\vert\vec{R_{\alpha}} - \vec{R_{\beta}}\vert} + \frac{e^{2}}{4\pi \epsilon_{0}}\sum\limits_{i\leqj}^{N}\frac{1}{\vert\vec{r_{i}} - \vec{r_{j}}\vert} 
\end{equation}

And the texmaker return the following error:

! Undefined control sequence.
l.15 ...{2}}{4\pi \epsilon_{0}}\sum\limits_{i\leqj
}^{N}\frac{1}{\vert\vec{r_...
The control sequence at the end of the top line
of your error message was never \def'ed. If you have
misspelled it (e.g., `\hobx'), type `I' and the correct
spelling (e.g., `I\hbox'). Otherwise just continue,
and I'll forget about whatever was undefined.
  • 3
    You wrote \leqj. It should be \leq j – Ignacio Correa Oct 6 '17 at 18:19
2

off-topic:

  • all \displaystyle and limits are superfluous
  • equation is to long that can be fit in text width, it should be break into two lines:

enter image description here

(red lines on image show text border)

\documentclass{article}
\usepackage{mathtools, amssymb}
%-------------------------------- show page layout, only for test
\usepackage{showframe}
\renewcommand\ShowFrameLinethickness{0.15pt}
\renewcommand*\ShowFrameColor{\color{red}}
%---------------------------------------------------------------%

\begin{document}
\begin{equation}
\begin{multlined}[0.8\linewidth]
\hat{H} = - \frac{\hslash^{2}}{2}
                \sum_{\alpha=1}^{n}\frac{\nabla_{\alpha}^{2}}{M_{\alpha}}
    - \frac{\hslash^{2}}{2m_{e}}\sum_{i=1}^{N}\nabla_{i}^{2}
    - \frac{e^{2}}{4\pi\epsilon_{0}}\sum_{\alpha=1}^{n}
                \sum_{i=1}^{N}\frac{Z_{\alpha}}{\vert\vec{r_{i}}
    - \vec{R_{\alpha}}\vert}    \\
%
    + \frac{e^{2}}{4\pi\epsilon_{0}}
                \sum_{\alpha\leq\beta}^{n}\frac{Z_{\alpha}Z_{\beta}}{\vert\vec{R_{\alpha}}
    - \vec{R_{\beta}}\vert}
    + \frac{e^{2}}{4\pi \epsilon_{0}}\sum_{i\leq j}^{N}\frac{1}{\vert\vec{r_{i}} - \vec{r_{j}}\vert}
\end{multlined}
\end{equation}
\end{document}

on-topic:

  • cause of error is described in above comment

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