# iopart class is not friend with physics package

I am using iopart class for my document. In an equation I wish to use ket. For this reason I use \usepackage{physics}. But when I type this package, I alarmed by Command \equation already defined. And proram does not compile!!!

How I write the symbol ket?

• physics is not a package too many are excited about. For \ket you can load the braket package. – user121799 Jun 28 '19 at 15:49
• iopart is not friend to many packages, including amsmath. – egreg Jun 28 '19 at 15:51
• @egreg: in French, there is this article (and a workaround) about incompatibilities between iopart and asmmath (the guilty is iopart): science.thilucmic.fr/index.php?c=la&p=iopart-amsmath. The translation in English provided by for example Google seems good. – quark67 Jun 28 '19 at 16:38

As suggested byquark67 we can use amsmath workaround by Thierry Masson on this occasion.

Note whilst IOPscience suggest not using amsmath because of the equation clash they do not exclude you using amsmath with other classes such as standard article.

"...using the IOP class file may help to speed the publication of accepted articles. Note that there is an incompatibility between amsmath.sty and iopart.cls. If your article relies on commands in amsmath.sty that are not available in iopart.cls, you may wish to consider using a different class file..."

\documentclass[12pt]{iopart} % Avoid iopams

% From http://science.thilucmic.fr/index.php?c=la&p=iopart-amsmath&l=en
$$\ket {\psi(t)}$$