# Why does not compile?

I have this equation

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


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.

• You wrote \leqj. It should be \leq j – Ignacio Correa Oct 6 '17 at 18:19

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:

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

on-topic:

• cause of error is described in above comment