Display math gets typeset as inline math

Question

When typesetting the below annotated equation my summations are being typeset as inline maths rather than display maths.

• If I wrap the sum and subscript with \smashoperator{} then they are typeset correctly as display math again.
• I've tried multiple outermost wrappers for the array of arrays,
  • \begin{equation},
  • \begin{align} starred and unstarred,
  • \[ ... \],
  • \begin{gather}.

I'm aware of the existance of the \displaystyle macro, I don't want to set it globally if at all posible, I have inline maths elsewhere in my document. In all honesty I'd like to find out what I'm doing wrong and fix that rather than stick a macro in there to fix my poor tex-ing).

I compile my document with LuaLaTeX, MWE follows.

\documentclass[a4paper]{book}
\usepackage{mathtools}
\usepackage{lipsum}
\begin{document}
\lipsum[1]
   \begin{equation}
   \begin{array}{ll} % array to ensure the force field and the braces are aligned, as the non-bonded terms are a little wider than the bonded.
      \begin{array}{rcl} % bonded terms.
         U  &=& \sum_{\text{bonds},\, i} K_{b,i} \left( b_i - b_{0,i} \right)^2 \\
            &+& \sum_{\text{angles},\, i} K_{\theta,i} \left( \theta_i - \theta_{0,i} \right)^2 \\
            &+& \sum_{\text{dihedrals},\, i}%
               K_{\phi,i} \left( 1 - \cos\left( n\phi_i - \phi_{0,i} \right) \right) \\
            &+& \sum_{%
               \substack{\text{improper},\, i\\ \text{dihedrals}}%
               } K_{\omega,i} \left( \omega_i - \omega_{0,i} \right)^2 \\
   \end{array}%
   & \left. \vphantom{% Phantom content to make brace correct size. \left. \right\} Havent worked when across two columns in array...
               \begin{array}{rcl}
                  U  &=& \sum_{\text{bonds},\, i} K_{b,i} \left( b_i - b_{0,i} \right)^2 \\
                     &+& \sum_{\text{angles},\, i} K_{\theta,i} \left( \theta_i - \theta_{0,i} \right)^2 \\
                     &+& \sum_{\text{dihedrals},\, i}%
                        K_{\phi,i} \left( 1 - \cos\left( n\phi_i - \phi_{0,i} \right) \right) \\
                     &+& \sum_{%
                        \substack{\text{improper},\, i\\ \text{dihedrals}}%
                        } K_{\omega,i} \left( \omega_i - \omega_{0,i} \right)^2 \\
               \end{array}
               }%
   \right\} \text{bonded} \\
   \begin{array}{rcl} % non-bonded terms.
      \hphantom{U}   &+& \sum_{\text{atoms},\, i,j} \epsilon_{ij}%
                        \left[%
                           \left(%
                              \frac{ r^{min}_{ij} } { r_{ij} }%
                           \right)^{12}%
                           -2 \left(%
                              \frac{ r^{min}_{ij} }{ r_{ij} }%
                           \right)^6%
                        \right] \\
                     &+& \sum_{\text{atoms},\, i,j}%
                        \frac{1}{4\pi\epsilon_0\epsilon_r}\frac{q_i q_j}{ r_{ij} }
   \end{array}%
   & \left. \vphantom{% Phantom content to make brace correct size. \left. \right\} Havent worked when across two columns in array...
               \begin{array}{rcl} % non-bonded terms.
                  \phantom{U} &+& \sum_{\text{atoms},\, i,j} \epsilon_{ij}%
                                 \left[%
                                    \left(%
                                       \frac{ r^{min}_{ij} } { r_{ij} }%
                                    \right)^{12}%
                                    -2 \left(%
                                       \frac{ r^{min}_{ij} }{ r_{ij} }%
                                    \right)^6%
                                 \right] \\
                              &+& \sum_{\text{atoms},\, i,j}%
                                 \frac{1}{4\pi\epsilon_0\epsilon_r}\frac{q_i q_j}{ r_{ij} }
               \end{array}
               }%
   \right\} \text{non-bonded}
   \end{array}
   \end{equation}

\lipsum[2-3]
\end{document}

Answer 1

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):

Answer 2

Here's a general option which addresses your concern. That is, a localized change from inline to display style mathematical typesetting. The easier option for the case you're using array is adjusting the argument thereof (as mentioned in the comments).

The solution opens a group, and redefines the macros you wish to be \displaystyle. After the group closes the redefinitions are also reset.

In your case, you may need to readjust the arraystretch to make sure no symbols overlap between lines. (\renewcommand\arraystretch{<factor>} somewhere between \begingroup and \endgroup).

Here's a screenshot from the output. Notice how the inline math below the array has the "standard inline" math style.

