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How can I increase the spacing between nodes A and B? Right now they are right beside each other. Also, is there a way to prevent node H from overlapping with the other nodes?

The above output is produced from the following commands.

documentclass[tikz]{standalone}

usetikzlibrary{arrows,quotes, positioning}

tikzset{block/.style={ellipse, draw, fill=gray!20, 

    text width=8em, text centered, rounded corners, minimum height=4em, width=4em},

    line/.style={draw, -latex'},}

begin{document}





begin{tikzpicture}[node distance = 3cm, auto]

     node [block] (A) {A};

node [block,right of=A] (B) {B};

node [block, above of=A] (C) {C};

node [block, above of=B] (D) {D};

node [block, below left of=B, below right of=A] (E) {E};

node [block, right of=E] (F) {F};

node [block, above right of=F, node distance=3cm] (G) {G};

node [block, above of=G] (H) {H};



% Draw edges

path [line] (C) edge["a"'] (A) ;

path [line] (D) edge["b"']  (B) ;

path [line] (A) -- (E);

path [line] (B) -- (E);

path [line] (F) --  (E);

path [line] (F) edge["g"'] (G) ;

path [line] (G) edge["h"']  (H);



end{tikzpicture}

end{document}

Edit: edited to include environment based on Skillmon's answer. Apologies for not making my code compilable initially

I added a minimal environment to your code so that I can work on it. You shouldn't use the right of=A syntax but instead use right=of A. I've also added a between key to place E, note that below right=of A, below left=of B will just result in below left=of B the former being ignored. Also the ysep and xsep keys seem to not exist in current TikZ versions (mine is 3.0.1a).

documentclass[tikz]{standalone}



usetikzlibrary{arrows,quotes,positioning,shapes,calc}



tikzset

  {

    block/.style=%

      {%

        ellipse, draw, fill=gray!20, text width=8em, text centered, rounded

        corners, minimum height=4em, minimum width=4em

      }

    ,line/.style={draw, -latex'}

    % https://tex.stackexchange.com/a/138828/117050

    ,between/.style args={#1 and #2}{at=($(#1)!0.5!(#2)$)}

  }



begin{document}

begin{tikzpicture}[node distance = 2cm, auto]

  node [block] (A) {A};

  node [block,right=of A] (B) {B};

  node [block, above=of A] (C) {C};

  node [block, above=of B] (D) {D};

  node [block, between=A and B, yshift=-2.5cm] (E) {E};

  node [block, right=of E] (F) {F};

  node [block, right=of B] (G) {G};

  node [block, right=of D] (H) {H};



  % Draw edges

  path [line] (C) edge["a"'] (A) ;

  path [line] (D) edge["b"']  (B) ;

  path [line] (A) -- (E);

  path [line] (B) -- (E);

  path [line] (F) --  (E);

  path [line] (F) edge["g"'] (G) ;

  path [line] (G) edge["h"']  (H);

end{tikzpicture}

end{document}