Re: [Phonopy-users] LDA vs PBE and phonons

[Sushil A. <sa...@gm...>]

Hi,
There are calculations that show that LDA gives better magnetic moments
than GGA.
Sushil Auluck

On 29 Oct 2016 7:23 a.m., "Huan Tran" <hu...@gm...> wrote:

> Bret,
>
> As mentioned in another post, a relaxed structure for an energy
> calculation may not be relaxed for a phonon calculation. Thus, a
> description of how to relax your structure is the first thing to look at.
> What DFT code did you use?
>
> Regards
> Huan
>
> On Fri, Oct 28, 2016 at 8:14 PM, Bret Hess <bre...@by...> wrote:
>
>> Haun, thanks so much for providing an explanation!
>>
>> To illustrate how different a result I get for  LDA vs PBE phonons in
>> rutile TiO2 (for the same degree of electronic convergence) in VASP, see
>> https://docs.google.com/presentation/d/1QmG3miCNhLmqIWEwTNfo
>> kajKVCaqmR3TdBM4mWlKDck/edit?usp=sharing
>>
>>
>> Bret
>>
>> On Fri, Oct 28, 2016 at 5:18 PM, Huan Tran <hu...@gm...> wrote:
>>
>>> Bret and All,
>>>
>>> LDA is not necessarily less accurate than GGA, especially in phonon
>>> calculations, due to numerical implementation. Here is a known reason,
>>> which can be found at http://www.quantum-espresso.or
>>> g/wp-content/uploads/Doc/ph_user_guide.pdf
>>>
>>> “The problem is [...] in the fact that the XC energy is computed in real
>>> space on a discrete
>>> grid and hence the total energy is invariant (...) only for translation
>>> in the FFT grid. Increasing
>>> the charge density cutoff increases the grid density thus making the
>>> integral more exact thus
>>> reducing the problem, unfortunately rather slowly...This problem is
>>> usually more severe for
>>> GGA than with LDA because the GGA functionals have functional forms that
>>> vary more
>>> strongly with the position; particularly so for isolated molecules or
>>> system with significant
>>> portions of “vacuum” because in the exponential tail of the charge
>>> density a) the finite cutoff
>>> (hence there is an effect due to cutoff) induces oscillations in rho and
>>> b) the reduced gradient
>>> is diverging.”(info by Stefano de Gironcoli, June 2008)
>>>
>>> Best
>>> Huan
>>>
>>>
>>> On Fri, Oct 28, 2016 at 4:49 PM, Bret Hess <bre...@by...> wrote:
>>>
>>>> Sushil, I understand that LDA is less accurate in general than PBE, but
>>>> I could not get the PBE to give good results for TiO2 phonons.  So for now
>>>> my strategy is to use LDA for phonons and PBE for everything else.
>>>>
>>>> Bret
>>>>
>>>>
>>>>
>>>> Message: 2
>>>> Date: Wed, 26 Oct 2016 06:36:38 +0530
>>>> From: Sushil Auluck <sa...@gm...>
>>>> Subject: Re: [Phonopy-users] LDA vs PBE and phonons
>>>> To: phonopy-users <pho...@li...>
>>>> Message-ID:
>>>>         <CAJ...@ma...
>>>> ail.com>
>>>> Content-Type: text/plain; charset="utf-8"
>>>>
>>>> Did you the equilibrium lattice constants obtained by GGA and PBE for
>>>> the
>>>> two respective calculations ?
>>>>
>>>> On 26 Oct 2016 5:08 a.m., "Bret Hess" <bre...@by...> wrote:
>>>>
>>>> > Hi, I really appreciate the great Phonopy code, Togo that I just
>>>> tried out
>>>> > this week.
>>>> >
>>>> > I benefited from the discussion on *Persistent imaginary (negative)
>>>> > frequency from phonon calculation
>>>> > <https://sourceforge.net/p/phonopy/mailman/message/34969163/>
>>>> *because I
>>>> > was getting lots of negative frequencies for TiO2 with PBE
>>>> > pseudopotentials.    I tried LDA, and it was much better.   I don't
>>>> > understand why, but I just wanted to add my datapoint to the
>>>> discussion.
>>>> >
>>>> > Thanks,
>>>> >
>>>> > Bret
>>>> >
>>>> > --
>>>> > Bret Hess
>>>> > Department of Physics and Astronomy
>>>> > Brigham Young University
>>>> > N251 ESC
>>>> > Provo, UT 84602
>>>> > 801-422-2108
>>>> > FAX 801-422-0553
>>>> >
>>>>
>>>> --
>>>> Bret Hess
>>>> Department of Physics and Astronomy
>>>> Brigham Young University
>>>> N251 ESC
>>>> Provo, UT 84602
>>>> 801-422-2108
>>>> FAX 801-422-0553
>>>>
>>>> ------------------------------------------------------------
>>>> ------------------
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>>>>
>>>>
>>>
>>> ------------------------------------------------------------
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>>>
>>
>>
>> --
>> Bret Hess
>> Department of Physics and Astronomy
>> Brigham Young University
>> N251 ESC
>> Provo, UT 84602
>> 801-422-2108
>> FAX 801-422-0553
>>
>> ------------------------------------------------------------
>> ------------------
>> The Command Line: Reinvented for Modern Developers
>> Did the resurgence of CLI tooling catch you by surprise?
>> Reconnect with the command line and become more productive.
>> Learn the new .NET and ASP.NET CLI. Get your free copy!
>> http://sdm.link/telerik
>> _______________________________________________
>> Phonopy-users mailing list
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>> https://lists.sourceforge.net/lists/listinfo/phonopy-users
>>
>>
>
> ------------------------------------------------------------
> ------------------
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> Did the resurgence of CLI tooling catch you by surprise?
> Reconnect with the command line and become more productive.
> Learn the new .NET and ASP.NET CLI. Get your free copy!
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