\documentclass[a4paper]{book}
\usepackage{mathtools}
\usepackage{lipsum}
\begin{document}
\lipsum[1]
\begingroup
\let\oldsum\sum
\let\oldfrac\frac
\def\sum{\displaystyle\oldsum}
\def\frac{\displaystyle\oldfrac}
   \begin{equation}
   \begin{array}{ll} % array to ensure the force field and the braces are aligned, as the non-bonded terms are a little wider than the bonded.
      \begin{array}{rcl} % bonded terms.
         U  &=& \sum_{\text{bonds},\, i} K_{b,i} \left( b_i - b_{0,i} \right)^2 \\
            &+& \sum_{\text{angles},\, i} K_{\theta,i} \left( \theta_i - \theta_{0,i} \right)^2 \\
            &+& \sum_{\text{dihedrals},\, i}%
               K_{\phi,i} \left( 1 - \cos\left( n\phi_i - \phi_{0,i} \right) \right) \\
            &+& \sum_{%
               \substack{\text{improper},\, i\\ \text{dihedrals}}%
               } K_{\omega,i} \left( \omega_i - \omega_{0,i} \right)^2 \\
   \end{array}%
   & \left. \vphantom{% Phantom content to make brace correct size. \left. \right\} Havent worked when across two columns in array...
               \begin{array}{rcl}
                  U  &=& \sum_{\text{bonds},\, i} K_{b,i} \left( b_i - b_{0,i} \right)^2 \\
                     &+& \sum_{\text{angles},\, i} K_{\theta,i} \left( \theta_i - \theta_{0,i} \right)^2 \\
                     &+& \sum_{\text{dihedrals},\, i}%
                        K_{\phi,i} \left( 1 - \cos\left( n\phi_i - \phi_{0,i} \right) \right) \\
                     &+& \sum_{%
                        \substack{\text{improper},\, i\\ \text{dihedrals}}%
                        } K_{\omega,i} \left( \omega_i - \omega_{0,i} \right)^2 \\
               \end{array}
               }%
   \right\} \text{bonded} \\
   \begin{array}{rcl} % non-bonded terms.
      \hphantom{U}   &+& \sum_{\text{atoms},\, i,j} \epsilon_{ij}%
                        \left[%
                           \left(%
                              \frac{ r^{min}_{ij} } { r_{ij} }%
                           \right)^{12}%
                           -2 \left(%
                              \frac{ r^{min}_{ij} }{ r_{ij} }%
                           \right)^6%
                        \right] \\
                     &+& \sum_{\text{atoms},\, i,j}%
                        \frac{1}{4\pi\epsilon_0\epsilon_r}\frac{q_i q_j}{ r_{ij} }
   \end{array}%
   & \left. \vphantom{% Phantom content to make brace correct size. \left. \right\} Havent worked when across two columns in array...
               \begin{array}{rcl} % non-bonded terms.
                  \phantom{U} &+& \sum_{\text{atoms},\, i,j} \epsilon_{ij}%
                                 \left[%
                                    \left(%
                                       \frac{ r^{min}_{ij} } { r_{ij} }%
                                    \right)^{12}%
                                    -2 \left(%
                                       \frac{ r^{min}_{ij} }{ r_{ij} }%
                                    \right)^6%
                                 \right] \\
                              &+& \sum_{\text{atoms},\, i,j}%
                                 \frac{1}{4\pi\epsilon_0\epsilon_r}\frac{q_i q_j}{ r_{ij} }
               \end{array}
               }%
   \right\} \text{non-bonded}
   \end{array}
   \end{equation}
\endgroup
$\sum_{\text{bonds},\, i} K_{b,i} \left( b_i - b_{0,i} \right)^2$
\lipsum[2-3]
\end{document}

Answer 3

Here's a solution that maintains your basic array setup, while simplifying it somewhat. Inside the equation, it first defines two scratch macros, \blocka and \blockb, and then uses each of them twice: first to typeset the material itself, then as the argument of the \vphantom instructions. Each "block" macro consists of an array environment that contains two columns. Extra care is taken to align the + symbols and the summation symbols and to provide a bit of vertical whitespace between the two big blocks.

\documentclass[a4paper]{book}
\usepackage{mathtools,array}
\usepackage{booktabs} % for "\addlinespace" macro
\begin{document}
\hrule % just to illustrate width of text block
\begin{equation}
\setlength\extrarowheight{1ex}
    %% Set up two scratch macros, \blocka and \blockb, for the bonded 
    %% and non-bonded terms, respectively
    \newcommand\blocka{% bonded terms
    \begin{array}{r @{} >{\displaystyle}l} % 2nd column in displaystyle
         U  ={}& \phantom{+\quad}\smashoperator[l]{\sum_{\text{bonds},\, i}} K_{b,i} ( b_i - b_{0,i} )^2 \\
               &+\quad \smashoperator[l]{\sum_{\text{angles},\, i}}
                  K_{\theta,i} (\theta_i -\theta_{0,i})^2 \\
               &+\quad \smashoperator[l]{\sum_{\text{dihedrals},\, i}}
                  K_{\phi,i} ( 1 - \cos( n\phi_i - \phi_{0,i} ) ) \\
               &+\quad \smashoperator[l]{\sum_{%
                  \substack{\text{improper},\, i\\ \text{dihedrals}}}} 
                  K_{\omega,i} ( \omega_i - \omega_{0,i} )^2 \\
    \end{array}}
    \newcommand\blockb{% non-bonded terms
    \begin{array}{r @{} >{\displaystyle}l} % 2nd column in displaystyle
        \phantom{U  ={}}
            &+\quad \smashoperator[l]{\sum_{\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] \\ \addlinespace
            &+\quad \smashoperator[l]{\sum_{\text{atoms},\, i,j}}
                        \frac{1}{4\pi\epsilon_0\epsilon_r}
                        \frac{q_i q_j}{ r_{ij} }
   \end{array}}
   %% Now typeset the material in the two big blocks
   \begin{array}{ll} 
       \blocka & \left. \vphantom{\blocka} \right\} \text{bonded} \\ 
       \addlinespace
       \blockb & \left. \vphantom{\blockb} \right\} \text{non-bonded}
   \end{array}
   \end{equation}

\hrule  % just to illustrate width of text block
\end{document}