multi-column row in align environment

Question

I would like to be able to use multicolumn inside an align environment, much like we can do in a tabular environment

MWE:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
    a &= e^{a+b+c+d+e+f+g+h+i+j+k+l+m+n+o+p} \\
    e^{a+b+c+d+e+f+g+h+i+j+k+l+m+n+o+p} &= a
\end{align}
\end{document}

In this case I would like to be able to put the & inside the exponent on the lhs of the second line, i.e. e^{a+b+c+d+e+ & f+g+h+i+j+k+l+m+n+o+p} but of course this is not allowed. I tried using multicolumn but this gave errors as well.

Of course I could align both equations to the left, but where I need it the situation is much more involving. Until now, I would put the ampersand in the second equation fully to the left, and manually adjust using negative spaces… But I wonder if there exists a better solution.

The most ideal situation would be something that allows me to put the & wherever I want, even inside brackets, but I would be happy with a multicolumn solution as well.

EDIT

What I try to achieve is something like

Answer 1

You should provide the spacing in a \phantom way:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
    &\phantom{e^{a+b+c+d+{}}}a = e^{a+b+c+d+e+f+g+h+i+j+k+l+m+n+o+p} \\
    &e^{a+b+c+d+e+f+g+h+i+j+k+l+m+n+o+p} = a
\end{align}
\end{document}

Answer 2

if i have understood correctly what you have described, this will give that result:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
    a &= e^{a+b+c+d+e+f+g+h+i+j+k+l+m+n+o+p} \\
    \rlap{$e^{a+b+c+d+e+f+g+h+i+j+k+l+m+n+o+p}$}\phantom{e^{a+b+c+d+e+{}}} &= a
\end{align}
\end{document}

but somehow, i don't think it's what you really want ...

Answer 3

You can use the optional argument of the\MoveEqLeft command, from mathtools to obtain experimetally what you want. In the equation where it's used, it replaces the ampersand and moves the equation a number of ems equal to the optional argument (2 by default).

\documentclass[12pt]{article}

\usepackage{mathtools, amssymb, graphicx}

\begin{document}

\begin{align}
    a &= e^{a+b+c+d+e+f+g+h+i+j+k+l+m+n+o+p} \\
\MoveEqLeft[4.5] e^{a+b+c+d+e+ f+g+h+i+j+k+l+m+n+o+p} = a
\end{align}

\end{document} 

Answer 4

can be written as

Code

\documentclass[12pt]{article}

\usepackage{mathtools, amssymb, graphicx}

\begin{document}

\begin{align}
    a &= e^{a+b+c+d+e+f+g+h+i+j+k+l+m+n+o+p} \\ 
 ~{}\quad e^{a+b+c+d+e+ f+g+h+i+j+k+l+m+n+o+p} 
 &= a
\end{align}

\end{document}