Drawing bonds between atoms with pgfplots

Question

I'm trying to draw a molecule using pgfplots. I have successfully drawn the atoms with the code below:

\documentclass{standalone}

\usepackage{pgfplots}
\pgfplotsset{compat=1.9}
\usepackage{tikz}

\begin{document}

\begin{tikzpicture}
\begin{axis}[
view = {0}{0},
axis lines=none,
axis equal
]
\addplot3[scatter,red,only marks, point meta=explicit symbolic,
          z buffer=sort, scatter/classes={H={mark size=3pt,blue},
          O={mark size=5pt,red},C={mark size=5pt,yellow}}]
  coordinates {
(0.483,0.593,0.525) [H]
(0.425,0.501,0.590) [H]
(0.510,0.580,0.640) [H]
(0.573,0.518,0.626) [H]
(0.517,0.463,0.322) [H]
(0.363,0.503,0.418) [H]
(0.538,0.422,0.558) [H]
(0.528,0.503,0.731) [H]
(0.559,0.505,0.428) [C]
(0.491,0.497,0.406) [C]
(0.449,0.518,0.455) [C]
(0.490,0.539,0.514) [C]
(0.478,0.497,0.578) [C]
(0.519,0.525,0.636) [C]
(0.610,0.491,0.397) [O]
(0.475,0.473,0.344) [O]
(0.382,0.521,0.459) [O]
(0.559,0.531,0.492) [O]
(0.490,0.428,0.566) [O]
(0.498,0.492,0.695) [O]
};
\end{axis}
\end{tikzpicture}

\end{document}

which looks like this:

Now I would like to add the bonds between atoms, as sticks. I need a method to be able to connect two of the scattered data points if they're closer than a certain distance. I'm interested in pgfplots automatically joining atoms whenever the interatomic distance falls below a certain cutoff. Of course I could write a python script to determine which atoms are bonded and generate all the \draw instances for me but I would prefer to be able to skip pasting a lot of extra code into the document.

Ideally I would also like to be able to change the color of the bond depending on which atom types are connected.

Is it possible to do this, at least the bond drawing?

Answer 1

I would definitely recommend using an external script. In any case, even as a mental exercise, here's something that you may find helpful. I expect it to be very slow when you give it a large number of atoms.

\documentclass{standalone}

\usepackage{pgfplots}
\pgfplotsset{compat=1.9}
\usepackage{tikz}

\usepackage{pdftexcmds} % for testing atom types
\usepackage{boolexpr}
\makeatletter\long\def\isequal#1#2{\pdf@strcmp{#1}{#2}}\makeatother

\begin{document}

\begin{tikzpicture}
\begin{axis}[
view = {0}{0},
axis lines=none,
axis equal
]
\def\COORDINATES{0.483/0.593/0.525/H,
                 0.425/0.501/0.590/H,
                 0.510/0.580/0.640/H,
                 0.573/0.518/0.626/H,
                 0.517/0.463/0.322/H,
                 0.363/0.503/0.418/H,
                 0.538/0.422/0.558/H,
                 0.528/0.503/0.731/H,
                 0.559/0.505/0.428/C,
                 0.491/0.497/0.406/C,
                 0.449/0.518/0.455/C,
                 0.490/0.539/0.514/C,
                 0.478/0.497/0.578/C,
                 0.519/0.525/0.636/C,
                 0.610/0.491/0.397/O,
                 0.475/0.473/0.344/O,
                 0.382/0.521/0.459/O,
                 0.559/0.531/0.492/O,
                 0.490/0.428/0.566/O,
                 0.498/0.492/0.695/O}
\newcounter{u}
\newcounter{v}
\foreach \ux/\uy/\uz/\ut in \COORDINATES {
  \stepcounter{u}
  \begingroup\edef\temp{\endgroup\noexpand%
      \addplot3[scatter,red,only marks, point meta=explicit symbolic,
            z buffer=sort, scatter/classes={H={mark size=3pt,blue},
            O={mark size=5pt,red},C={mark size=5pt,yellow}}]
      coordinates { (\ux,\uy,\uz) [\ut] };
  }\temp
  \setcounter{v}{0}
  \foreach \vx/\vy/\vz/\vt in \COORDINATES {
    \stepcounter{v}
    \ifnum\value{u}<\value{v}
    \pgfmathparse{less(sqrt((\ux-\vx)^2+(\uy-\vy)^2+(\uz-\vz)^2), 0.1)}
    \ifnum\pgfmathresult=1
      \edef\bond{\switch
        \case{\isequal{\ut}{O} \OR \isequal{\vt}{O}}  thick,color=red
        \case{\isequal{\ut}{\vt}}                     color=green
        \otherwise                                    color=black
        \endswitch}
      \begingroup\edef\temp{\endgroup\noexpand%
        \draw[\bond] (axis cs:\ux,\uy,\uz) -- (axis cs:\vx,\vy,\vz);
      }\temp
    \fi\fi
  }
}
\end{axis}
\end{tikzpicture}
\end{document}

The result is: