[Tex/LaTex] Underbrace outside parentheses

alignatequationsmath-modeunderbrace

How do I move the underbrace out of the big parentheses while still keeping it underneath the term?

Image

\begin{alignat*}{4}
& \frac{E_{0-0}}{hc} &&= C+\frac{E'(\upsilon'=0)}{hc} &\\
&                    &&= C+\frac{1}{hc}\left(\hbar\omega'_e\left(0+\frac{1}{2}\right)-\underbrace{\hbar\chi'\omega'_e\left(0+\frac{1}{2}\right)^2}_{\approx0}\right) &  \quad\Big|\quad\parbox{9em}{\small{Vernachlässigung\\der Anharmonizität}}\\
&                    &&\approx C+\frac{\omega'_e}{4\pi c}
\end{alignat*}

Best Answer

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I replaced alignat* with just align* as others also said and \Bigl,\Bigr are also better. The \frac{1}{2} looks unnecessarily big, so I used \tfrac. Finally, to lower the underbrace you can add a virtual rule like \vphantom{\rule[-1em]{0pt}{0pt}} noting that a \smash is required to keep line spacing consistent.

\documentclass{article}
\usepackage{amsmath}
\begin{document}

\begin{align*}
 \frac{E_{0-0}}{hc} &= C+\frac{E'(\upsilon'=0)}{hc} &\\
                    &= C+\frac{1}{hc}\Bigl(\hbar\omega'_e\left(0+\tfrac{1}{2}\right)-\smash{\underbrace{\vphantom{\rule[-1em]{0pt}{0pt}}\hbar\chi'\omega'_e\left(0+\tfrac{1}{2}\right)^2}_{\approx0}}\Bigr) &  \quad\Big|\quad\parbox{9em}{\small{Vernachlässigung\\der Anharmonizität}}\\
                    &\approx C+\frac{\omega'_e}{4\pi c}
\end{align*}

\end{document}
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