\documentclass{standalone}
\usepackage{amsmath,amssymb,amsfonts,pgfpages,graphicx,subfigure,xcolor,bm,multirow,microtype,wasysym,multimedia,hyperref,tabularx,amscd,pgfgantt,mhchem,physics}
\usetikzlibrary{shapes.gates.logic.US,trees,positioning,arrows}

\usepackage{tgchorus}
\usepackage[T1]{fontenc}

\begin{document}

\begin{tikzpicture}
	\begin{scope}[very thick,
		node distance=5cm,on grid,>=stealth',
		block1/.style={rectangle,draw,fill=red!20},
		block2/.style={rectangle,draw,fill=orange!20},
		comp1/.style={rectangle,draw,fill=green!40},
		comp2/.style={circle,draw,fill=green!40},
		comp3/.style={rectangle,fill=white!40}]
		
		\node [block1, text width=7cm, align=center]	(KS)		
		{\textbf{\LARGE Kohn-Sham DFT}
			$$
				\qty[ -\frac{\nabla^2}{2} + v_\text{ext} + V^{\text{Hxc}} ] \phi_p^{\text{KS}} = \varepsilon^{\text{KS}}_p \phi_p^{\text{KS}}
			$$	
		};									

		\node [block2, text width=7cm, align=center]	(GW)	[below=of KS, yshift=2cm]				
		{\textbf{\LARGE $GW$ approximation} 
			$$   
				\varepsilon_p^{GW} = \varepsilon_p^{\text{KS}} +
				\mel{\phi_p^{\text{KS}}}{\Sigma^{GW}(\varepsilon_p^{GW}) - V^{\text{xc}}}{\phi_p^{\text{KS}}}
			$$

		};												

		\node [comp1, text width=7cm, align=center]	(BSE)	[below=of GW, yshift=2cm]									
		{\textbf{\LARGE Bethe-Salpeter equation}
			$$
				\begin{pmatrix}
					R & C  \\
					-C^*  & -R^{*}
				\end{pmatrix}
				\begin{pmatrix}
					X_m  \\
					Y_m
				\end{pmatrix}
				=
				\Omega_{m}
				\begin{pmatrix}
					X_m  \\
					Y_m
				\end{pmatrix}
			$$
		};
								

%		\node [comp2, align=center] 					(phys)	[left=of BSE, xshift=-2cm]										
%		{\LARGE Molecules};

%		\node [comp2, align=center] 					(chem)	[below=of BSE, xshift=0cm]			
%		{\LARGE Materials};
		
%		\node [comp2, align=center] 					(bio)	[right=of BSE, xshift=2cm]		
%		{\LARGE Clusters};
		
		\path 
		(KS) 			edge [->,color=black] 			node [right,black] {\LARGE Fundamental gap}		(GW)
		(GW) 			edge [->,color=black] 			node [right,black] {\LARGE Excitonic effect}	(BSE)
%		(BSE) 			edge [->,color=black] 			node [above,black] {}	(phys)
%		(BSE) 			edge [->,color=black] 			node [above,black] {}	(chem)
%		(BSE) 			edge [->,color=black] 			node [above,black] {}	(bio)
		;

		
	\end{scope}
\end{tikzpicture}

\end{document}  
