[Pw_forum] Relaxation giving negative relative coordinates.

[Christopher Heard]

Hello,

I am running QE for a small atomic cluster, in a single cubic unit cell, with the 'relax' keyword, just a simple local minimisation.
The atomic coordinates are fractional coordinates of the unit cell, but the problem is that the output gives some negative values.
I'm new to QE, so perhaps this is easily explained (if so, sorry), but it seems strange that the simulation would converge, and how does the code know what positions/energies negative values correspond to?

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INPUT
&CONTROL
     calculation  = "relax",
     prefix       = "pwscf",
     pseudo_dir   = ".",
     outdir       = ".",
     etot_conv_thr=  1.D-3,
     forc_conv_thr=  1.D-2,
     nstep        =  200,
/
&SYSTEM
     ibrav        =  0,
     nat          =  4,
     ntyp         =  2,
     ecutwfc      =  20.D0,
     occupations  = "smearing",
     smearing     = "mp",
     degauss      =  0.6,
/
&ELECTRONS
     electron_maxstep     =  100,
     conv_thr     =  1.D-6,
     mixing_beta  =  0.3,
/
&IONS
/
CELL_PARAMETERS cubic
 6.70  0.00  0.00
 0.00  6.70  0.00
 0.00  0.00  6.70
ATOMIC_SPECIES
Cu   63.55  Cu.pbe-d-rrkjus.upf
Ag  107.80  Ag.pbe-d-rrkjus.upf
ATOMIC_POSITIONS {crystal}
 Cu  3.352950000000000E-002  0.231848250000000       0.909548000000000
 Cu  0.385636250000000       0.891478500000000       9.939174999999999E-002
 Cu  0.205962500000000       0.843170750000000       0.568072500000000
 Ag  0.109203000000000       5.366124999999999E-002  5.051524999999999E-002
K_POINTS {Gamma}
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SAMPLE OF FINAL OUTPUT

     the Fermi energy is    19.5731 ev

!    total energy              =    -336.07301310 Ry
     Harris-Foulkes estimate   =    -336.07301304 Ry
     estimated scf accuracy    <          2.4E-09 Ry

     The total energy is the sum of the following terms:

     one-electron contribution =     -15.71264533 Ry
     hartree contribution      =      65.46190146 Ry
     xc contribution           =     -55.48851306 Ry
     ewald contribution        =    -330.01856694 Ry
     - averaged Fock potential =       0.00000000 Ry
     + Fock energy             =       0.00000000 Ry
     smearing contrib. (-TS)   =      -0.31518923 Ry

     convergence has been achieved in   6 iterations

     Forces acting on atoms (Ry/au):

     atom   1 type  1   force =    -0.00120005   -0.00208895   -0.00171136
     atom   2 type  1   force =    -0.00716149   -0.00098208    0.00204276
     atom   3 type  1   force =     0.00892231    0.00566633    0.00154815
     atom   4 type  2   force =    -0.00056077   -0.00259530   -0.00187956

     Total force =     0.013779     Total SCF correction =     0.000064

     bfgs converged in  41 scf cycles and  30 bfgs steps
     (criteria: energy < 0.10E-02, force < 0.10E-01)

     End of BFGS Geometry Optimization

     Final energy   =    -336.0730131033 Ry
Begin final coordinates

ATOMIC_POSITIONS (crystal)
Cu      -0.050437709   0.504314433   0.629069336
Cu       0.445520384   0.504573287   0.216995562
Cu       0.419644090   1.004288078   0.669422976
Ag      -0.080395515   0.006982952   0.112039626
End final coordinates

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Many thanks,
Chris