Read a checkpoint file into Gaussview.
Select the Menu: Results --> Surfaces/Contours...
On the Surfaces and Contours window, in the Cubes Available: section, select the pulldown: Cube Actions --> New Cube --> Type --> ESP and click Ok (change the Grid if you want a better picture).
Repeat these Cube Actions, but now select Total Density instead of ESP.
In a few minutes (longer for large molecules and basis sets), two entries
will appear in the Cubes Available section.
Select (highlight) Electron density from Total SCF Density.
Then, in the Surfaces Available: section, select the pulldown: Surface Actions - New Mapped Surface.
This opens the Surface Mapping window.
On that window, choose Electrostatic potential from Total SCF Density in the Cubes Available field, and check the box to Use an existing cube. Then click Ok. It may take a few minutes before the display appears.
Adjust the surface size by adjusting the Isovalue.
In the upper left and right corners of the window displaying the surface, there are fields for the upper and lower bounds for charges. You can set these so they are the same for different molecules, so that the color regions can be compared between molecules.