[Pw_forum] Re : Learning TDDFT calculation in Atomic label (CH4 molecule) using turbo_lanczos.x and turbo_davidson.x

[dario rocca]

Dear Prem,


I was trying in different way to converge scf calculation in case of carbon
> dimer without "smearing". First I converge the system using "smearing" then
> again I performed the scf calculation by changing the "restart_mode" from
> "from_scratch" to "restart". In this condition, the scf calculation for
> carbon dimer is converging & also turbo_davidson.x is possible. Do you
> think, this is a right approach ??
>

At equilibrium distance I would guess that this dimer has a finite gap and
smearing shouldn't be necessary. At large interatomic distances the gap
will go to 0
and you might need a smearing to converge. Are you interested in large
interatomic distances?
Your procedure is not too clear but as far as I understand you perform a
calculation with a smearing and then
you restart it removing the smearing. I'm surprised you don't get an error
message.
I think that as long as the ground state calculation converges without
smearing you can safely use the turbo-tddft code.



>
>
> Also I have observed that if I am using hybrid functional, the system is
> converging normally (keeping the restart_mode = from_scratch & without
> "smearing").
>


As Iurii said it is possible that the hybrid functional increases the value
of the gap and the smearing is not anymore necessary. However, if you keep
increasing
the interatomic distance you will have the same problem




>
>
> Can I compute the absorption spectra for a molecule having triplet ground
> state using turbo_davidson.x or turbo_lanczos.x ??
>

I don't think so since triplet states require spin polarized calculations
and the structure of the TDDFT Liouvillian is different in the case of
triple states.

You should first of all compute the PBE and hybrid gap for you system. If
your C2 is at equilibrium it should not have a ~0 gap
Best,
Dario Rocca





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> Thanks & regards,
> Prem Sen
> Ph.D. Student,
> IIT Bombay,
> Mumbai,India.
>
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