Equation refuses to move leftward, past the first line

Question

I suspect people have already asked this question, but:

I have a really long multiline equation, inside an align environment, which is inside a subequation environment.

Each line of the equation is just a tiny bit too long for both the equation and the equation line number to fit into one line. So, I would like to move the entire multiline equation leftward a little bit, further left than the left edge of the text. However, the equation would only ignore the left margin and move further left for the first line. Any other line in the equation is instead aligned with the left edge of the text.

How should we tell LaTeX to move the entire equation leftwards ignoring the left margin?

Specifically, neither \hspace{-5in} nor \hspace*{-5in} would work.

Below are the first two lines of the equation: (the original equation is 10+ lines long, so)

\documentclass[11pt, oneside]{article}
\usepackage{geometry}
\geometry{letterpaper}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[english]{babel}
\usepackage{amsmath}

\begin{document}

\begin{subequations}
\begin{align}
\hspace*{-20cm}
\langle\tilde{\Theta}(\mathbf{l_1})...\tilde{\Theta}{(\mathbf{l_4})}\rangle&=\langle
\Theta(\mathbf{l_1})
\Theta(\mathbf{l_2})
\Theta(\mathbf{l_3})
\Theta(\mathbf{l_4})\rangle\\
+(
\iint\frac{d^2\mathbf{l_1'}d^2\mathbf{l_2'}}{(2\pi)^4}&\langle
\Theta(\mathbf{l_3})
\Theta(\mathbf{l_4})
\Theta(\mathbf{l_1'})
\Theta(\mathbf{l_2'})
\phi(\mathbf{l_1}-\mathbf{l_1'})
\phi(\mathbf{l_2}-\mathbf{l_2'})\rangle
[(\mathbf{l_1}-\mathbf{l_1'})\cdot \mathbf{l_1'}] 
[(\mathbf{l_2}-\mathbf{l_2'})\cdot \mathbf{l_2'}]
\end{align}
\end{subequations}

\end{document}

Answer 1

Locally you can enlarged text width with \adjustwidth environment from changepage package and with it move equation to the left with centered in the wide environment:

    \documentclass[11pt, oneside]{article}
    \usepackage[showframe]{geometry}
    \geometry{letterpaper}
    \usepackage[utf8]{inputenc}
    \usepackage[T1]{fontenc}
    \usepackage[english]{babel}
    \usepackage{amsmath}

    \usepackage[strict]{changepage}

    \begin{document}

    \begin{adjustwidth}{-1em}{}
    \begin{subequations}
    \begin{align}
    \langle\tilde{\Theta}(\mathbf{l_1})...\tilde{\Theta}{(\mathbf{l_4})}\rangle&=\langle
    \Theta(\mathbf{l_1})
    \Theta(\mathbf{l_2})
    \Theta(\mathbf{l_3})
    \Theta(\mathbf{l_4})\rangle\\
    +(
    \iint\frac{d^2\mathbf{l_1'}d^2\mathbf{l_2'}}{(2\pi)^4}&\langle
    \Theta(\mathbf{l_3})
    \Theta(\mathbf{l_4})
    \Theta(\mathbf{l_1'})
    \Theta(\mathbf{l_2'})
    \phi(\mathbf{l_1}-\mathbf{l_1'})
    \phi(\mathbf{l_2}-\mathbf{l_2'})\rangle
    [(\mathbf{l_1}-\mathbf{l_1'})\cdot \mathbf{l_1'}]
    [(\mathbf{l_2}-\mathbf{l_2'})\cdot \mathbf{l_2'}]
    \end{align}
    \end{subequations}
    \end{adjustwidth}

    \begin{adjustwidth}{-2em}{}
    \begin{equation}
    \begin{multlined}
    \langle\tilde{\Theta}(\mathbf{l_1}) \dots \tilde{\Theta}{(\mathbf{l_4})}\rangle
    = \langle
        \Theta(\mathbf{l_1})\Theta(\mathbf{l_2})\Theta(\mathbf{l_3})\Theta(\mathbf{l_4})
      \rangle\\
    +(
    \iint\frac{d^2\mathbf{l_1'}d^2\mathbf{l_2'}}{(2\pi)^4}
    \langle
        \Theta(\mathbf{l_3})\Theta(\mathbf{l_4})\Theta(\mathbf{l_1'})\Theta(\mathbf{l_2'})
    \phi(\mathbf{l_1}-\mathbf{l_1'})\phi(\mathbf{l_2}-\mathbf{l_2'})
    \rangle
    [(\mathbf{l_1}-\mathbf{l_1'})\cdot \mathbf{l_1'}]
    [(\mathbf{l_2}-\mathbf{l_2'})\cdot \mathbf{l_2'}]
    \end{multlined}
    \end{equation}
    \end{adjustwidth}
    \end{document}

Note, so far I didn't see that part of one equations is numbered. See, if the use `multlined` is better suit to what you like to have.

\documentclass[11pt, oneside]{article}
\usepackage[showframe]{geometry}
\geometry{letterpaper}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[english]{babel}
\usepackage{mathtools}

\usepackage[strict]{changepage}

\begin{document}

\begin{adjustwidth}{-1em}{-1em}
\begin{subequations}
\begin{align}
\langle\tilde{\Theta}(\mathbf{l_1})...\tilde{\Theta}{(\mathbf{l_4})}\rangle&=\langle
\Theta(\mathbf{l_1})
\Theta(\mathbf{l_2})
\Theta(\mathbf{l_3})
\Theta(\mathbf{l_4})\rangle\\
+(
\iint\frac{d^2\mathbf{l_1'}d^2\mathbf{l_2'}}{(2\pi)^4}&\langle
\Theta(\mathbf{l_3})
\Theta(\mathbf{l_4})
\Theta(\mathbf{l_1'})
\Theta(\mathbf{l_2'})
\phi(\mathbf{l_1}-\mathbf{l_1'})
\phi(\mathbf{l_2}-\mathbf{l_2'})\rangle
[(\mathbf{l_1}-\mathbf{l_1'})\cdot \mathbf{l_1'}]
[(\mathbf{l_2}-\mathbf{l_2'})\cdot \mathbf{l_2'}]
\end{align}
\end{subequations}
\end{adjustwidth}

\begin{adjustwidth}{-1em}{-1em}
\begin{equation}
\begin{multlined}
\langle
    \tilde{\Theta}(\mathbf{l_1}) \dots \tilde{\Theta}(\mathbf{l_4})
\rangle
= \langle
    \Theta(\mathbf{l_1})\Theta(\mathbf{l_2})\Theta(\mathbf{l_3})\Theta(\mathbf{l_4})
  \rangle\\
+(
\iint\frac{d^2\mathbf{l_1'}d^2\mathbf{l_2'}}{(2\pi)^4}
\langle
    \Theta(\mathbf{l_3})\Theta(\mathbf{l_4})\Theta(\mathbf{l_1'})\Theta(\mathbf{l_2'})
\phi(\mathbf{l_1}-\mathbf{l_1'})\phi(\mathbf{l_2}-\mathbf{l_2'})
\rangle
[(\mathbf{l_1}-\mathbf{l_1'})\cdot \mathbf{l_1'}]
[(\mathbf{l_2}-\mathbf{l_2'})\cdot \mathbf{l_2'}]
\end{multlined}
\end{equation}
\end{adjustwidth}
\end{document}

Another possibilities is to use smaller font for this equation.

Answer 2

Try with \begin{flalign}...\end{flalign} which comes with the package amsmath.sty