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MD step update and mixing weight; specification or problem?
Date: 2017/08/01 18:10
Name: Asako Terasawa

Dear those who concerned,

I found the following issue about the update of mixing weight in standard output log.

The mixing-related parameters in the input file is as follows.

scf.Mixing.Type rmm-diisk # Simple|Rmm-Diis|Gr-Pulay
scf.Init.Mixing.Weight 0.0020 # default=0.30
scf.Min.Mixing.Weight 0.0001 # default=0.001
scf.Max.Mixing.Weight 0.3000 # default=0.40


As intended, scf calculation started from initial mixing weight:

******************* MD= 1 SCF= 1 *******************
...
<DFT> Mixing_weight= 0.002000000000


Then, scf calculation in MD = 1 ended at a larger mixing weight:

******************* MD= 1 SCF=114 *******************
...
<DFT> Mixing_weight= 0.030000000000


However, when updated, the mixing weight starts from the maximum weight:

******************* MD= 2 SCF= 1 *******************
...
<DFT> Mixing_weight= 0.300000000000


It is understandable that this may cause just a minor effect if a single MD update causes a very small change in electronic states, which may not be the case sometimes.
So, I want to know that if this behavior is intended or not.

Thanks,
AT
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Re: MD step update and mixing weight; specification or problem? ( No.1 )
Date: 2017/08/01 23:11
Name: T. Ozaki

Hi,

During MD, the initial charge density at each MD step is obtained by extrapolation using
charge densities at previous steps. So, in many cases the estimated charge density
can be a good approximation for the converged one, and we expect that convergence can be
obtained even using a large mixing weight. That's why Mixing_weight is set to the maximum
weight.

However, as you worry, such a treatment may cause charge sloshing problem again for some
cases. For such cases it might be better to use a relatively smaller scf.Max.Mixing.Weight.

If you do not want such a behavior, you only have to comment out the following line
of 905 in DFT.c:

if (MD_iter!=1) Mixing_weight = Max_Mixing_weight;

If you do that, Mixing_weight is not set to Max_Mixing_weight.

Regards,

TO
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Re: MD step update and mixing weight; specification or problem? ( No.2 )
Date: 2017/08/02 12:33
Name: Asako Terasawa

Hello,

Thank you for your very helpful answer.
Now I and one of colleague are trying both suggestions and checking the behavior of NormRD and mixing_weight, because we are treating systems which easily diverge.
I will let you know again if I get any suggestive results.

Regards,
AT
メンテ

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