VASP Forum

Dear VASP team, I have a question regarding the simulation of charged systems with VASP. Lets say for example that I want to study the behavior of Na+ which has 10 electrons, if I want to tell VASP that I have this net charge present in my system I do this in the INCAR under the command NELECT=10. Wont this create an infinite charged system under periodic boundery conditions? And therefore introduce a lot of artifacts in the simulation?
And if the VASP software would automatically add a charge of -1 to the system, am I then not just simulating Na, a radical with completely different behavior than Na+? Thank you for your time!

Dear stijn_devos,

We have a very detailed guide on how to deal with charged cells on our wiki.
But in short, if you change NELECT from default and your cell becomes charged, VASP will add a homogeneous background charge to compensate it.

Best,
Alexey

Hi Alexey, thank you for your explanation and in the meantime i have been reading more about the background charge added. i do have one more question:
Members of our team say the best way to simulate a charged system is to add a counter ion of opposite charge. In the example of Na+ this could be for example Cl- that you place far away so that it doesnt interact with your desired Na+ simulation. The downside of this technique is that you have to enlarge your unit cell. But i personally think that you need to set NELECT=10 so that a background charge of -1 is added. Which technique do you think is best? Regards, Stijn

By placing an ion of the opposite charge you would still get an interaction between the charges themselves for any practically possible cell sizes. Furthermore, we will get a dipole-dipole interaction between periodic images, which needs to be corrected for. Thus, I don't see what would be the advantage of this approach.