We will sometimes distinguish between CRNs with different initial configurations, and to this end define a chemical reaction system (CRS) as a tuple \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$S=(\varLambda ,R,x_0)$$\end{document}S=(Λ,R,x0) where \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$(\varLambda ,R)$$\end{document}(Λ,R) is a CRN and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$x_0 \in {\mathbb {N}}^{|\varLambda |}$$\end{document}x0∈N|Λ| represents its initial configuration, and we sometimes use the terms CRN and CRS interchangeably.