[Pw_forum] Relaxation giving negative relative coordinates.

[Stefano de Gironcoli]

there is nothing strange in negative positions
anyway the system is periodic so you can remap them on the  positive 
axes if you like,

stefano

On 05/16/2011 12:17 PM, Christopher Heard wrote:
> 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?
>
> -----------------------------------------------------------------------------------------------------------------
> 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}
> -----------------------------------------------------------------------------------------------------------------
>
>
> -----------------------------------------------------------------------------------------------------------------
> 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
>
> -----------------------------------------------------------------------------------------------------------------
>
>
> Many thanks,
> Chris
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