[Pw_forum] Single molecule relaxation vs changing the angle and performing SCF

[sarashs]

Hi,

I'm using quantum espresso v6.1 to relax a molecule (OH)3ZrOSi(OH)3. 
After the relaxation I change the ZrOSi angle around the equilibrium and 
perform SCF and compute the final energy value. However, to my surprise 
the minimum energy that I find by plotting the SCF data is 10 degrees 
larger than what I calculate with a relax calculation (the relax 
calculation matches the empirical data.) I tried fixing this issue by 
increasing the ecutwfc and ecutrho but I had no success. What is my 
problem?

Here's the input file:

&CONTROL
   calculation = "scf",
   outdir       = "outputs",
   prefix     ="ZrSi3",
   restart_mode="from_scratch"
   pseudo_dir = "pseudos",
/
&SYSTEM
   nosym       = .TRUE.,
   ibrav       = 0,
   nat         = 15,
   ntyp        = 4,
   ecutwfc     = 600,
   ecutrho = 6000,
   occupations = "smearing",
   smearing    = "gauss",
   degauss     = 0.05,
   vdw_corr    = "grimme-d2",
/
&ELECTRONS
   conv_thr    = 1.D-11,
   mixing_beta = 0.5D0,
/
ATOMIC_SPECIES
O   15.9994 O.pz-mt.UPF
H   1.00794 H.pz-vbc.UPF
Si  28.0855 Si.pz-vbc.UPF
Zr  91.224 Zr.pz-hgh.UPF
ATOMIC_POSITIONS { angstrom }
H       -0.217652040   2.144165004  -0.967842103
O       -0.644926428   1.299751587  -1.173517450
H       -0.763682000  -2.102234442  -0.637701677
O       -0.631971073  -1.266428593  -1.106959840
H       -2.244617658   0.059203828   1.041286276
O       -1.287708239   0.028876445   1.183017241
Si      -0.341076411   0.043743634  -0.163179170
O        1.183131599   0.097692502   0.401792709
Zr       2.959296806   0.743741575   0.268340451
H        3.161927401   3.483669731   0.077078720
O        2.777805281   2.601338899   0.023924414
H        4.228044154   0.110551898   2.670358608
O        4.003457303   0.315517992   1.757071324
H        3.597999032  -0.225640653  -2.168546302
O        3.766434111  -0.029092443  -1.238766538
K_POINTS { gamma }
CELL_PARAMETERS { angstrom }
13 0 0
0 13 0
0 0 13

Regards,

S. Arash Sheikholeslam
PhD candidate at electrical engineering department, University of 
British columbia