\begin{tikzpicture}[scale=0.75, every node/.style={rounded corners = 3mm,draw=black}]

\tikzstyle{comment}=[fill=none,draw=none,midway,anchor=west]
\tikzstyle{dbnode}=[top color=white, bottom color = blue!70]
\tikzstyle{filenode}=[top color=white, bottom color = green!60!black!40]

\node [filenode] (CIF) at (0,0){CIF};

\node [dbnode] (COD) at (-4,-3) {COD};

\draw [->] (CIF) -- node [comment,anchor=south east] {
	\parbox{2cm}{\scriptsize\raggedright
		\textbf{experimental:} refined against $F_{obs}$
	}
}(COD);

\node [filenode] (theoretical) at (4,-2) {
	\parbox{2.0cm}{
		\centering
		theoretical structure
	}
};

\draw [->] (CIF) -- node [comment,anchor=south west,midway] {no $F_{obs}$} (theoretical);

\node [dbnode] (PCOD) at (1.5,-6) {PCOD};

\draw [->] (theoretical) -- node [comment,anchor=south east,near end] {
	\parbox{2.5cm}{\scriptsize\raggedright
		\textbf{predicted:} from first principles,
		no crystallographic information used at all
	}
} (PCOD);

\node [dbnode] (TCOD) at (6.5,-6) {TCOD};

\draw [->] (theoretical) -- node [comment,anchor=south west,near end] {
	\parbox{2.5cm}{\scriptsize\raggedright
		\textbf{theoretical:} uses experimental cell constants, composition, etc.
	}
}(TCOD);

\end{tikzpicture}