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

[Ari P Seitsonen]

Hello Prem Sen et alia,

   Just a tiny note: If I remember correctly, C_2 has a level crossing 
(very?) close to the minimum distance. So one might need to be careful to 
choose the "correct" electronic ground state; I found this:

[doi: 10.1063/1.3624383]

"A particularly challenging example of this is the dissociation of the 
carbon dimer, where due to its multiple bond and significant 
multireference character even at equilibrium geometries, many methods 
struggle.3

   So Kohn-Sham method (the usual, single-determinant variant) might not be 
very accurate - it depends on which level of accuracy you need.

     Good Luck, :)

        apsi

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   Ari Paavo Seitsonen / Ari.P.Seitsonen at iki.fi / http://www.iki.fi/~apsi/
     Ecole Normale Supérieure (ENS), Département de Chimie, Paris
     Mobile (F) : +33 789 37 24 25    (CH) : +41 79 71 90 935


On Wed, 1 Feb 2017, Timrov Iurii wrote:

> 
> Dear Prem Sen,
> 
> 
> > 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 ??
> 
> I am not sure, but I would probably not rely on this approach. I think that if the system is gapped, then it should be possible to converge the
> ground state calculation without any intermediate calculation with a smearing.
> 
> > Also I have observed that if I am using hybrid functional, the system is converging normally (keeping the restart_mode = from_scratch & without
> "smearing").
> 
> This seems to indicate that with a hybrid functional you open a gap, and hence no smearing is needed in the calculation.
> 
> > Can I compute the absorption spectra for a molecule having triplet ground state using turbo_davidson.x or turbo_lanczos.x ??
> 
> 
> If this amounts to perform a spin-polarized calculation (and include magnetism), then these codes cannot be used because it is not
> implemented (currently only spin-unpolarized calculations can be performed with turbo_davidson.x and turbo_lanczos.x).
> 
> 
> Regards,
> 
> Iurii
> 
> 
> --
> Dr. Iurii Timrov
> Postdoctoral Researcher
> Swiss Federal Institute of Technology Lausanne (EPFL)
> Laboratory of Theory and Simulation of Materials (THEOS)
> CH-1015 Lausanne, Switzerland 
> 
> __________________________________________________________________________________________________________________________________________________
> From: pw_forum-bounces at pwscf.org <pw_forum-bounces at pwscf.org> on behalf of prem sen <premsen1502 at gmail.com>
> Sent: Tuesday, January 31, 2017 7:18 PM
> To: pw_forum at pwscf.org
> Subject: [Pw_forum] Re : Learning TDDFT calculation in Atomic label (CH4 molecule) using turbo_lanczos.x and turbo_davidson.x  
> Dear Dr. Iurii Timrov,
>                             Thanks for the reply.
> 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 ??
> 
> 
> Also I have observed that if I am using hybrid functional, the system is converging normally (keeping the restart_mode = from_scratch & without
> "smearing").
> 
> 
> Can I compute the absorption spectra for a molecule having triplet ground state using turbo_davidson.x or turbo_lanczos.x ??
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> Thanks & regards,
> Prem Sen
> Ph.D. Student,
> IIT Bombay,
> Mumbai,India.
> 
>