Drawing circular arrows between nodes

Question

I want to draw something like this:

So far I have this latex:

\documentclass[border=5mm]{standalone}

\usepackage{tikz}
\usetikzlibrary{arrows}
\begin{document}
\begin{tikzpicture}

\node (species_1) at (-3,0.5) 
    {$S_1$};

\node (species_2) at (0,0) 
    {$S_2$};

\node (species_3) at (2,-2) 
    {$S_3$};

\node (species_4) at (0,-4) 
    {$S_4$};

\node (species_5) at (-2,-2) 
    {$S_5$};

\node (species_6) at (-3,-4.5) 
    {$S_6$};

\node(equation_1) at (6,-1)
    {$S_1 + S_5 \rightarrow S_2 $}; 
\node(equation_2) at (6,-2)
    {$S_2 + S_3 \rightarrow S_4 $}; 
\node(equation_3) at (6,-3)
    {$S_4 + S_5 \rightarrow S_6 $}; 


\end{tikzpicture}
\end{document}

Which produces this:

I've had a few attempts but I can't seem to get the arrows to be circular. They just end up square, nevermind the bifurcating arrows.

How do I do this?

Thanks

Answer 1

What about that:

 \documentclass[border=5mm]{standalone}

 \usepackage{tikz}
 \usetikzlibrary{arrows}
 \begin{document}
 \begin{tikzpicture}[paths/.style={->, thick, >=stealth'}]

 \node (species_1) at (-3,0.5) 
     {$S_1$};

 \node (species_2) at (0,0) 
     {$S_2$};

 \node (species_3) at (2,-2) 
     {$S_3$};

 \node (species_4) at (0,-4) 
     {$S_4$};

 \node (species_5) at (-2,-2) 
     {$S_5$};

 \node (species_6) at (-3,-4.5) 
     {$S_6$};

 \node(equation_1) at (6,-1)
     {$S_1 + S_5 \rightarrow S_2 $}; 
 \node(equation_2) at (6,-2)
     {$S_2 + S_3 \rightarrow S_4 $}; 
 \node(equation_3) at (6,-3)
     {$S_4 + S_5 \rightarrow S_6 $}; 

 \draw [paths] (species_2) to [bend left=40] node {} (species_3);
 \draw [paths] (species_3) to [bend left=40] node {} (species_4);
 \draw [paths] (species_4) to [bend left=40] node {} (species_5);
 \draw [paths] (species_5) to [bend left=40] node {} (species_2);

 \draw [paths] (species_1) to [bend left=-12] node {} (species_2);
 \draw [paths] (species_4) to [bend left=-12] node {} (species_6);

 \end{tikzpicture}
 \end{document}

So as you can see the main idea is to use the [bend left=XX] option to bend the arrows in the direction you want them.

Answer 2

To guarantee a circular shape for the arrows, I borrowed some code from Heiko Oberdiek's answer to Draw nodes with path in tikz.

The code:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{bending,positioning}

\begin{document}

% code from Heiko Oberdiek
% https://tex.stackexchange.com/a/250270/3954
\begin{tikzpicture}[
  ->,   
  thick,
  main node/.style={},
]
  \newcommand*{\MainNum}{4}
  \newcommand*{\MainRadius}{1.5cm} 
  \newcommand*{\MainStartAngle}{90}

  % Print main nodes, node names: p1, p2, ...
  \path
    (0, 0) coordinate (M)
    \foreach \t [count=\i] in {2,...,5} {
      +({\i-1)*360/\MainNum + \MainStartAngle}:\MainRadius)
      node[main node, align=center] (species_\i) {$S_\t$}
    }
  ;  

  % Calculate the angle between the equal sides of the triangle
  % with side length \MainRadius, \MainRadius and radius of circle node
  % Result is stored in \p1-angle, \p2-angle, ...
  \foreach \i in {1, ..., \MainNum} {
    \pgfextracty{\dimen0 }{\pgfpointanchor{species_\i}{north}} 
    \pgfextracty{\dimen2 }{\pgfpointanchor{species_\i}{center}}
    \dimen0=\dimexpr\dimen2 - \dimen0\relax 
    \ifdim\dimen0<0pt \dimen0 = -\dimen0 \fi
    \pgfmathparse{2*asin(\the\dimen0/\MainRadius/2)}
    \global\expandafter\let\csname p\i-angle\endcsname\pgfmathresult
  }

  % Draw the arrow arcs
  \foreach \i [evaluate=\i as \nexti using {int(mod(\i, \MainNum)+1}]
  in {1, ..., \MainNum} {  
    \pgfmathsetmacro\StartAngle{   
      (\i-1)*360/\MainNum + \MainStartAngle
      + \csname p\i-angle\endcsname
    }
    \pgfmathsetmacro\EndAngle{
      (\nexti-1)*360/\MainNum + \MainStartAngle
      - \csname p\nexti-angle\endcsname
    }
    \ifdim\EndAngle pt < \StartAngle pt
      \pgfmathsetmacro\EndAngle{\EndAngle + 360}
    \fi
    \draw[<-]
      (M) ++(\StartAngle:\MainRadius)
      arc[start angle=\StartAngle, end angle=\EndAngle, radius=\MainRadius]
    ;
  }

\node[above left=1.4cm and 10pt of species_2] (species_0) 
  {$S_1$};
\node[below left=1.4cm and 10pt of species_2] (species_5) 
  {$S_6$};
\node[right=2cm of species_4](equation_2) 
    {$S_2 + S_3 \rightarrow S_4 $}; 
\node[above=10pt of equation_2] (equation_1)
    {$S_1 + S_5 \rightarrow S_2 $}; 
\node[below=10pt of equation_2] (equation_3) 
    {$S_4 + S_5 \rightarrow S_6 $}; 

\draw[->]
  (species_0) to[out=-20,in=180] (species_1);
\draw[<-]
  (species_5) to[out=20,in=180] (species_3);
\end{tikzpicture}

\end{document}