[Pw_forum] Constrained magnetic calculation
varadharajan.srinivasan at gmail.com
Wed Mar 5 05:25:19 CET 2014
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?
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.
> 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.
>> Respected Lorenzo sir,
>> Thanks for your replay but my question was how to fix the
>> proper LAMBDA value for any constrained system as the energy is increasing
>> with increase in lambda value.Please suggest me something regarding lambda.
>> On Mon, Mar 3, 2014 at 5:54 PM, Lorenzo Paulatto <
>> lorenzo.paulatto at impmc.upmc.fr<mailto:lorenzo.paulatto at impmc.upmc.fr>>
>> On 03/03/2014 12:39 PM, paresh rout wrote:
>> Dear all,
>> I am doing spin polarized calculations on a multiferroic
>> compound . In some cases I am doing constrained magnetic calculations to
>> get the Low-spin and High-spin state . For this I am using
>> starting_magnetization(1) = 5.0
>> starting_magnetization(2) = -3.0
>> starting_magnetization(3) = 0.0
>> starting_magnetization(4) = 0.0
>> lambda =0,5,10,20,25,........etc
>> Variable: starting_magnetization(i), i=1,ntyp
>> Type: REAL
>> Description: starting spin polarization on atomic type 'i' in a spin
>> polarized calculation. Values range between -1 (all
>> down for the valence electrons of atom type 'i') to 1
>> (all spins up). Breaks the symmetry and provides a
>> point for self-consistency. The default value is zero,
>> BUT a
>> value MUST be specified for AT LEAST one atomic type
>> in spin
>> polarized calculations, unless you constrain the
>> (see "tot_magnetization" and
>> Note that if you start from zero initial
>> magnetization, you
>> will invariably end up in a nonmagnetic (zero
>> state. If you want to start from an antiferromagnetic
>> you may need to define two different atomic species
>> corresponding to sublattices of the same atomic type.
>> starting_magnetization is ignored if you are
>> performing a
>> non-scf calculation, if you are restarting from a
>> run, or restarting from an interrupted run.
>> If you fix the magnetization with "tot_magnetization",
>> you should not specify starting_magnetization.
>> kind regards
>> Dr. Lorenzo Paulatto
>> IdR @ IMPMC -- CNRS & Université Paris 6
>> phone:+33 (0)1 44275 084 / skype: paulatz
>> www: http://www-int.impmc.upmc.fr/~paulatto/
>> mail: 23-24/4é16 Boîte courrier 115, 4 place Jussieu 75252 Paris Cédex 5
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