[QE-users] Problem obtaining reliable cell constants from vc-relax (molecular crystals)

[Christoph Wolf]

Just out of curiosity I ran it over night using the recommended pseudos
from materialscloud.com (accuracy) and grimme-D3 vdw correction
(vdw_corr='grimme-d3', ) and get this

VC-Relax
In:
######################################################################################

     bravais-lattice index     =           14
     lattice parameter (alat)  =      23.9372  a.u.
     unit-cell volume          =    5001.5647 (a.u.)^3

     celldm(1)=  23.937161  celldm(2)=   0.723613  celldm(3)=   0.558380
     celldm(4)=   0.000000  celldm(5)=  -0.430669  celldm(6)=   0.000000

     crystal axes: (cart. coord. in units of alat)
               a(1) = (   1.000000   0.000000   0.000000 )
               a(2) = (   0.000000   0.723613   0.000000 )
               a(3) = (  -0.240477   0.000000   0.503944 )

######################################################################################
Out:

     bfgs converged in  57 scf cycles and  55 bfgs steps
     (criteria: energy <  1.0E-04 Ry, force <  1.0E-03Ry/Bohr, cell <
1.0E-01kbar)

     End of BFGS Geometry Optimization

     Final enthalpy =   -1047.0318536891 Ry
Begin final coordinates
     new unit-cell volume =   5050.42134 a.u.^3 (   748.39522 Ang^3 )
     density =      1.34154 g/cm^3

CELL_PARAMETERS (alat= 23.93716093)
   1.009807822   0.000229006   0.013789675
   0.000164505   0.724538992  -0.000018063
  -0.235821946  -0.000070070   0.500059155

unfortunately I am not sure if that is an improvement or not as I am too
lazy to analyze the structure in detail but all in all it seems to agree
rather well for such a complex unit cell.

The pseudos used were
ATOMIC_SPECIES
C 12.0107 C.pbe-n-kjpaw_psl.1.0.0.UPF
H 1.00794 H.pbe-rrkjus_psl.1.0.0.UPF
N 14.0007 N.pbe-n-radius_5.UPF
O 15.9994 O.pbe-n-kjpaw_psl.0.1.UPF

HTH!
Chris
-- 
Postdoctoral Researcher
Center for Quantum Nanoscience, Institute for Basic Science
Ewha Womans University, Seoul, South Korea
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