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

[Francesco Antoniella]

Il mer, 2004-01-07 alle 12:54, 贺伟 ha scritto:
> 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. 
Try to take a bigger dx and more points in your FD to improve the
accuracy of the derivative.
F. 
> 
> Best Wishes
> Wei He