Hydrogen Molecule Dissociation energy calculation

[AsifShah]

I am calculating the dissociation energy of Hydrogen molecule.
The formula used is E(disso)=E(H2)-2E(H)
where E(H2) is the total energy calculated using two methods as:
1. Spin unpolarised/polarized using B3LYP functionals giving energies - 32, -32 respectively.
2. Spin unpolarised/polarised using PBE functionals giving energies - 31.651, -27.181 respectively.
Since H2 is spin unpolarised molecule & when choosing unpolarised/polarised using B3LYP both give same energy -32 which makes sense but why does PBE give different results for spin polarised & spin unpolarised?

[Anders Blom]

All numbers are very far from the right answer (around 4.5 eV), although you didn't specify the unit. You do need spin polarization to get the right total energy of the isolated H atom, because the ground state is strong spin polarized (single unpaired electron). I have seen discussions where people mention that H2, however, should be run spin-unpolarized, but I haven't tried myself so you had better check this.

As for B3LYP, I have little to no experience with it, but given the odd numbers in general, I tend to think something else is wrong...

[AsifShah]

Dear Admin,
All units are in eV.
You probably misunderstood my question.

The mentioned values are not the dissociation energy values of H2 molecule but that of whole system i.e H2 molecule using PBE/B3LYP functionals.
As you can see when I calculate energy of H2 molecule using B3LYP whether unpolarized/polarized, I get -32 eV as total energy. Now, when I calculate same using PBE, I get different results for polarized/ unpolarized as -27.181 eV, -31.651 eV which should not happen because H2 anyways is unpolarized.

This questions the software because here in this case although I know H2 is unpolarized I can keep the unpolarized results & move on. However, when probing some unknown system for which I don't know prior whether its polarized or unpolarized, I may get incorrect results because PBE for polarized/unpolarized shows different results for H2 which should otherwise show same results.

[filipr]

When you calculate the H2 using polarized spin treatment the initial density will be maximally polarized (both electrons will be put in the UP state) and the implementation of PBE in the libXC library that we use for calculating XC functionals can't correctly handle this situation and it yields wrong results. This only happens for systems where the initial density is maximally polarized, i.e. for atoms/molecules that do not have any full shells, so basically only H and H2 when using spin-polarized PBE. As a workaround you should adjust the initial spin to anything smaller than strictly 1.0, i.e. 0.999 - or since you know H2 is unpolarized you might as well choose 0.0 (or do an unpolarized calculation). You can set the initial spin in the GUI by choosing the "Initial State" block in the Script Generator, insert it after the calculator, open its settings, choosing "polarized spin" and setting the spin polarization for each atom.

We have run into this problem before, but unfortunately it is not so easy to fix in general without implications in other parts of the program, so setting the initial spin != 1.0 is currently the best solution.

[AsifShah]

That was helpful.
Thanks for your clarification.