<div dir="ltr"><div><div><div><div><div>Dear all,<br><br></div>I'm trying to optimize the atomic structure of GaP using QE. When I use a unit cell only containing two atoms, I have no problem in SCF convergence and the structure get optimized in the first step. But when I make my unit cell bigger (let's say a unit cell with 64 atoms) just by repeating my smaller optimized unit cell in real space and run relax calculation, the SCF calculation never get converged. Does anyone know where the problem is?<br><br></div><div>Following is my input:<br><br>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~<br>&CONTROL<br>  calculation = "relax",<br>  dt          = 30.D0,<br>  prefix = 'gap',<br>  pseudo_dir = './',<br>  outdir='./',<br>/<br> &system    <br>    ibrav= 6, celldm(1) =20.52, celldm(3) =1, nat=  64, ntyp= 2,<br>    ecutwfc = 12.0,   <br>  degauss     = 0.05D0,<br>  occupations = "fixed",<br>  smearing    = "methfessel-paxton",<br> /<br> &ELECTRONS<br>  conv_thr    = 1.D-7,<br>  mixing_beta = 0.3D0,<br>  electron_maxstep= 200,<br>/<br>&IONS<br>  pot_extrapolation = "second_order",<br>  wfc_extrapolation = "second_order",<br> /<br>ATOMIC_SPECIES<br>Ga  69.72  Ga.blyp-d-hgh.UPF<br>P  30.97  P.blyp-hgh.UPF<br>ATOMIC_POSITIONS (angstrom)<br>Ga          2.7146780000          2.7146780000          0.0000000000<br>Ga          0.0000000000          0.0000000000          0.0000000000<br>Ga          2.7146780000          0.0000000000          2.7146780000<br>Ga          0.0000000000          2.7146780000          2.7146780000<br>P          4.0720170000          4.0720170000          1.3573390000<br>P          1.3573390000          1.3573390000          1.3573390000<br>P          4.0720170000          1.3573390000          4.0720170000<br>P          1.3573390000          4.0720170000          4.0720170000<br>Ga          2.7146780000          2.7146780000          5.4293560000<br>Ga          0.0000000000          0.0000000000          5.4293560000<br>Ga          2.7146780000          0.0000000000          8.1440340000<br>Ga          0.0000000000          2.7146780000          8.1440340000<br>P          4.0720170000          4.0720170000          6.7866950000<br>P          1.3573390000          1.3573390000          6.7866950000<br>P          4.0720170000          1.3573390000          9.5013730000<br>P          1.3573390000          4.0720170000          9.5013730000<br>Ga          2.7146780000          8.1440340000          0.0000000000<br>Ga          0.0000000000          5.4293560000          0.0000000000<br>Ga          2.7146780000          5.4293560000          2.7146780000<br>Ga          0.0000000000          8.1440340000          2.7146780000<br>P          4.0720170000          9.5013730000          1.3573390000<br>P          1.3573390000          6.7866950000          1.3573390000<br>P          4.0720170000          6.7866950000          4.0720170000<br>P          1.3573390000          9.5013730000          4.0720170000<br>Ga          2.7146780000          8.1440340000          5.4293560000<br>Ga          0.0000000000          5.4293560000          5.4293560000<br>Ga          2.7146780000          5.4293560000          8.1440340000<br>Ga          0.0000000000          8.1440340000          8.1440340000<br>P          4.0720170000          9.5013730000          6.7866950000<br>P          1.3573390000          6.7866950000          6.7866950000<br>P          4.0720170000          6.7866950000          9.5013730000<br>P          1.3573390000          9.5013730000          9.5013730000<br>Ga          8.1440340000          2.7146780000          0.0000000000<br>Ga          5.4293560000          0.0000000000          0.0000000000<br>Ga          8.1440340000          0.0000000000          2.7146780000<br>Ga          5.4293560000          2.7146780000          2.7146780000<br>P          9.5013730000          4.0720170000          1.3573390000<br>P          6.7866950000          1.3573390000          1.3573390000<br>P          9.5013730000          1.3573390000          4.0720170000<br>P          6.7866950000          4.0720170000          4.0720170000<br>Ga          8.1440340000          2.7146780000          5.4293560000<br>Ga          5.4293560000          0.0000000000          5.4293560000<br>Ga          8.1440340000          0.0000000000          8.1440340000<br>Ga          5.4293560000          2.7146780000          8.1440340000<br>P          9.5013730000          4.0720170000          6.7866950000<br>P          6.7866950000          1.3573390000          6.7866950000<br>P          9.5013730000          1.3573390000          9.5013730000<br>P          6.7866950000          4.0720170000          9.5013730000<br>Ga          8.1440340000          8.1440340000          0.0000000000<br>Ga          5.4293560000          5.4293560000          0.0000000000<br>Ga          8.1440340000          5.4293560000          2.7146780000<br>Ga          5.4293560000          8.1440340000          2.7146780000<br>P          9.5013730000          9.5013730000          1.3573390000<br>P          6.7866950000          6.7866950000          1.3573390000<br>P          9.5013730000          6.7866950000          4.0720170000<br>P          6.7866950000          9.5013730000          4.0720170000<br>Ga          8.1440340000          8.1440340000          5.4293560000<br>Ga          5.4293560000          5.4293560000          5.4293560000<br>Ga          8.1440340000          5.4293560000          8.1440340000<br>Ga          5.4293560000          8.1440340000          8.1440340000<br>P          9.5013730000          9.5013730000          6.7866950000<br>P          6.7866950000          6.7866950000          6.7866950000<br>P          9.5013730000          6.7866950000          9.5013730000<br>P          6.7866950000          9.5013730000          9.5013730000<br>K_POINTS {automatic}<br>2 2 2 0 0 0<br>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~<br><br></div><div><br></div><div>Thanks in advance.<br></div><div><br></div>Regards,<br></div>Reza Vatan<br></div>School of Electrical, Computer and Energy Engineering<br></div>Arizona State University<br></div>