Isodensity surfaces colour-coded with the electrostatic potential can be used to produce qualitative insights into a number of different chemical situations. This article shall refer to these as "ESP maps", although this term is less accurate it is certainly a lot easier to say. The software used is Gaussian and VMD, although the process is similar for a number of different packages and visualisers.
Gaussian cube files
The first step is to obtain the isodensity and electrostatic potential information.
- Add %Chk=foo.chk to the Gaussian input file.
- Using the formchk utility: formchk foo.chk foo.fchk
- Create a cube file containing the density information: cubegen 0 density foo.fchk foo-dens.cube 80
- And a cube file containing the electrostatic information: cubegen 0 potential foo.chk foo-esp.cube 80
The following guide is purposefully vague as producing images is a creative process:
- Open the density cube as a new molecule.
- Open the ESP cube as data for the existing molecule.
- In Graphics/Representations, change the drawing method to Licorice.
- Create a new Represenation, with a drawing method of Isosurface.
- Change Draw to Solid Surface.
- Change Show to Isosurface.
- Adjust the isovalue to personal preference. 0.04 is a decent starting point.
- Change the Isosurface coloring method to Volume, selecting the ESP cube in the adjacent box.
- Adjust the color data range under the Trajectory tab.
- A color scale bar can be added through the Extensions menu.
With some further adjustments you can end up with an image that looks something like this:
Some of the settings below have worked well for us.
Colors: An RWB color scale with a midpoint of 0.50 to 0.65.Material Settings: