I'm getting this error:

```
! Undefined control sequence.
l.64 ...p(- \frac{E_j - E_i}{k_B T}) & \quad \text
{: E_j > E_i}\\
?
! Undefined control sequence.
l.65 1 & \quad \text
{: E_j<E_i}\\
?
```

which is from this:

```
\begin{equation}\label{eq:hop}
w_{i,j}=w_0 exp(-2 \gamma R_{i,j})\left\{
\begin{array}{l l}
exp(- \frac{E_j - E_i}{k_B T}) & \quad \text{: E_j > E_i}\\
1 & \quad \text{: E_j<E_i}\\
\end{array} \right.
\end{equation}
```

But I don't understand why I get the error. This is my header:

```
\documentclass[11pt]{iopart}
%\newcommand{\gguide}{{\it Preparing graphics for IOP journals}}
%Uncomment next line if AMS fonts required
\usepackage{iopams}
\usepackage{graphicx, wrapfig}
\usepackage{fixltx2e}
\graphicspath{{images/}}
\DeclareGraphicsExtensions{.pdf,.png,.jpg,.gif}
\makeatletter
\g@addto@macro\@floatboxreset\centering
%\def\@cite#1{\textsuperscript{[#1]}}
\makeatother
```

And my class file is the standard IoP one from here

Can anyone help me get read of this error?

## Best Answer

The mistake is, as TeX also informs. It cannot find your

`\text`

command. The error lies probably in package`iopams`

and should be noticed there. It seems that it does not load the ams math package (only its fonts) and thus you need to explicitly include that package as well.On a side note you should also include

`$`

in your`\text`

command.Solution: