[QE-users] Pressure/Stress in vc-relax calculation

[Inna Martha Nangoi]

Dear all,
I'm proceeding with a vc-relax calculation with conv_thr = 10-6 ; energy and force default values. The pressure goes to zero but at the last structure the pressure increases to ~ -8 kbar. Since the pressure is negative one must think that the cell is expanding, but what it is observed (checking the cell parameters) is that the cell is contracting.Why is this happening? 
Thank you in advance,
Dr. Inna NangoiGrupo de Físico-Química de Sólidos e Interfaces (GFQSI)http://www.ufjf.br/gfqsi/Universidade Federal de Juiz de Fora - UFJF

INPUT: / &SYSTEM                       ibrav = 0,                   celldm(1) = 1.8897265,                         nat = 408,                        ntyp = 2,                     ecutwfc = 40 ,                     ecutrho = 400 , / &ELECTRONS                    conv_thr = 1.D-6 , / &IONS / &CELL /CELL_PARAMETERS cubic  16.467196779   0.003155548   0.004141842   0.003137055  16.453425288  -0.000761157   0.004171643  -0.000775474  16.446571996ATOMIC_SPECIES    O   16.00000  O.pbe-n-kjpaw_psl.1.0.0.UPF    H    1.01000  H.pbe-kjpaw_psl.1.0.0.UPFATOMIC_POSITIONS angstromO        9.321137481  14.266560377   9.327818241H        9.682105309  14.676153052   8.481823531H       10.049781829  14.333756337  10.015860139O       11.166283791   6.044399971   2.975926238..  .  K_POINTS gamma  


OUTPUT: 
     End of BFGS Geometry Optimization
     Final enthalpy =   -5995.1370960771 RyBegin final coordinates     new unit-cell volume =  28794.22023 a.u.^3 (  4266.86316 Ang^3 )     density =      0.95375 g/cm^3
CELL_PARAMETERS (alat=  1.88972650)  16.229014053   0.002449560   0.005745845   0.002429308  16.219588894   0.000479082   0.005772101   0.000461059  16.209758644
ATOMIC_POSITIONS (angstrom)O        9.180431407  14.143089383   9.191719262H        9.561522937  14.567241497   8.352842759H        9.908913197  14.170313644   9.891402550O       11.026600724   5.996092777   2.933422632H       11.877618746   6.446014321   2.622523705H       10.719968316   6.437111173   3.792164500O        5.141235135  15.134646177   1.987199104H        4.933636936  15.342422258   1.017207703H        5.853001846  14.414601411   1.987420436 . .   .    H       13.338033020   5.155154854  13.251553812O       11.079195321   9.183121298   6.146092164H       10.379755422   9.912994593   6.143398011H       11.912644022   9.559263869   5.711046728End final coordinates   .
   .     .    Total force =     0.008044     Total SCF correction =     0.000070

     Computing stress (Cartesian axis) and pressure

     negative rho (up, down):  9.616E-06 0.000E+00          total   stress  (Ry/bohr**3)                   (kbar)     P=   -8.32  -0.00005717  -0.00000010  -0.00000025         -8.41     -0.01     -0.04  -0.00000010  -0.00005654   0.00000006         -0.01     -8.32      0.01  -0.00000025   0.00000006  -0.00005604         -0.04      0.01     -8.24

          total   stress  (Ry/bohr**3)                   (kbar)     P=  -12.00          total   stress  (Ry/bohr**3)                   (kbar)     P=  -10.74          total   stress  (Ry/bohr**3)                   (kbar)     P=   -7.11          total   stress  (Ry/bohr**3)                   (kbar)     P=   -2.92          total   stress  (Ry/bohr**3)                   (kbar)     P=    0.65          total   stress  (Ry/bohr**3)                   (kbar)     P=    0.70          total   stress  (Ry/bohr**3)                   (kbar)     P=    0.30          total   stress  (Ry/bohr**3)                   (kbar)     P=    0.08          total   stress  (Ry/bohr**3)                   (kbar)     P=   -0.19          total   stress  (Ry/bohr**3)                   (kbar)     P=   -0.43          total   stress  (Ry/bohr**3)                   (kbar)     P=   -0.58          total   stress  (Ry/bohr**3)                   (kbar)     P=   -0.40          total   stress  (Ry/bohr**3)                   (kbar)     P=   -0.06          total   stress  (Ry/bohr**3)                   (kbar)     P=    0.25          total   stress  (Ry/bohr**3)                   (kbar)     P=    0.42          total   stress  (Ry/bohr**3)                   (kbar)     P=    0.43          total   stress  (Ry/bohr**3)                   (kbar)     P=    0.26          total   stress  (Ry/bohr**3)                   (kbar)     P=   -0.03          total   stress  (Ry/bohr**3)                   (kbar)     P=   -0.29          total   stress  (Ry/bohr**3)                   (kbar)     P=   -0.53          total   stress  (Ry/bohr**3)                   (kbar)     P=   -0.37          total   stress  (Ry/bohr**3)                   (kbar)     P=   -0.19          total   stress  (Ry/bohr**3)                   (kbar)     P=    0.18          total   stress  (Ry/bohr**3)                   (kbar)     P=    0.36          total   stress  (Ry/bohr**3)                   (kbar)     P=    0.50          total   stress  (Ry/bohr**3)                   (kbar)     P=    0.33          total   stress  (Ry/bohr**3)                   (kbar)     P=    0.06          total   stress  (Ry/bohr**3)                   (kbar)     P=   -0.09          total   stress  (Ry/bohr**3)                   (kbar)     P=   -0.34          total   stress  (Ry/bohr**3)                   (kbar)     P=   -0.28          total   stress  (Ry/bohr**3)                   (kbar)     P=   -0.10          total   stress  (Ry/bohr**3)                   (kbar)     P=    0.03          total   stress  (Ry/bohr**3)                   (kbar)     P=    0.25          total   stress  (Ry/bohr**3)                   (kbar)     P=    0.25          total   stress  (Ry/bohr**3)                   (kbar)     P=   -8.32



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