Relative coordinates & Calc library

Question

Attached is the code to draw a simple tree.

I wish to get the NEGF node to appear in a middle location between the "west" of the \node {Without Illumination}, and "east" of the {Geometric} node. I want this centering to happen just on the x axis while with the y axis I want it to appear "below " the {-QTBM} \node.

The issue is twofold:

• 1. I am trying the \x1 with let command but the compiler fails with the message:

"No shape named 0 is known"

only when I try to introduce a more complecated mathematical expression no matter I try curly brackets, paranthese, I use 0.5*, 1/2,... if I want to center between both x values.

• 2. If I went with a simplified version and just wanted functionality to the righmost x (NEGFr) in my coordinates, it does not appear at the exact location in the plot. The code in its current status runs with no errors but just introducing more complecated math as above breaks it.

What am I doing wrong here? & Many thanks in advance for your help N.B: Please excuse any bad coding experience here as I am completely new to these LaTeX, TikZ things

\documentclass{article}
\usepackage{tikz}
\usepackage{lipsum}
\usetikzlibrary{trees,positioning, graphs, calc}

\begin{document}

\tikz [font=\footnotesize, grow=down, 
level 1/.style={sibling distance=2.6cm},
level 2/.style={sibling distance=2.5cm}, level distance=1cm]{
    \node   [align =center](TOO){Theory of\\Operation } % root
    child { node {Tunnelling}
        child { coordinate (NEGFl) node [align=center](NI) {Without \\Illumination}}
        child {node [align=center](I) {Under\\ Illumination}}
    }
    child { coordinate (NEGFr) node (Geo) {Geometric}
    }
    child { node {Hybrid}
    }
    ;% This comma is for the parent \node DON'T REMOVE

    % Now we list all the nodes down of each tree branch
    \node[below right][inner sep = 0.4, minimum height =0.6cm] at (NI.south west)(Simmons){-Simmons};
    \node[below right][inner sep = 0.4, minimum height =0.6cm] at (Simmons.south west)(Sanchez){-Sanchez};
    \node[below right][inner sep = 0.4, minimum height =0.8cm] at (Sanchez.south west)(TMM){-TMM};
    \node[below right][inner sep = 0.4, minimum height =0.6cm] at (TMM.south west)(QTBM){-QTBM};
    %\node[below right][inner sep = 0.4, minimum height =0.6cm, minimum width =10pt, fit = (NEGFl)(NEGFr)] at (QTBM.south west)(NEGF){-NEGF};

    % Now let's list the nodes below the "With Illumination" option
    \node[below right][inner sep = 0.4, minimum height =0.6cm] at (I.south west)(PAT){-PAT};
    \node[below right][inner sep = 0.4, minimum height =0.6cm] at (PAT.south west)(Floquet){-Floquet};
    \node[below right][inner sep = 0.4, minimum height =0.6cm] at (Floquet.south west)(Mayer){-Mayer};
    \path
        let 
                \p1 = ($(NEGFr)$),
                \p2 = ($(NEGFl)$)
            in     
                node (NEGF) at (\x1,\y1) [below, draw] {{$\x1$}}; %<=== where mistakes come from
    ;

    %Now let's put some explanatory text around
    %Hint: You can use below above, right, left,... OR use anchor = east, west,...
    %Single insulator models text
    \node [below left, rotate=270, inner sep = 0cm, align= center, text width =1.5cm] at (Sanchez.south west)(Ss){\tiny Single barrier insulator};
    %Multi insulator text
    \node [below right, rotate=270, inner sep = 0cm, align= center, text width =2cm] at (Sanchez.south west)(Ms){\tiny Single \& multi barrier insulator};
    %Draw dashed line to separate between single & Multi insulator models
    \draw [dashed] (Ss.south east) -- ($(Sanchez.south east) + (0.5cm, 0)$);

} %This bracket closes the \tikz command
\end{document}

Answer 1

edit: something like this:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{trees, positioning, decorations.pathreplacing, calc}

\begin{document}
    \begin{tikzpicture}[
 node distance = 2mm and 0mm,
font=\footnotesize,
          grow = down,
sibling distance=22mm,
level distance = 11mm,
%
      B/.style = {decorate,
                  decoration={brace, amplitude=3pt,
                  raise=1mm, mirror},
                  thick},
every node/.append style = {align=center, anchor=north},
                    ]
\node (TOO) {Theory of\\Operation } % root
    child { node {Tunnelling}
        child {node (NI) {Without \\Illumination}}
        child {node (I) {Under\\ Illumination}}
            }
    child { coordinate (NEGFr) node (Geo) {Geometric}}
    child { node {Hybrid}}
    ;% end of basic treee
\begin{scope}[every node/.append style={align=left}]
    % Now we list all the nodes down of each tree branch
\node[below right=of NI.south west] (Simmons)   {- Simmons\\[1ex]
                                                 - Sanchez};
\node[below right=of Simmons.south west] (TMM)  {- TMM\\[1ex]
                                                 - QTBM};
\node[below right=of I.south west] (PAT)        {- PAT\\[1ex]
                                                 - Floquet};
\node[below right=of PAT.south west] (Mayer)    {- Mayer};
    \end{scope}
% NEGF node
\path (TMM.south west) -- coordinate (aux) (TMM.south -| Geo.east);
\path   let \p1 = ($(TMM.west) - (TMM -| Geo.east)$), 
            \n1 = {veclen(\x1,\y1)} in
        node (NEGF) [draw, minimum width=\n1, below=of aux] {- NEGF};  
% explanatory text
\draw[B]    (Simmons.north west) --
    node[below=2mm,font=\tiny, sloped] {Single\\ barrier\\ insulator}
                                    (Simmons.south west);
 \draw[B]    (TMM.north west) --
    node[below=2mm,font=\tiny, sloped] {Single \& \\ multi barrier\\ insulator}
                                    (TMM.west |- NEGF.south);
    \end{tikzpicture}
\end{document}

Answer 2

Something like this?

