# [Pw_forum] Constrained magnetic calculation

paresh rout paresh.rout88 at gmail.com
Tue Mar 4 13:57:09 CET 2014

Respected Sclauzero   sir,
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),
> 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
> gabriele.sclauzero at mat.ethz.ch<mailto:gabriele.sclauzero at mat.ethz.ch>
> www.theory.mat.ethz.ch
>
>
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