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

[贺伟]

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 	
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