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 & Universit? Paris 6
phone:+33 (0)1 44275 084 / skype: paulatz
www: http://www-int.impmc.upmc.fr/~paulatto/
mail: 23-24/4?16 Bo?te courrier 115, 4 place Jussieu 75252 Paris C?dex 5
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