I used Forest because it makes doing things with trees much easier, generally. I used fit to create the spanning node.

\documentclass[border=10pt]{standalone}
\usepackage{forest}
\usetikzlibrary{positioning,fit,calc}
\begin{document}
\begin{forest}
  for tree={%
    align=center,
    font=\footnotesize,
  },
  before typesetting nodes={%
    where={ > Ow+P {n children} {isodd(#1)} }{%
      calign=child edge,
      if n children=1{%
        parent anchor=south west,
        for children={%
          no edge,
          align=left,
          content/.wrap value={\textbullet{} #1},
          child anchor=north west,
          anchor=north west,
          before computing xy={l'=1.75\baselineskip},
        },
      }{%
        calign child/.process={ Ow+n {n children} {(#1+1)/2} },
      },
    }{},
  }
  [Theory of\\Operation, name=to % names are optional - needed only if you want to refer to nodes by name later
    [Tunnelling, name=t
      [Without\\Illumination, name=wi
        [Simmons, name=si
          [Sanchez, name=sa
            [TQM, name=tq
              [QTBM, name=qtbm]
            ]
          ]
        ]
      ]
      [Under\\Illumination, name=i
        [PAT, name=p
          [Floquet, name=f
            [Mayer, name=m]
          ]
        ]
      ]
    ]
    [Geometric, name=g]
    [Hybrid, name=h]
  ]
  \path (current bounding box.south west) coordinate (b) -- (b -| g.east) node (n) [midway, below, font=\footnotesize] {\textbullet{} NEGF};
  \node [draw, inner sep=0pt, fit=(n) (b) (b -| g.east)] {};
  \begin{scope}[every node/.append style={font=\tiny, rotate=270, inner sep =0cm, align=center}]
    \node (sb) [anchor=north east] at (sa.south west) {Single barrier\\insulator};
    \node (sbm) [anchor=north west] at (tq.north west) {Single \& multi\\barrier insulator};
  \end{scope}
  \draw [densely dashed] (current bounding box.west |- {$(sa.south)!1/2!(tq.north)$}) coordinate (a) -- (wi.east |- a);
\end{forest}
\end{document}

Answer 3

Following is the working code. Altered a tiny bit of that of @Zarko. A very enlighting note is gievn by @cfr. So many thanks to both of you.

\documentclass{article}
\usepackage{tikz}
\usepackage{lipsum}
\usetikzlibrary{trees,positioning, graphs, calc}

\begin{document}


\tikz [font=\footnotesize, grow=down, 
level 1/.style={sibling distance=2.6cm},
level 2/.style={sibling distance=2.5cm}, level distance=1cm]{
    \node   [align =center](TOO){Theory of\\Operation } % root
    child { node {Tunnelling}
        child { coordinate (NEGFl) node [align=center](NI) {Without \\Illumination}}
        child {node [align=center](I) {Under\\ Illumination}}
    }
    child { coordinate (NEGFr) node (Geo) {Geometric}
    }
    child { node {Hybrid}
    }
    ;% This comma is for the parent \node DON'T REMOVE

    % Now we list all the nodes down of each tree branch
    \node[below right][inner sep = 0.4, minimum height =0.6cm] at (NI.south west)(Simmons){-Simmons};
    \node[below right][inner sep = 0.4, minimum height =0.6cm] at (Simmons.south west)(Sanchez){-Sanchez};
    \node[below right][inner sep = 0.4, minimum height =0.8cm] at (Sanchez.south west)(TMM){-TMM};
    \node[below right][inner sep = 0.4, minimum height =0.6cm] at (TMM.south west)(QTBM){-QTBM};
    %\node[below right][inner sep = 0.4, minimum height =0.6cm, minimum width =10pt, fit = (NEGFl)(NEGFr)] at (QTBM.south west)(NEGF){-NEGF};

    % Now let's list the nodes below the "With Illumination" option
    \node[below right][inner sep = 0.4, minimum height =0.6cm] at (I.south west)(PAT){-PAT};
    \node[below right][inner sep = 0.4, minimum height =0.6cm] at (PAT.south west)(Floquet){-Floquet};
    \node[below right][inner sep = 0.4, minimum height =0.6cm] at (Floquet.south west)(Mayer){-Mayer};

    \draw (QTBM.south west) -- coordinate (aux) (QTBM.south -| Geo.east);
    \path   let 
                \p1=($(QTBM.west) - (QTBM -| Geo.east)$),
                \n1= {veclen(\x1, \y1)}
            in
                node (NEGF) [below right, draw, minimum width=\n1 ] at (QTBM.south west) {-NEGF}
    ;

    %Now let's put some explanatory text around
    %Hint: You can use below above, right, left,... OR use anchor = east, west,...
    %Single insulator models text
    \node [below left, rotate=270, inner sep = 0cm, align= center, text width =1.5cm] at (Sanchez.south west)(Ss){\tiny Single barrier insulator};
    %Multi insulator text
    \node [below right, rotate=270, inner sep = 0cm, align= center, text width =2cm] at (Sanchez.south west)(Ms){\tiny Single \& multi barrier insulator};
    %Draw dashed line to separate between single & Multi insulator models
    \draw [dashed] (Ss.south east) -- ($(Sanchez.south east) + (0.5cm, 0)$);

} %This bracket closes the \tikz command
\end{document}