About QATK Slater-Koster parameter setting rule?

[faxer92]

Dear Prof.
I followed tutorials of Silicon and tried building my own InSe TB model, however, I didn't completely catch physical meaning in Silicon case, its about determination of occupations in spds* basis and orbital-chosen in hatree potentials. For example, Bassani's parameters of As atom in library(basis: spds*) do not show same Si occupation of # number_of_valence_electrons = 4, occupations= [0.4, 1.2, 2.0, 0.4] in other elements?  if so how to attain, e.g. As, occupations=[ 1.577 , 3.012 , 0.204 , 0.067 ]?

arsenic_onsite_term = SlaterKosterOnsiteParameters(
    element=PeriodicTable.Arsenic,
    angular_momenta=[ 0 , 1 , 2 , 0 ],
    occupations=[ 1.577 , 3.012 , 0.204 , 0.067 ], <====Q1.
    filling_method=SphericalSymmetric,
    ionization_potential=[ -5.9801*eV , 3.5813*eV , 12.1954*eV , 17.8411*eV ],
    onsite_hartree_shift=[ 7.81664*eV , 7.81664*eV , 7.81664*eV , 7.81664*eV ], <====Q2
    onsite_spin_split=[[-0.619, -0.43145, -0.43145, -0.619], [-0.43145, -0.3804, -0.3804, -0.43145], [-0.43145, -0.3804, -0.3804, -0.43145], [-0.619, -0.43145, -0.43145, -0.619]]*eV,
    onsite_spin_orbit_split=[0.0, 0.3526, 0.0, 0.0]*eV,     <=====Q3
    vacuum_level=0.0*Hartree,
    )

Second and third Quiz is. Why/How hartree or spin_split are determined by[ "?p", "?p", "?p", "?s"]
instead of basis assumption of ["3s", "3p", "3d", "3s"] in ionization_potential, and how to read ["3p"], 'ncp' in Si tutorials?
+++++++++++++++++++++++++++++++++++++++++++++
onsite_hartree_shift=ATK_U(PeriodicTable.Silicon, ["3p"], 'ncp')[0],
onsite_spin_split=ATK_W(PeriodicTable.Silicon, [ "3p", "3p", "3p", "3s" ]),
+++++++++++++++++++++++++++++++++++++++++++++

thank you for your kind assistance and waiting for your reply

[faxer92]

After reading old QATK2014 manual, I think Q2 and Q3 do no longer exist owing to lowest occupation. However, it remains unclear for me to reproduce occupations values of Q1. Would it be possible to escalate to creator  and clarify it? thank you very much   

[Petr Khomyakov]

Which version of QuantumATK are you using?

[faxer92]

Now I'm using latest QATK2019.3, however, in this version, build-in onsite-/off-site parameters of Vog, Bassian, Boykin TB parameters do not  reveal in code (why cannot be seen in 2019? I had chosen full script in generator,is proper method valid to call this from library?), so I roll back to 2017 (it indeed worked out~but not every time)  and listed associated SK parameters as mentioned above. 

[faxer92]

Sorry, Is there any further information I should provide?

[gpenazzi]

Hi faxer92,
I am not the creator of the set, but I am familiar with the tight binding behavior in QuantumATK, hopefully I can shed some light.

As you can read in the documentation, in QuantumATK we always support self-consistent calculations. For binary compounds you will end up in some charge transfer between anion and cation during an scf calculation. The occupations are chosen by running reference bulk calculations and inspecting the mulliken population, to make sure that the starting point of an scf calculation is close to this solution. If you run an scf calculation for bulk GaAs and check the Mulliken population, you will get numbers which are very close. The end result is not very sensible to initial population, therefore as long as the total occupation is good, you can avoid to worry about it too much. For monoatomic systems where all atoms are chemically equivalent (like Silicon), no charge transfer is expected and the single occupations of the orbitals make no difference, as long as the total occupation is correct.

For the shifts (hartree and spin) the choices are to some extent arbitrary. While orbital resolved quantities can be taken, it is not uncommon to use the values corresponding to the highest occupied orbital (this is for example the typical approach in some SCF-TB schemes like doi:10.1103/PhysRevB.58.7260). Using a single quantity can be a very close approximation which gives better behavior in terms of convergence. If I'd be setting up a parametrization myself, I'd probably start with a single value for the highest occupied orbital and look at the result.