The Quantum Theory of Atoms in Molecules (QTAIM) is the quantum mechanical way of looking at molecular structures and bonding. Developed by Richard Bader, QTAIM is built on concepts proper to the molecular structure hypothesis in which a functional grouping of atoms has an additive and characteristic set of properties, and uses the topology of the electron density to investigate the interactions in molecular systems. We propose to use QTAIM as a detecting tool in order to identify small changes in the electron density due to weak forces. In particular we would like to test the power of topological analysis in revealing small effects of H/D isotopic replacement. Single crystal neutron diffraction data are vital for the correct description of atoms in the analysis.