I think it should be quite straightforward. Change the charge and multiplicity, calculate its wavefunction, save the checkpoint file, use cubegen to get ESP in the points you would like to.

What problems have you faced so far with the ion?

Thank you very much Dr  Kolář  for your valuable answer. I am calculating the wavefunction by changing charge and mutiplicity and also get ESP.  But when I am changing the range such as (-4.000e-2 to 4.000e-2 a.u) the potential surface get either totally red or totally bule. How I get ESP of a charge species within the range such as (-4.000e-2 to 4.000e-2) a.u

Thank you in advance

It may be that the ESP is calculated correctly, but there is something weird with vizualization. Have you tried to play with the range? It may be to wide, to narrow, or out of the actual ESP values...

Thank you Dr  Kolář  again for your valuable answer. I am trying to change the range but one time the surface get red or blue it will not further change by changing the values.

Then I can't really help. What vizualization program do you use? Do you visualize cube files? The cube file is human readable with the ESP values in the 4th column. Are these values meaningful?

Yes , I am visualize the ESp by cube file. What is the meaning of "4th column"  ?

It means that you can open the cube file in a text editor such as vim. The you will see data in this format: http://www.gaussian.com/g_tech/g_ur/u_cubegen.htm (section output file format). Below a header, there are four columns - x, y, z and ESP. You should check if the ESP values are meaningful. If yes, change the ESP range in your visualization program accordingly...

Thank You very much  Dr Kolář for your  help.

I normally generate  cube file form fromated chk point file (.fchk) and save them as .tiff or .jpg file. will you able to send any  ESP cube file so that  I visualize the  four columns and able to set the range of ESP plot ?

Dr. Kolář , you gave very useful answer about ESP. I really appreciate all of your answers . Now I am able to visualize ESP of  neutral, cationic and anionic species.  One of my question about ESP is that  there is any rule about ESP value range such as two value in positive and negative part always same  ( e.g -4.000e-2 to 4.000e-2)  a.u or it is possible to give different value ( e.g -4.000e-2 to 2.000e-2 i.e. different value ) a.u during ESP plot. In some of my calculation when I give different value it looks good

Thanking you

Which software you are using to visualize ESP. I am using GaussView 5.0.8 but it is showing error surface map failed executing Gaussian cubegen job.

Shilendra