[Pw_forum] Phonons in 2D boron nitride

[Lorenzo Paulatto]

On 11/12/17 23:21, Francesco Delodovici wrote:
> 1) the out-of-plane acoustic branch at low wave vector is not quadratic, 
> istead is linear.

This would indicate that your mono-layer structure is strained, although 
you say you did a vc-reax, you may try to double-check this part. What 
if you relax the bulk and then you use the bulk parameter? Anyway, the 
branch will always be linear very close to Gamma, because we are using 
are really doing a 3D calculation, with vacuum, using more q-points may 
also help.

> 2) Since I don't include the flag    epsil='true'     there is no LO-TO 
> splitting, but the slope of the LO branch is vanishing at Gamma

Otherwise, you may want to check the work of Thibault Sohier, Matteo 
Calandra, and Francesco Mauri. Phys. Rev. B 91, 165428. It is 
implemented in the development version of the code, and it should be in 
in the upcoming 6.2.1 version of QE, check PHonon/examples/example17

hth


-- 
Lorenzo Paulatto - Paris