fractionsmath-mode
I was wondering if there is a way to have LaTeX interpret $\frac{a}{b}$ as $a/b$, but \[\frac{a}{b}\] as \[\frac{a}{b}\].
It is a little tedious to change a inline equation $a/b$ to a display equation \[\frac{a}{b}\] when doing changes to my text.
These two ways should work also in ConTeXt:
$\displaystyle\sum_{i=1}^n a_i$
$\sum\limits_{i=1}^n a_i$
Probably the second form is preferable, although I usually advise not to put limits above and below in in-line math formulas.
You can add \displaystyle to the columns you like in your array:
% arara: pdflatex
\documentclass[a4paper]{book}
\usepackage{mathtools}
\usepackage{lipsum}
\usepackage{array}
\begin{document}
\lipsum[1]
\begin{equation}
\begin{aligned}
&\left.\begin{array}{r@{\;}>{{}\displaystyle}l}
U =& \sum_{\mathclap{\text{bonds},\,i}} K_{b,i} ( b_i - b_{0,i})^2 \\
&+ \sum_{\mathclap{\text{angles},\, i}} K_{\theta,i} ( \theta_i - \theta_{0,i} )^2 \\
&+ \sum_{\mathclap{\text{dihedrals},\, i}} K_{\phi,i} \bigl( 1 - \cos( n\phi_i - \phi_{0,i} ) \bigr) \\
&+ \mathrlap{\sum_{\mathclap{\substack{\text{improper},\, i\\ \text{dihedrals}}}} K_{\omega,i} ( \omega_i - \omega_{0,i} )^2 }
\hphantom{\sum_{\mathclap{\text{atoms},\,i,j}}\epsilon_{ij}\Biggl[\biggl(\frac{r^{\min}_{ij}}{r_{ij}}\biggr)^{12}-2\biggl(\frac{r^{\min}_{ij}}{r_{ij}}\biggr)^6\Biggr]}
\end{array}\right\}\text{bonded}\\
&\left.\begin{array}{r@{\;}>{{}\displaystyle}l}
\hphantom{U =}
&+\sum_{\mathclap{\text{atoms},\,i,j}}\epsilon_{ij}\Biggl[\biggl(\frac{r^{\min}_{ij}}{r_{ij}}\biggr)^{12}-2\biggl(\frac{r^{\min}_{ij}}{r_{ij}}\biggr)^6\Biggr] \\
&+\sum_{\mathclap{\text{atoms},\, i,j}}(4\pi\epsilon_0\epsilon_r)^{-1}\frac{q_i q_j}{ r_{ij}}
\end{array}\right\}\text{non-bonded}
\end{aligned}
\end{equation}
\lipsum[2-3]
\end{document}
If you were just concerned about the limits of your sum-symbols, you could add the command \limits to each of them. Safes you some space.
\begin{equation}
\begin{aligned}
&\left.\begin{array}{r@{\;}>{{}}l}
U =&\sum\limits_{\mathclap{\text{bonds},\,i}} K_{b,i} ( b_i - b_{0,i})^2 \\
&+ \sum\limits_{\mathclap{\text{angles},\, i}} K_{\theta,i} ( \theta_i - \theta_{0,i} )^2 \\
&+ \sum\limits_{\mathclap{\text{dihedrals},\, i}} K_{\phi,i}\bigl( 1 - \cos( n\phi_i - \phi_{0,i} ) \bigr) \\
&+ \mathrlap{\sum\limits_{\mathclap{\substack{\text{improper},\, i\\ \text{dihedrals}}}} K_{\omega,i} ( \omega_i - \omega_{0,i} )^2 }
\hphantom{\sum\limits_{\mathclap{\text{atoms},\,i,j}}\epsilon_{ij}\biggl[\Bigl(\frac{r^{\min}_{ij}}{r_{ij}}\Bigr)^{12}-2\Bigl(\frac{r^{\min}_{ij}}{r_{ij}}\Bigr)^6\biggr]}
\end{array}\right\}\text{bonded}\\
&\left.\begin{array}{r@{\;}>{{}}l}
\hphantom{U =}
&+\sum\limits_{\mathclap{\text{atoms},\,i,j}}\epsilon_{ij}\biggl[\Bigl(\frac{r^{\min}_{ij}}{r_{ij}}\Bigr)^{12}-2\Bigl(\frac{r^{\min}_{ij}}{r_{ij}}\Bigr)^6\biggr] \\
&+\sum\limits_{\mathclap{\text{atoms},\, i,j}}(4\pi\epsilon_0\epsilon_r)^{-1}\frac{q_i q_j}{ r_{ij}}
\end{array}\right\}\text{non-bonded}
\end{aligned}
\end{equation}
This would look like
However, personally I would not set such things as an array, as those are meant for matrices and alike. You could just nest two aligned which will result in display style as well.
\begin{equation}
\begin{aligned}
&\left.\begin{aligned}
U ={}& \sum_{\mathclap{\text{bonds},\,i}} K_{b,i} ( b_i - b_{0,i})^2 \\
&+ \sum_{\mathclap{\text{angles},\, i}} K_{\theta,i} ( \theta_i - \theta_{0,i} )^2 \\
&+ \sum_{\mathclap{\text{dihedrals},\, i}} K_{\phi,i} \bigl( 1 - \cos( n\phi_i - \phi_{0,i} ) \bigr) \\
&+ \mathrlap{\sum_{\mathclap{\substack{\text{improper},\, i\\ \text{dihedrals}}}} K_{\omega,i} ( \omega_i - \omega_{0,i} )^2 }
\hphantom{\sum_{\mathclap{\text{atoms},\,i,j}}\epsilon_{ij}\Biggl[\biggl(\frac{r^{\min}_{ij}}{r_{ij}}\biggr)^{12}-2\biggl(\frac{r^{\min}_{ij}}{r_{ij}}\biggr)^6\Biggr]}
\end{aligned}\right\}\text{bonded}\\
&\left.\begin{aligned}
\hphantom{U ={}}
&+\sum_{\mathclap{\text{atoms},\,i,j}}\epsilon_{ij}\Biggl[\biggl(\frac{r^{\min}_{ij}}{r_{ij}}\biggr)^{12}-2\biggl(\frac{r^{\min}_{ij}}{r_{ij}}\biggr)^6\Biggr] \\
&+\sum_{\mathclap{\text{atoms},\, i,j}}(4\pi\epsilon_0\epsilon_r)^{-1}\frac{q_i q_j}{ r_{ij}}
\end{aligned}\right\}\text{non-bonded}
\end{aligned}
\end{equation}
You can see that the vertical spacing is much more pleasant (but you could treat that manually for all cases, of course):
Best Answer
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amsmathnaturally switches\fracto\tfracwhen in "inline math" (or text style) and\dfracunder "display math" (or display style). This provides a better layout:However, you can update
\fracto condition based on the style using\mathchoice:Given that (say)
\frac{x+y}{x-y}will be set asx+y/x-ywhen in text style, and technically have an incorrect mathematical representation, one could venture one step further. That is, measure the numerator and denominator width, and set it inside brackets if necessary: