[QE-users] Error with IFC generation

[Lorenzo Paulatto]

>> Using, shengBTE I generate my displaced super-cell files, and then use
>> these as input files by Quantum Espresso for calculating the forces
>> arising from atomic displacements. Then, I generate the force
>> constants using those files. But, the force constants which I am
>> getting tends not to decay after a point.


Hello Aliya,
this is more likely a problem in ShengBTE, which is not associated with 
Quntum-ESPRESSO, I recommend you contact its authors to seek help. If I 
remember correctly, the 3 body force constants produced by ShengBTE are 
periodic, hence it will take many neighbors before observing a 
significant decay.

That said, you can compute three body force constants in QE via "2n+1" 
perturbation theory using the d3q package for a fraction of the cost 
http://sf.net/p/d3q
this code also includes some tools to "play" with 3-body FCs, which can 
be expressed in a periodic form or in a localized form, which assumes 
that forces go to zero for interactions further than the supercell. It 
can import (but not export) ShengBTE force constant files.

Anyway, it is quite complicated, even defining precisely what 
"localized" means, when dealing with three bodies, is a bit of an headache.

kind regards


-- 
Lorenzo Paulatto - Paris