Regarding specification of atom in oxidation state

[Roshan Pudasaini]

While optimizing any system at any level, we have the option of only mentioning overall charge on the system not on a particular atom. In such case, if I want to represent the oxidation state of an atom like (metal center), then how would i go for mentioning the oxidation state?
In the input file here, I want to specify Fe (In line number 58) is in (+3) or (+2) state then how can I do that?


Best regards,
Roshan Pudasaini

[Bushra Alam]

Hello,

I think for any quantum chemistry method, you have to account for the total charge and multiplicity as a whole (since the calculation is conducted on the system as a whole). I am unaware if NWChem offers a way to specify atomic charges, but many codes require you to adjust overall charge and multiplicty. For example, an Fe+3 centre will have 5 unpaired electrons, making the overall multiplicity 6 (unless there are other centres with unpaired electrons). Hope this helps.

[Edoardo Aprà]

Even though you cannot set charges on specific atoms, you can try to direct the SCF towards a solution with an Fe in a 3+ oxidation state by using a fragment guess that combines the isolated Fe atom with a 3+ charge with the ligands in a 3- charge as in the attached input file.

https://nwchemgit.github.io/Hartree-Fock-Theory-for-Molecules.html?h=fragm#vectors-fragment-superposition-of-fragment-molecular-orbitals

In this case, 

the first fragment (labeled as "fe" geometry) is the isolated Fe atom with charge equal 2 and a multiplicity set equal to 2,

while the second fragment (labeled as "ligand" geometry) are remaining atoms in molecule other than Fe with a charge of -2 and multiplicity set equal to 1.

Keep in mind that that I moved the Fe atom as first atom in the "all" geometry to be compatible with the fragment composition.

[Roshan Pudasaini]

Thank you for your answer. 

Best Regards,
Roshan