\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}
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
\DeclarePairedDelimiterX
mathtools
\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}
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}
解释:
\begin{align} ... &= ... \\ ... &= ... \end{align}
将线对齐在等号处- 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。 - 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。