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

[Wei He]

Dear Francesco Antoniella

	Thank you very much for your suggestion. 

	But increasing dx to 2/1000 or 1/100 or using 4-order FD 
do not improve the result or even give a worse result. Another thing
puzzling me is that the energies are not strictly equal for movements 
along x and -x direction.

Sincerely,
Wei He
	

======= 2004-01-07 20:19:11 =======

>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 	
>
>
>
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　　　　　　　　致
礼！
 
				 
　　　　　　　　Wei He
　　　　　　　　weihe at mail.ustc.edu.cn
　　　　　　　　　　2004-01-07