[Pw_forum] Constrained magnetic calculation
Varadharajan Srinivasan
varadharajan.srinivasan at gmail.com
Sat Mar 8 20:02:36 CET 2014
Dear Cyrille,
This is what we ended up doing setting all lambdas except the two magnetic
ions to zero. However, the constrained moments still seem to converge
rather slowly with lambda. I attach the plot below. We are also testing
this on a Fe-Cr system. We set the target moments to 5 for Fe and -3 for Cr
(antiferromagnetic arrangement). Is there any physical reason why this
would happen?
Best,
Vardha
On Thu, Mar 6, 2014 at 3:31 PM, BARRETEAU Cyrille
<cyrille.barreteau at cea.fr>wrote:
> In fact it can be very useful to constrain just some atoms and not all
> of them. We used that option a few years ago when studying the magnetic
> properties of Cr atoms in a Fe matrix. At that time we did it by hacking
> ad_bfield.f90. An alternative solution would be to define a different
> lambda(i) for each type of atom and set lambda(i)=0 for the atom which do
> not need to be constrained.
>
> regards
> Cyrille
>
>
> -----------------------------------------------------------------------------------------------------------
> *Cyrille Barreteau*
> CEA Saclay, IRAMIS, SPCSI, Bat. 462
> 91191 Gif sur Yvette Cedex, FRANCE
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> phone: +33 (0)1 69 08 29 51 / +33 (0)6 47 53 66 52
> fax : +33 (0)1 69 08 84 46
> email: cyrille.barreteau at cea.fr
> Web: http://iramis.cea.fr/Pisp/cyrille.barreteau/
>
> -----------------------------------------------------------------------------------------------------------
> ------------------------------
> *De :* pw_forum-bounces at pwscf.org [pw_forum-bounces at pwscf.org] de la part
> de Gabriele Sclauzero [gabriele.sclauzero at mat.ethz.ch]
> *Envoyé :* mercredi 5 mars 2014 10:38
> *À :* PWSCF Forum
> *Objet :* Re: [Pw_forum] Constrained magnetic calculation
>
> Dear Varadharajan,
>
> I do not know if it make sense or not in general, anyway this
> possibility is not available at the moment. All atoms of atomic type i will
> be constrained to have local magnetization equal to
> starting_magnetization(i). However, you can try to edit the magnetic
> constraint subroutine in PW/src/add_bfield.f90 to suit your needs.
>
> Regards
>
>
> GS
> Materials Theory, ETH Zurich
>
>
> On 03/05/2014 05:25 AM, Varadharajan Srinivasan wrote:
>
> Dear Gabriele,
>
> To add to Paresh's question is it possible (and does it make sense) to
> constrain the magnetisation of only a few atoms and not the others? While
> the target here seems to be atoms 3 and 4 the other atoms are being made to
> pay the price so to speak.
>
> One option, in the present framework, is to constrain the values of
> magnetisation of all other atoms to their respective lambda=0 values. Could
> this speed up the convergence with lambda?
>
> Thanks,
> Vardha.
>
>
> On Tue, Mar 4, 2014 at 7:18 PM, Gabriele Sclauzero <
> gabriele.sclauzero at mat.ethz.ch> wrote:
>
>>
>> What about the evolution of the constrained magnetization?
>> (Please also make sure that the values specified in
>> starting_magnetization make sense, as suggested by L. Paulatto Sir).
>>
>> My suggestion was to vary lambda in small steps (say 0.5). I'm surprised
>> that you managed to converge the calculation with such high lambda values.
>>
>> Anyway, the constrain energy looks way too large, your system is probably
>> still far from the target.
>>
>>
>> GS
>>
>>
>> On 03/04/2014 01:57 PM, paresh rout wrote:
>>
>> Respected Sclauzero sir,
>> Thanks for your reply. According to your suggestion, I varied my
>> Lambda value from 0,5,.......150 ry. Although calculated constrained
>> energy are decreasing but upto 150 ry the constrained energy and the
>> estimated scf accuracy are not the same order. Here I am providing my
>> constrained energy with various lambda value.
>>
>> Lambda Constraint_Energy
>> 0 0.00000000
>> 5 36.93411685
>> 10 69.54815816
>> 15 6.65653915 estimated scf accuracy < 7.6E-13 Ry
>> 20 7.88546052
>> 25 8.88385707
>> 30 9.71513061
>> 35 10.42697250
>> 40 11.05006563
>> 45 11.60072229
>> 50 12.08887057
>> 70 13.54966033
>> 80 14.05546257
>> 90 14.45159513
>> 100 14.75974550
>> 110 14.99680383
>> 120 15.17624003
>> 130 15.30876396
>> 140 15.40310437 estimated scf accuracy <
>> 9.9E-13 Ry
>> 150 15.46632278 estimated scf accuracy <
>> 9.9E-13 Ry
>>
>>
>>
>>
>> On Tue, Mar 4, 2014 at 3:06 PM, Sclauzero Gabriele <
>> gabriele.sclauzero at mat.ethz.ch> wrote:
>>
>>> Dear Paresh,
>>>
>>> in my understanding you should start with a very small lambda value
>>> (e.g. 0.1), make sure the calculation has converged (not always trivial),
>>> then restart with a larger value.
>>> It is important to tune the steps by which you increase lambda.
>>> Increasing it by steps of 5 seems too much to me, I would suggest you to
>>> try much smaller steps, say between 0.1 and 0.5.
>>>
>>> There are two reasons why the energy increases: the first is because you
>>> are constraining your system out of its ground state, but that's exactly
>>> what one would expect. The other is the contribution from the penalty
>>> energy (E_constrain, it should be printed after each scf step), which is
>>> used to impose the constraint.
>>> An important thing is that this energy term is not physical and becomes
>>> negligible once your system reaches the target state.Therefore one should
>>> monitor this constraint energy, together with the constrained quantity, and
>>> make sure it goes to zero at some point.
>>>
>>> Once lambda is large enough and you reached the targeted state,
>>> E_constrain should be negligible w.r.t. the total energy and of the same
>>> order of the estimated scf accuracy. From that point on, the energy should
>>> not change if you further increase lambda, because your system fulfills
>>> (almost) exactly the constraint, so that E_constrain should stay to a very
>>> low value.
>>>
>>> HTH
>>>
>>>
>>> GS
>>>
>>
>
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