对齐环境中的拆分环境的方程标签未居中

对齐环境中的拆分环境的方程标签未居中
\documentclass{article}
\usepackage{amsmath,amssymb,physics}

\begin{document} 

\begin{align}
\mel{Q,t}{\rho_{S}(t)}{Q',t}&=
Tr_{B}\mel{Q,X,t}{\rho_{SB}(t)}{Q',X',t}\\ 
\begin{split} 
&=\int \underbrace{dQ_0\;dQ_0'}_{\text{initial system state differentials}}\;\underbrace{dX_0\;dX_0'}_{\text{initial bath state diferentials}}\;\underbrace{dX}_{\text{final bath state differential}}\times \\ 
& \underbrace{\mel{Q,X,t}{T \qty[e^{-\frac{i}{\hbar}\int_{0}^t H(\tau) d\tau}]}{Q_0,X_0,0}}_{\text{ right propagator}}\underbrace{\mel{Q_0',X_0',0}{T\qty[ e^{\frac{i}{\hbar}\int_{0}^t H(\tau) d\tau}]}{Q',X,t}}_{\text{left propagator}} \times\\ &\underbrace{\mel{Q_0,X_0,0}{\rho_{SB}(0)}{Q_0',X_0',0}}_{\text{initial state density matrix element}}
\end{split}
\end{align}

\end{document}

快照

如您所见,分割环境的标签未居中。如何修复此问题?

答案1

您的三行方程式不可能适合边距,除非您加载geometry以获得更合理的边距(如果您不使用边距注释),所以我建议将其写在四行上。此外,我会使用fleqn来自的环境稍微改变布局nccmath,并且我定义了一个命令,它与具有更好的水平间距和可变大小分隔符的命令\mymel相同,来自包:\mel\DeclarePairedDelimiterXmathtools

\documentclass{article}
\usepackage{showframe}
\renewcommand\ShowFrameLinethickness{0.3pt}
\usepackage{mathtools, nccmath, amssymb, physics}%
\DeclarePairedDelimiterX\mymel[3]{\langle}{\rangle}%
{#1\,\delimsize\vert\,\mathopen{}#2\,\delimsize\vert\,\mathopen{}#3}
\usepackage[usestackEOL]{stackengine}

\begin{document}
\begin{fleqn}
\begin{align}
\mymel{Q,t}{\rho_{S}(t)}{Q',t} & =
\Tr_{B}\mymel{Q,X,t}{\rho_{SB}(t)}{Q',X',t}\\
\begin{split}
&=\int \hspace{-1em}\underbrace{dQ_0\;dQ_0'}_{\scriptsize\Shortstack{initial system state\\ differentials}}\;\underbrace{dX_0\;dX_0'}_{\scriptsize\Shortstack{initial bath state\\ differentials}}\;\underbrace{dX_{\vphantom{0}}^{\vphantom{'}}}_{\scriptsize\Shortstack{final bath state\\ differential}}\hspace{-1.75em}\times \\
&\hphantom{{}\times{}} \underbrace{\mymel*{Q,X,t}{T \qty[e^{-\frac{i}{\hbar}\int_{0}^t H(\tau) d\tau}]}{Q_0,X_0,0}}_{\text{ right propagator}} \times \\
 &\hphantom{{}\times{}} \underbrace{\mymel*{Q_0',X_0',0}{T\qty[ e^{\frac{i}{\hbar}\int_{0}^t H(\tau) d\tau}]}{Q',X,t}}_{\text{left propagator}} \times\\
  &\hphantom{{}\times{}} \underbrace{\mymel{Q_0,X_0,0}{\rho_{SB}(0)}{Q_0',X_0',0}}_{\text{initial state density matrix element}}
\end{split}
\end{align}
\end{fleqn}

\end{document}

在此处输入图片描述

答案2

不完全了解您的要求,这里我提供了一个建议性的解决方案。

\documentclass{article}
\usepackage{amsmath, amssymb}
\usepackage{physics}

\begin{document} 

\begin{align}
\mel{Q,t}{\rho_{S}(t)}{Q',t}
    &= Tr_{B}\mel{Q,X,t}{\rho_{SB}(t)}{Q',X',t} \\ 
    &= \int \underbrace{dQ_0\;dQ_0'}_{\text{initial system state differentials}}\;
            \underbrace{dX_0\;dX_0'}_{\text{initial bath state diferentials}}\;
            \underbrace{dX}_{\text{final bath state differential}}\times \notag\\ 
    &\mathrel{\phantom{=}} 
     \underbrace{\mel{Q,X,t}{T \qty[e^{-\frac{i}{\hbar}\int_{0}^t H(\tau) d\tau}]}{Q_0,X_0,0}}_{\text{ right propagator}}
      \underbrace{\mel{Q_0',X_0',0}{T\qty[ e^{\frac{i}{\hbar}\int_{0}^t H(\tau) d\tau}]}{Q',X,t}}_{\text{left propagator}} \times \notag\\
    &\mathrel{\phantom{=}} 
     \underbrace{\mel{Q_0,X_0,0}{\rho_{SB}(0)}{Q_0',X_0',0}}_{\text{initial state density matrix element}}
\end{align}

\end{document}

输出: 解决方案的输出,其中的线以等号对齐

解释:

  1. \begin{align} ... &= ... \\ ... &= ... \end{align}将线对齐在等号处
  2. 对于跨多行的长表达式,为了实现右对齐,在\mathrel{\phantom{=}}没有真正等号的行前添加一个数学类型为“关系”( ) 的幻像等号(占用等号的宽度,但不排版)。请注意,默认情况下,\phantom{...}数学类型为“普通”,其开头和后面的自动添加间距与数学类型“关系”(“普通”等号所属的类型)不同。
  3. 对于跨越数行的长表达式,为了获得正确的标记(逻辑上的一行表达式通常只有一个标记,无论它跨越多少行纸),\notag添加了一些命令。

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