# [Pw_forum] LDA+U in md calculations

Jiayu Dai daijiayu at nudt.edu.cn
Sat Apr 3 13:08:40 CEST 2010

Dear Matteo,

Thanks for your comments. I am using the BOMD (pwscf). In my case, at high density, i would expect the U should be different with the change of ionic structures. In particular, i am concerning about the pressure. I found a strange thing: the pressure with LDA+U is lower than the pressure without U adding. As my experience, th pressure should be larger when we added the U parameter, is it right?

Besides, all the cases using LDA+U is about the transition metals, which has the d or f electrons. How about the s or p electrons?

Thanks.

Jiayu

>

Dear Jiayu,

are you talking about Car-Parrinello MD or Born-Oppenheimer MD (run with
pwscf)?
In any case lda+U MD is implemented in the sense that the code computes
the forces and the stress
as derivatives of  the lda+U energy functional. However it is not able
to account for the variation
of U with atomic position or with the cell. So it is a dynamics at fixed U.
To account for this variation one would need to evaluate dU/dR or
dU/d\epsilon (R and \epsilon being atomic
positions and strains). Some work in this direction has been recently
done (not sure it's
published yet) by Kulik and Marzari.
If the structure you are interested in doesn't change dramatically
during the MD run I would expect
that keeping the same U is not too bad. but this is just my guess.

regards,

Matteo

Jiayu Dai wrote:
> Dear developers,
>
> I have some doubts about the molecular dynamics calculations using LDA+U. First, is LDA+U performed in MD calculations? Because i did not find the Hubbard energy in the output file of MD. Second, i think the value of U be dependent on the temperature and density, also on the ionic structures. Therefore, since ionic positions are different in each time step, is U different at each time step? If so, the performance of LDA+U should be very difficult in MD.
>
> I did not know if what i said is right. Hope experts comment on this problem.
>
> Thanks a lot.
>
> Jiayu
> -------------------------------------------
> Jiayu Dai
> Department of Physics
> National University of Defense Technology,
> Changsha, 410073, P R China
> -----------------------------------------
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