Stretching of text beyond underbrace

Question

The current behavior when using \underbrace is the following (Tex inserts whitespace to avoid streching the text underneath the underbrace beneith the neighboring symbols):

The behavior I want instead is the following:

How can this be achieved?

EDIT: The code I used for this image is as follows

Y_{2,2}(\theta,\varphi) - Y_{2,-2}(\theta,\varphi) = \dfrac{2i}{\sqrt{4\pi}}\sqrt{\dfrac{15}{8}} \underbrace{\sin^2\theta\sin(2\varphi)}_{=\frac{1}{2}\sin^2\theta\sin\varphi\cos\varphi={\frac{1}{2}d_{xy}/C_{xy}}}

Answer 1

Is this what you want? Using \mathclap from the mathtools package:

\documentclass{article}
\usepackage{amsmath}
\usepackage{mathtools}

\begin{document}

\[
  Y_{2, 2}(\theta, \varphi) - Y_{2, -2}(\theta, \varphi) =
  \frac{2i}{\sqrt{4\pi}} \sqrt{\dfrac{15}{8}}
  \underbrace{\sin^{2}\theta \sin(2\varphi)}_{\mathclap{= \frac{1}{2}
    \sin^{2}\theta \sin\varphi \cos\varphi = \frac{1}{2}d_{xy}/C_{xy}}}
\]

\end{document}

Answer 2

Use \mathclapfrom mathtools (needless to load amsmath in this case), and possibly \substack:

\documentclass{article}
\usepackage{mathtools}

\begin{document}

\[ \Upsilon_{2,2}(\theta,\varphi) - \Upsilon_{2,-2}(\theta,\varphi) = \dfrac{2i}{\sqrt{4\pi}}\sqrt{\dfrac{15}{8}} \underbrace{\sin^2\theta\sin(2\varphi)}_{\mathclap{=\frac{1}{2}\sin^2\theta\sin\varphi\cos\varphi = {\frac{1}{2}d_{xy}/C_{xy}}}} \]

\vspace{4ex}
\[ \Upsilon_{2,2}(\theta,\varphi) - \Upsilon_{2,-2}(\theta,\varphi) = \dfrac{2i}{\sqrt{4\pi}}\sqrt{\dfrac{15}{8}} \underbrace{\sin^2\theta\sin(2\varphi)}_{\substack{\mathclap{=\frac{1}{2}\sin^2\theta\sin\varphi \cos\varphi}\\=\frac{1}{2}d_{xy}/C_{xy}}} \]

\end{document}