3. Plotting color-filled map of IRI on system plane of guanine-cytosine dimer

Boot up Multiwfn and input

GC.wfn
4  // Plot plane map
24  // IRI
1  // Color-filled map
[Press ENTER button directly to use default grid setting]
0  // Set extension distance around boundary atoms
2  // 2 Bohr is proper for present case
1  // XY plane
0  // Z=0. The current plotting plane is exactly identical to the system plane

Click right-mouse button on the graph to close it, then input:

8  // Enable showing bonds
14  // Brown
19  // Set color transition
2  // Reversed rainbow
4  // Enable showing atom labels
1  // Red
1  // Set lower&upper limit of color scale
0,2
-1  // Plot again

Now you can see the following map. You can also use other options to further improve the map.