[Pw_forum] about the precision of energy and some extend problem.

[xu yuehua]

hi ,uesers here :
i relax one crystal lattice vs energy:
i have compute several energy list :

c          E(RY)
2.795  -137.79266
2.78   -137.79248
2.77   -137.79233
2.8     -137.7927
2.81  -137.79275
2.82  -137.79275
2.83  -137.79278
2.84  -137.79276
2.85  -137.79279
2.86  -137.79272
2.87  -137.79265
2.88  -137.79258
2.9   -137.79229
2.95  -137.79122
these energy points are smooth and  can be plotted  parabola.
the system has 12 atoms.the energy is different most at the third digit
after radix point. so i think the average energy/atom is less 0.0001.

i want to ask  the  within the precision of energy(my thinking is based on
that :my above calculation is set ecutwfc:29, ecutrho:200,and the  criterion
is 35,350, the convergence is 0.0049345 ry.)
so this small difference of energy can skip,i.e we can choose 2.77 which is
regarded as the best structure?
 as why i ask this question,for the the structure of 2.85 has two  big
negative frequencies  near -100cm-1, even if i add asr .
but 2.77 has no negative frequency after i add asr.


-- 
Xu Yuehua
physics Department of Nanjing university
China
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20071212/942e2fee/attachment.html>