[Pw_forum] why the results obtained from pwscf have so big difference with experiment

[xu yuehua]

hello :
 i am doing relax calculation, and my cell lattice parameter is obtained
from experiment.
The hexagonal crystal has unit cell dimensions 4.5181 Å (a) and 7.3560 Å (c)
(90º, 90º, 120º, 4 molecules, at 250 K) <ref4.html#r382>
the force on every atoms is OK
but the STRSS SECTION is:
 entering subroutine stress ...

          total   stress  (Ry/bohr**3)                   (kbar)     P=
-13.55
  -0.00009429  -0.00000007  -0.00000009        -13.87     -0.01     -0.01
  -0.00000007  -0.00008866  -0.00000020         -0.01    -13.04     -0.03
  -0.00000009  -0.00000020  -0.00009344         -0.01     -0.03    -13.75
why P is so big ?
I add K Pionts, but the results is still like that ?
should i do vc-relax?
thank you !


-- 
Xu Yuehua
physics Department of Nanjing university
China
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