Okay, here's my first attempt. It needs a little tweaking with spacing either side, and I've only tested it with the 2x2 matrix given and without any font size changes, so some of the choices for lengths might be amiss.
The vertical centring is handled by setting the baseline option on the enclosing tikzpicture. We want the actual centre of the matrix to be a bit above the baseline (about the middle of an =, I guess). Trial and error gave me a height of .7ex. Next is the delimiters. TikZ can handle them itself, so I figured it best to let it deal with them rather than putting stuff in boxes or minipages (the node text is already in boxes and so forth so putting in more seems overkill). I found the delimiters to be vertically a little off, so I shifted them down a point, and also a little far out. I didn't want to touch the inner sep as that changes both vertical and horizontal, so I simply shifted them in a little. These are the every (left|right|) delimiter styles. I wanted a little inner sep on the matrix itself but not on the entries themselves. I also adjusted the row and column separations a little to better match the AMS styles.
Next was the ampersand. I'm a little wary of this, but what I do is to change it to an active character. It checks to see if it is inside a TikZ matrix. If it is, then it produces \pgfmatrixnextcell and if not, it produces the old ampersand. This could be made a little more robust by only checking for if we're in one of our special TikZ matrices. I don't know if this is a good solution or not.
After that, it's simply a matter of wrapping it all up in an environment.
Let's start with the result. In each row, the first is a tikzpicture, the second is the same but wrapped in to an environment, the third is the AMS original.
And now the code:
\documentclass{standalone}
%\url{http://tex.stackexchange.com/q/26866/86}
\usepackage{amsmath,tikz}
\usetikzlibrary{matrix}
\newlength\mtxrowsep \setlength\mtxrowsep{1.5ex}
\newlength\mtxcolsep \setlength\mtxcolsep{2\arraycolsep}
\tikzset{
ams/.style={
baseline=-.7ex,
every delimiter/.style={yshift=-1pt},
every left delimiter/.style={xshift=2pt},
every right delimiter/.style={xshift=-2pt},
every node/.style={inner sep=0pt},
},
ams matrix/.style={
inner sep=1pt,
column sep=\mtxcolsep,
row sep=\mtxrowsep,
% ampersand replacement=\&,
matrix of math nodes,
},
bmatrix/.style={
ams,
every matrix/.style={
ams matrix,
left delimiter={[},
right delimiter={]},
}
},
Bmatrix/.style={
ams,
every matrix/.style={
ams matrix,
left delimiter={\lbrace},
right delimiter={\rbrace},
}
},
pmatrix/.style={
ams,
every matrix/.style={
ams matrix,
left delimiter={(},
right delimiter={)},
}
},
vmatrix/.style={
ams,
every matrix/.style={
ams matrix,
left delimiter={|},
right delimiter={|},
}
},
Vmatrix/.style={
ams,
every matrix/.style={
ams matrix,
left delimiter={\|},
right delimiter={\|},
}
},
}
\let\matamp=&
\catcode`\&=13
\makeatletter
\def&{\iftikz@is@matrix
\pgfmatrixnextcell
\else
\matamp
\fi}
\makeatother
%\usepackage{environ}
\def\endtikzmatrix{\\\egroup;\end{tikzpicture}}
\foreach \mtype in {b,B,p,v,V} {
\expandafter\xdef\csname tikz\mtype matrix\endcsname{%
\noexpand\begin{tikzpicture}[\mtype matrix]
\noexpand\matrix \noexpand\bgroup}
\expandafter\global\expandafter\let\csname endtikz\mtype matrix\endcsname=\endtikzmatrix
}
\begin{document}
\foreach \mtype in {b,B,p,v,V} {
\edef\metype{\mtype matrix}
\edef\tmetype{tikz\mtype matrix}
\begin{gather*}
\begin{tikzpicture}[\metype]
\matrix (m) {
a & b \\ c & d \\};
\end{tikzpicture}
\begin{\tmetype} a & b \\ c & d \end{\tmetype}
\begin{\metype} a & b \\ c & d \end{\metype}
\end{gather*}
}
\end{document}
(The \foreach loops are just to save me cut-and-pasting a lot.)
Best Answer
Instead of trying to modify the inner workings of the
vmatrix*environment, it may be more straightforward to start over with a basicarrayenvironment and simply fine-tune some of its settings.I gather that your main concern is with the spacing between the vertical bars and the contents of the arrays. The following example shows how the spacing may be adjusted. The adjustments amounts used in the example --
2muon the left and5muon the right -- can, of course, be changed to suit your needs and tastes. (The spacing used internally byvmatrixis0muon both sides.)