Aligning terms of two line equation with exponentials

Question

I have an equation split into two lines and I'd like to align the terms so that the exponentials and + signs are on top of each other. I have tried as usual with align and &, but I cannot get the desired result.

\documentclass{article}
\usepackage{amsmath}
\pagestyle{empty}
\begin{document}
\begin{align*}
\varepsilon(\vec{k})=
\varepsilon_{0} -\alpha - \gamma 
&(
%{\phantom{=}}
e^{-i \vec{k} \cdot \vec{\rho}_{1}} 
&+ e^{-i \vec{k} \cdot \vec{\rho}_{2}}
&+ e^{-i \vec{k} \cdot \vec{\rho}_{3}}
&+ e^{-i \vec{k} \cdot \vec{\rho}_{4}}
&+ e^{-i \vec{k} \cdot \vec{\rho}_{5}}
&+ e^{-i \vec{k} \cdot \vec{\rho}_{6}}\\
&+ e^{i \vec{k} \cdot \vec{\rho}_{1}}
&+ e^{i \vec{k} \cdot \vec{\rho}_{2}}
&+ e^{i \vec{k} \cdot \vec{\rho}_{3}}
&+ e^{i \vec{k} \cdot \vec{\rho}_{4}}
&+ e^{i \vec{k} \cdot \vec{\rho}_{5}}
&+ e^{i \vec{k} \cdot \vec{\rho}_{6}}
) 
\end{align*}
\end{document}

This is what I get. Not all the + signs are aligned in the second line, any clue for a solution?

Answer 1

I'd use alignedat:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{equation*}
\varepsilon(\vec{k})=
\varepsilon_{0} -\alpha - \gamma 
\begin{alignedat}[t]{6}
(&e^{-i \vec{k} \cdot \vec{\rho}_{1}} 
&&+ e^{-i \vec{k} \cdot \vec{\rho}_{2}}
&&+ e^{-i \vec{k} \cdot \vec{\rho}_{3}}
&&+ e^{-i \vec{k} \cdot \vec{\rho}_{4}}
&&+ e^{-i \vec{k} \cdot \vec{\rho}_{5}}
&&+ e^{-i \vec{k} \cdot \vec{\rho}_{6}}\\
\makebox[0pt][r]{${}+{}$}
&e^{i \vec{k} \cdot \vec{\rho}_{1}}
&&+ e^{i \vec{k} \cdot \vec{\rho}_{2}}
&&+ e^{i \vec{k} \cdot \vec{\rho}_{3}}
&&+ e^{i \vec{k} \cdot \vec{\rho}_{4}}
&&+ e^{i \vec{k} \cdot \vec{\rho}_{5}}
&&+ e^{i \vec{k} \cdot \vec{\rho}_{6}}
&)\end{alignedat} 
\end{equation*}

\end{document}

Answer 2

In my opinion, using multlined and bringing the second line slightly closer would look as nice:

\documentclass{article}
\usepackage{mathtools}

\begin{document}

\begin{equation*}
\varepsilon(\vec{k})=
\varepsilon_{0} -\alpha - \gamma
\begin{multlined}[t]
\Bigl(e^{-i \vec{k} \cdot \vec{\rho}_{1}}
+ e^{-i \vec{k} \cdot \vec{\rho}_{2}}
+ e^{-i \vec{k} \cdot \vec{\rho}_{3}}
+ e^{-i \vec{k} \cdot \vec{\rho}_{4}}
+ e^{-i \vec{k} \cdot \vec{\rho}_{5}}
+ e^{-i \vec{k} \cdot \vec{\rho}_{6}}\\[-1ex]
{}+ e^{i \vec{k} \cdot \vec{\rho}_{1}}
+ e^{i \vec{k} \cdot \vec{\rho}_{2}}
+ e^{i \vec{k} \cdot \vec{\rho}_{3}}
+ e^{i \vec{k} \cdot \vec{\rho}_{4}}
+ e^{i \vec{k} \cdot \vec{\rho}_{5}}
+ e^{i \vec{k} \cdot \vec{\rho}_{6}}
\Bigr)\end{multlined}
\end{equation*}

\end{document}