Command already defined - IOP publication with mhchem

I am using iopart class for IOP publication, and I want use the mhchem package for writing chemical equations. I ran into the same problem as here and there, old topics without answer. I compile with pdflatex in Texmaker the following code:

documentclass[12pt]{iopart}
%Uncomment next line if AMS fonts required
%usepackage{iopams} 
usepackage[version=3]{mhchem}

begin{document}
title[mytitle]{mytitle}
author{myauthor}

begin{abstract}
A minimal non-working iopart + mhchem example
end{abstract}

ce{P ->[sigma_{P}] T}

end{document}

This throws back the error

! LaTeX Error: Command equation* already defined.
               Or name end... illegal, see p.192 of the manual.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...                                              

l.2664 }

I added the advice in here:

makeatletter
@namedef{[email protected]}{}
makeatother
usepackage{amstext}

before usepackage[version=3]{mhchem}. I have no clue what it means.
But it raises plenty of errors :

! Undefined control sequence.
<argument> rightarrowfill@ 
                            displaystyle 
l.18 ce{P ->[sigma_{P}] T}

? 
! Missing $ inserted.
<inserted text> 
                $
l.18 ce{P ->[sigma_{P}] T}

? 
! Extra }, or forgotten $.
@begin@tempboxa ...begingroup #2color@endgroup }
                                                  def width {wd @tempbox...
l.18 ce{P ->[sigma_{P}] T}

? 
! Missing $ inserted.
<inserted text> 
                $
l.18 ce{P ->[sigma_{P}] T}

? 
! Missing } inserted.
<inserted text> 
                }
l.18 ce{P ->[sigma_{P}] T}

EDIT: Following comments below I tried :

documentclass[12pt]{iopart}
%Uncomment next line if AMS fonts required
%usepackage{iopams} 

makeatletter
@namedef{[email protected]}{}
makeatother
usepackage{amstext}
usepackage{chemformula}

begin{document}
title[mytitle]{mytitle}
author{myauthor}

begin{abstract}
A minimal working iopart + chemformula example
end{abstract}

ch{P ->[$sigma$] T} par

end{document}

And it does the job !

expandafterletcsname equation*endcsnamerelax

expandafterletcsname endequation*endcsnamerelax

@namedef{equation*}{[}

@namedef{endequation*}{]}