[Pw_forum] Anharmonic terms - 3rd order derivatives of energy wrt position

[Nicola Marzari]

Dear Derek,

I think it's only 0 -q and q. The only way around it is to do a
supercell calculation, to fold into Gamma of the supercell points
outside Gamma of the primitive unit cell.

Also, it's norm-conserving only.


			nicola





stewart at cnf.cornell.edu wrote:

> Hi all,
>  I am interested in anharmonic properties of crystals and I would like 
> to calculate the third order derivatives of energy with respect to 
> changes in lattice position.  PWscf does a great job of calculating the 
> interatomic force constants.  I was wondering if it is possible to 
> calculate the 3rd order terms for arbitrary q,q', and q''.  I have 
> looked at the d3.x code, but I believe this program only calculates 
> terms of the form (0,q,-q).  I would be interested in any suggestions 
> people have on calculating these terms.  Is this feature in the works 
> for PWscf?  Also, can d3.x now handle ultrasoft pseudopotentials?
> Thanks in advance for your help!
> Best regards,
> Derek
> 
> ################################
> Derek Stewart, Ph. D.
> Scientific Computation Associate
> 250 Duffield Hall
> Cornell Nanoscale Facility (CNF)
> Ithaca, NY 14853
> stewart (at) cnf.cornell.edu
> (607) 255-2856
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