changing POMASS in POTCAR

[pooja_goddard1]

I have a question regarding the POMASS tag in the POTCARS.
We are trying to model Pu 239 for one of our projects however the Pu POTCAR
available is for Pu244.
Can I simply change the POMASS term in the POTCAR to become 239 so that the
number of neutrons is 239 or do we need a whole new POTCAR for this?
In which case, how can we generate a new POTCAR and or is there one
available for Pu239 please?
Many thanks,

[christopher_sheldon1]

Dear Pooja,

Thank you for your question. The POTCAR for Pu can be used for any isotope of Pu. You can change the POMASS in the INCAR, defining it for each species in your POSCAR, e.g., plutonium oxide:

Code: Select all

POMASS = 239.0521616 16.000 

This is the recommended approach, rather than modifying the POTCAR. For a single structure, this will not change the energy of the system, due to the Born-Oppenheimer approximation. However, it will affect molecular dynamics. For nuclear effects, e.g., NMR, you define the isotope of interest separately.

Does this answer your question?

Best wishes,

Chris

[pooja_goddard1]

yes thank you. So would an MD - NVE work better rather than doing a geometry optimisation?

[christopher_sheldon1]

Not necessarily, if you want to optimise the geometry, then an MD is not suitable. The difference in mass between isotopes does not change the electronic structure, only those parts that depend on the nuclear mass, e.g., vibrational frequencies. What sort of calculation do you want to run?

Chris

[pooja_goddard1]

Hi Chris,
We are interested in seeing how the structure PuO2 changes when certain Pu decay further. There are 3 different isotopes of Pu in PuO2 bulk. So it would be nice to even check how the ratios of these affects the structure. Hope that makes sense?
Thanks

[christopher_sheldon1]

Hi Pooja,

Yes, I understand now. For this, you could try MD. However, the electronic structure is independent of the mass of the nuclei. This means that, even if you place an isotope with a different mass there, it will not affect the electronic structure. This is a result of the Born-Oppenheimer approximation. The mass will change the phonon frequencies, the zero-point vibrational energy, and the MD trajectories, as the frequency is inversely proportional to the square root of the mass (cf. simple harmonic oscillators). The equilibrium geometries of the nuclei will not change; structure is independent of isotopic mass. I hope that this answers your question.

If you are interested in how Pu's decay affects the structure, then it would likely be best to investigate the decay product, i.e., the element that Pu239 decays into. A different element in that position will change the electronic structure and therefore the geometry of the system.

Does this answer your question?

Best wishes,

Chris

[pooja_goddard1]

ok thank you Chris. I understand your point now.

Best Wishes,
-Pooja

[christopher_sheldon1]

Great, let me know if there's anything else that I can help you with. Otherwise, I will close this topic on Wednesday next week.

Best wishes,

Chris