# Formatting long equation with iopart

I need to write the following long equation in my paper with the template given by IOP, iopart:

$$\begin{split} \frac{dN}{d(\hbar\omega)} &= \frac{\alpha}{12\pi}\, \frac{4+\beta(K)+\beta(K)^2}{1+\beta(K)} \frac{|\vec{\Delta\beta}|^2}{\hbar\omega}=\\ &=\frac{\alpha}{12\pi}\frac{4+\beta(K)+\beta(K)^2}{\hbar\omega(1+\beta(K))}\left( \frac{2mc^2 K+ K^2}{(mc^2+K)^2} + \frac{2mc^2 (K-\hbar\omega)+ (K-\hbar\omega)^2}{(mc^2+K-\hbar\omega)^2} + 2\frac{m^2c^4-K(K-\hbar\omega)}{m^2c^4} \right) \end{split}$$


Normally I would use split as above, but with this template it is not possible. Their guidelines say that one should use \fl in order to align multiline equations, so I have tried with:

$$\fl \frac{dN}{d(\hbar\omega)} = \frac{\alpha}{12\pi}\, \frac{4+\beta(K)+\beta(K)^2}{1+\beta(K)} \frac{|\vec{\Delta\beta}|^2}{\hbar\omega}=\\ \fl =\frac{\alpha}{12\pi}\frac{4+\beta(K)+\beta(K)^2}{\hbar\omega(1+\beta(K))}\left( \frac{2mc^2 K+ K^2}{(mc^2+K)^2} + \frac{2mc^2 (K-\hbar\omega)+ (K-\hbar\omega)^2}{(mc^2+K-\hbar\omega)^2} + 2\frac{m^2c^4-K(K-\hbar\omega)}{m^2c^4} \right)$$


but it gives this result:

• The \fl command what is it, \lefteqn? Commented Aug 1, 2020 at 15:39
• @Sebastiano It seems their own implementation of \lefteqn. IOP has defined its own package that is incompatible with the one by ams Commented Aug 1, 2020 at 16:11

As you've noted, the iopart document class doesn't let you use the amsmath package and its multi-line display-math environments. However, it provides a re-implementation of LaTeX's venerable eqnarray environment, which seems suitable for the job at hand.

\documentclass{iopart}
\usepackage{esvect} % for \vv macro (alternative for \vec)
\begin{document}
\begin{eqnarray}
\frac{\mathrm{d}N}{\mathrm{d}(\hbar\omega)}
&= \frac{\alpha}{12\pi} \, \frac{4+\beta(K)+\beta(K)^2}{1+\beta(K)}
\frac{|\vv{\Delta\beta}|^2}{\hbar\omega} \nonumber\\
&= \frac{\alpha}{12\pi} \, \frac{4+\beta(K)+\beta(K)^2}{\hbar\omega(1+\beta(K))}
\biggl( \frac{2mc^2 K+ K^2}{(mc^2+K)^2}\\

• @Eddymage - Do you employ the iopart document class? If not, do please say which document class you use.
• Yes, I am using the iopart class (unfortunately). Actually, I am using this workaround and everything works fine with iopart, and I hope that the journal will accept this in the preamble. Commented Sep 8, 2022 at 13:59