General question about using NELECT

[jess_white]

Hi,

I am currently doing some charged cells as I am looking at ligand and metal bonding. However I have found some issues with two of the calculations I am doing. I am using NELECT tag in INCAR.

1) The ligand is CO3 (charge of -2) so when the cation (2+) is added the charge changes from negative to neutral

2) The ligand is an acid (charge of -1) so when the cation (2+) is added the charge changes from negative to positive

3) All my other ligands which have a charge of -3 and -4 and then are still negative with the addition of the cation are all gaining reasonable results.

So as a general question I was wondering if the issue with the points in 1 and 2 is because of the change of charge of the box? I am calculating the adsorption/binding energy so do a calculation with the ligand by it self and then with the ligand and metal (and of course the plain metal).

I can attach the calculations but thought a good starting point would be to see if this is an issue already known and if I should add Na and Cl to balance the charge for these cases?

Looking forward to your reply

Cheers,
Jess

[manuel_engel1]

Hi Jess,

Thanks for reaching out. I'm not sure I understand your question yet. But I will try my best to give some initial feedback.

Unless you are changing NELECT, the total charge of the entire simulation cell will always be zero. Now, what you are looking at are oxidation states of the molecules. Here, the electrons are borrowed from somewhere else in the cell. In principle, VASP will "know" where the put the extra or missing electrons during electronic minimization. So setting NELECT should be enough to simulate such systems.

[jess_white]

Hi Manuel,

Thank you for your response! Sorry that my question is a bit confusing - I kind of just put too much information in the text for sure.

So I have been using NELECT but I have been getting weird results. I would be expecting to see favourable binding of the metal and ligand and instead I am seeing it to be unfavourable. This leads me to believe that I am having some issues with the charged cells. I have attached Ca (2+) and Co (2+), carbonate (2-) and Ca/Co-carbonate together to see if my input files are reasonable.

[jess_white]

Here are the rest of the files

[manuel_engel1]

When you change the number of electrons in your system, it is sometimes necessary to add an additional spin degree of freedom by setting ISPIN=2. Make sure to choose reasonable defaults for ISPIN, or else the calculation might not find the right ground state. Apart from this, I didn't find any obvious mistakes in your input files.