[Pw_forum] problem of the second derivative of energy of DFPT

[Stefano de Gironcoli]

using your input I get:

0.190 Ryd/bohr2 from energy vs displacement of frozen phonon
0.193 Ryd/bohr2 from force vs displacement of frozen phonon
0.191 Ryd/bohr2 from diagonal element of dynamical matrix computed by
phonon code.

pay attention that in your input alas.dx.scf.in the atom is displaced by
0.001 in the 111 direction so the lenght of your displacement is 10.60 *
0.001 * sqrt(3) bohr. I guess you overlooked that (0.571/3=0.190).
I don't know why you get 0.203 from the phonon calculation.

Stefano de Gironcoli


贺伟 wrote:

>Dear PWSCF user
>
>	I want to get the second derivatives of Born-Oppenheimer energy surface,
>but I found that dinsity functional perturbation mothod gave different result 
>than the stupid finite difference method.
>
>	My testing system is the AlAs bulk. In finite difference method, I first
>did a scf calculation with the relaxed geometry. Then I moved Al atom 1/1000 
>of celldm(1) along x and -x directions, and did scf calculations again. The 
>resulting three energies gave a FD second derivative of energy as 0.571 Ryd/Bohr^2. 
>But DFTP gave the second derivative as 0.203 Ryd/Bohr^2 (the first element of 
>the dynamic matrix).
>
>	Can anybody give me some suggestion? Thanks! I have attach the input files 
>with this email. 
>
>Best Wishes
>Wei He 	
>