[Pw_forum] Regarding structural optimization

[kondaiah samudrala]

Dear Sir,

(1) Here i am running Black phosphorous, which is layered compound
crystallizes in orthorhombic structure and reported that having huge van
der waals forces. Generally systems having van der waals forces shows
huge volume change( either 10-12% underestimate in LDA or 5-6% overestimate
in GGA)  with experimental volume in DFT calculations. In my case, CASTEP
and VASP i got similar variation in volume. But in PwSCF, i got 2%
volume change with experimental volume. Here i used 50Ry cutoff and
10X10X10 K-mesh is used for my calculations. For energy minimum i used
up to 10e-8 . Where as in "vc-relax" calculations i got 10% volume
change with experimental volume.

 please give your suggestions for my problem. If it is need i will send
my b/a, c/a and a calculation input  and output files to your mail.

(2) What is the difference in structural optimization between normal
process( optimizain c/a, b/a and a with energy) and "vc-relax"?



Thanks in advance.


with regards
S.Appalakondaiah
Research scholar
University of Hyderabad
Hyderabad
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