[Pw_forum] a complex and old question about why the erengy is down when i enlarge one of the crystal lattice

[xu yuehua]

hi everyone:
my question is somewhat old ,i have seen a lot of related discussion in this
forum .but my results does not like previous instance.
i only want to relax c of crystal lattice ,and fixed c,computes it,and then
change it  ,compute it,and .....
i get the  E VS C tabular
so strange is that :

c               E
2.655     -137.78747
2.725     -137.7912
2.76      -137.79219
2.77       -137.79233
2.78      -137.79248
2.795      -137.79266
2.8         -137.7927
2.81       -137.79275
2.82       -137.79275
2.83       -137.79278
2.84       -137.79276
2.85       -137.79279
(1)the energy is down  before 2,81,and c=2.83 is like local minimum
,and then the energy is go on to down. the graph is smooth ,but   i can not
get the parabola shape .so the best relaxed structure is unkown to me.
  and  i saw many discuss about the down energy ,maybe the  ecutrho
issue.  but i am not sure .if ti is ,i should test the ecutrho?i post my
input file here:
&CONTROL
calculation  = "relax",
  prefix       = "xyh",
  pseudo_dir   = "/home/xyh/intel_espresso-3.2/pseudo",
  outdir       = "/raid/xyh/pwscf/tmp/",
  etot_conv_thr=1.D-5
  forc_conv_thr=1.D-4
  nstep=50
  tstress=.TRUE.
/
&SYSTEM
  ibrav     = 4,
  a=20,b=20,c=2.7950,cosab=-0.5,cosac=0,cosbc=0,
  nat       = 12,
  ntyp      = 2,
  ecutwfc   =29.9876 ,
  ecutrho   =200
/
&ELECTRONS
  conv_thr    = 1.D-9,
  mixing_beta = 0.2D0,
/
&IONS
  pot_extrapolation = "second_order",
  wfc_extrapolation = "second_order",
/
 ATOMIC_SPECIES
H  1.0  H.pbe-van_bm.UPF
O 15.999 O.pbe-van_bm.UPF
ATOMIC_POSITIONS { crystal }
H  0.0373836515057430  0.9503896146857272  0.0102606085503574
H  0.1115317646909714  0.9930799089192295  0.3344160938275307
H  0.0789318950169224  0.0718359337861800  0.9884868039481935
H  0.0724243415282819  0.1328116898677112  0.6645236821235255
H  0.9626484865634389  0.0496365043168296  0.0101367101334339
H  0.8884876867936907  0.0069387333102180  0.3342097796899214
H  0.9210924986163674  0.9281832378776828  0.9882981504770443
H  0.9276176392628244  0.8672101853812219  0.6643346513014633
O  0.0926597580972745  0.9929556282114975
......


..
(2)and another question is although i can not get the lowest erengy,but i
can get the parabola of  the stress. maybe someone the stress is not
consistent with energy,for i only want to    know  which  structure is best
,so the exactly stress is not so import for me, i only want to know the
trend .in this point,whether i can rely on the stress to determine the best
structure.

i need your help.
thanks
 --
Xu Yuehua
physics Department of Nanjing university
China
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