How to move Equation Number?

Question

I'm trying to write a paper and I can't get some of the equation numbers to fit into a nice place. Here's a minimal working example:

\documentclass[%
reprint,
 amsmath,amssymb,
 aps,
]{revtex4-1}

\begin{document}
\begin{multline}
    \omega=\frac{Y\left(x_1+x_2+2x_1x_2\right)+Z\left(x_2-x_1\right)}{2Z}\\
    +\frac{Q}{L}\sum_{f=8}^R\frac{ uZ\left(1+x_1+x_3\right)\left(1+o 
    U_f^2\right)-2V\left(x_1-x_2\right)A_f}{B_f^2-Q^2}\\
\end{multline}
\end{document}

What happens is that the number appears below the second line to the right. I'd like it to be on the upper right line if possible... or at least in the middle somewhere. It's adding like a centimeter of blank space to my document.

I've tried the multlined environment as well and I can't get it to , but it's just as possible that it's operator error

Answer 1

Here are two possibilities, with mathtools: one with multlined nested in an equation, and an aligned environment in the numerator of the 2nd fraction, and another with the \splitfraccommand. I removed all the unnecessary \left( … \right) pairs:

\documentclass[reprint, amsmath,xamssymb, aps]{revtex4-2}
\usepackage{mathtools}

\begin{document}

\begin{equation}
\begin{multlined}[t]%
    \omega=\frac{Y(x_1+x_2+2x_1x_2)+Z(x_2-x_1)}{2Z}\\
    +\frac{Q}{L}\sum_{f=8}^R\frac{\begin{aligned}uZ(1+x_1+x_3)(1+o
    U_f^2)\\[-0.5ex]-2V(x_1-x_2)A_f\end{aligned}}{B_f^2-Q^2}
\end{multlined}
\end{equation}

\begin{multline}\raisetag{3ex}
    \omega=\frac{Y(x_1+x_2+2x_1x_2)+Z(x_2-x_1)}{2Z}\\
    +\frac{Q}{L}\sum_{f=8}^R\frac{\splitfrac{uZ(1+x_1+x_3)\bigl(1+o
    U_f^2\bigr)}{-2V(x_1-x_2)A_f}}{B_f^2-Q^2}
\end{multline}

\end{document}