Please search ESPiso_eV.txt in Multiwfn manual, you will find using ESPiso_eV.bat and ESPiso_eV.txt instead of ESPiso.bat and ESPiso.txt will make the ESP map use eV as the unit. Similarly, you can duplicate ESPiso_eV.txt as ESPiso_kcal.txt file, in which you modify 27.2114 to 627.51, and create ESPiso_kcal.bat script to run Multiwfn according to the commands in the file. Then the unit in the ESP map plotted by VMD will be kcal/mol, and you can correspondingly modify the upper/lower limits of coloring scale and color bar.

Best,

Tian

However, when I included the color bar via VMD, the value of the upper and lower limits was expressed in a.u. To convert it to more suitable "kcal/mol" values, I opened the ESPiso.vmd script that contains the following lines:

#Lower and upper limit of color scale of ESP (a.u.)
set colorlow -0.03
set colorhigh 0.03

and changed the respective values of the limits to -50 and 50 manually, as in the ESPpt.vmd script:

#Lower and upper limit of color scale of ESP (kcal/mol in most case)
set colorlow -50
set colorhigh 50

After this action, the gray transparent surface was generated. I understand that discoloration of the MEP surface is caused by changing upper and lower limits values while the resulting estimation of the limits is still expressed in a.u. (in other words, the program "sees" -50/50 a.u. instead of -50/50 kcal/mol).
I've analyzed your video tutorial on YouTube, where the following statement is present (on the second screenshot). But I don't clearly understand how to fix it manually (perhaps, it can be done using .bat files..) without external software like Photoshop.
So, I would highly appreciate your help.
Best regards,
Alex