[Pw_forum] Constrained magnetic calculation

[Sclauzero Gabriele]

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

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>> wrote:
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
    constrained_magnetization='atomic'
    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 spins
                   down for the valence electrons of atom type 'i') to 1
                   (all spins up). Breaks the symmetry and provides a starting
                   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 magnetization
                   (see "tot_magnetization" and "constrained_magnetization").
                   Note that if you start from zero initial magnetization, you
                   will invariably end up in a nonmagnetic (zero magnetization)
                   state. If you want to start from an antiferromagnetic state,
                   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 previous
                   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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Dr. Gabriele Sclauzero
Materials Theory - ETHZ
ETH Zurich, HIT G 43.2
Wolfgang-Pauli-Str. 27
8093 Zürich, Switzerland

Phone +41 44 633 94 10
Fax +41 44 633 14 59
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