what is the physical meaning of electrostatic potential contour

[deepwave]

I plot the electrostatic potential contour, but I don’t know what is the physical meaning of the picture. Anders Blom told the electrostatic difference potential is the solution to the Poisson equation where on the right-hand side you have the electron difference density, which is the (total) electron density minus the density of the neutral atoms on this forum.
In my picture, why the region is yellow on left side while red on the right side around the molecular, however it is opposite on the molecular. Can anybody tell me the physical meaning of the picture?

[Nordland]

In my picture, why the region is yellow on left side while red on the right side around the molecular, however it is opposite on the molecular. Can anybody tell me the physical meaning of the picture?

It is because the voltage drop happens across the molecule, as it should.

[deepwave]

thanks a lot!  but it is not the voltage drop,it is the  the electrostatic difference potential under zero bias. so i donot think it is phsical meaning of "why the region is yellow on left side while red on the right side around the molecular, however it is opposite on the molecular."