[Pw_forum] Constrained magnetic calculation

[Gabriele Sclauzero]

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 
> <mailto: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
>>     <mailto: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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