In chemical reactions, the electron density of a molecular system undergoes significant changes. For simple reactions, such as identity S N 2 and Menshutkin reactions, electron density decreases around broken bonds, whereas the density increases at newly-formed chemical bonds. For other reactions, such as Claisen rearrangement and Diels-Alder reactions, density changes are also expected around double bonds converted into single bonds (or vice versa). It would be highly desirable to be able to directly visualize such electron density changes (EDC) during a chemical reaction. However, the EDC visualization is not as straightforward as it seems. In computational chemistry, the electron density of each stationary structure (reactant, transition state, and product) of a reaction can be readily computed on a rectangular grid, and then visualized via an isosurface representation. Naively, one can compute the EDC from one stationary structure to another by subtracting the density values on the same rectangular grid. But, the corresponding isosurface plot tells us more about the displacement of each atom (which we already know) than the actual change in the density around key atoms and bonds.