I'd like to finetune the big rectangle in the last column and its content, i.e. that it's all aligned to the third column's center. It is currently too far on the right:

So far I have been unable to find the missing option.
MWE
\documentclass[
11pt
]{scrartcl}
\usepackage{
tikz,
relsize,
tgheros
}
\usetikzlibrary{
calc,trees,shadows,positioning,arrows,chains,shapes.geometric,
decorations.pathreplacing,decorations.pathmorphing,shapes,
matrix,shapes.symbols,patterns,intersections%,fit
}
\pgfdeclarelayer{background layer}
\pgfdeclarelayer{foreground layer}
\pgfsetlayers{background layer,main,foreground layer}
\tikzset{
rolle/.style=
{
rectangle,
drop shadow={opacity=0.5},
minimum width=4em,
minimum height=4em,
text width=4cm,
draw=black!80,
very thick,
fill=black!40,
font=\sffamily\bfseries,
align=center
},
einheit/.style=
{
rectangle,
rounded corners,
minimum height=2em,
minimum width=4em,
draw=black!80,
fill=black!20,
font=\sffamily
},
}
\begin{document}
\begin{center}
\begin{tikzpicture}[font=\sffamily\small,node distance=0.5cm]
\draw[thick] (-7.5,-4) rectangle (7.5,4);
\draw[thick,fill=black!20] (-7.5,4) rectangle (7.5,5) node[midway] {something};
\matrix (m) [matrix of nodes,nodes in empty cells,row sep=5mm,column sep=5mm] {
%
\node[rolle] {123}; & \node[rolle] {456}; & \node[rolle] {789}; \\
%
\node[einheit] {words}; & & \node[minimum width=5cm] (abc) {}; \\
%
& \node[einheit] {words}; & \\
%
& \node[einheit] {words}; & \\
%
& \node[einheit] {companies}; & \\
%
\node[einheit] {thing}; & & \\
};
\draw[einheit] (abc.north east) rectangle (m-6-3.south west) node[midway]{abc};
\end{tikzpicture}
\end{center}
\end{document}




minimum widthinstead oftext widthfor therollenodes you can get rid of theUnderful \hboxwarnings. Related: Alignment problem of empty cells when doing multirow/multicolumn cells in TikZ matrices – Qrrbrbirlbel Jul 20 '13 at 15:51