<html><head></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">Dear Masoud,<div><br></div><div> first I would suggest you to use bfgs as the algorithm for both ions and cell dynamics. Excepted particular cases, it should reach the minimum much faster. </div><div><br></div><div>Also, why do you specify the cell with such an unusual way. You simply need celldm(1) and celldm(3) with ibrav=4 if you want to describe an hexagonal lattice. Other suggestions: your ecutrho looks really large to me, do you really need it. On the other hand, degauss might be too large to describe a spin-polarized system.</div><div><br></div><div>Then, are you sure that you have built correctly your supercell? It looks like there are some C-C bonds much shorter that others in the central graphene plane (1.2 instead of 1.4 angs). Please check again your structure. </div><div>In general, you can expect that if you relax the atoms in the supercell some kind of surface-reconstruction may appear, since you leave more freedom to atoms to rearrange in structures with larger periodicity. I don't think this is the case for graphite, but you may find some ripples (as you mentioned in your earlier emails, if I am not wrong) if the C-C bonds are at a distance shorter than the theoretical equilibrium distance (I guess).</div><div><br></div><div>HTH</div><div><br></div><div>GS</div><div><br></div><div><br><div><div>Il giorno 12/lug/2010, alle ore 20.24, Masoud Nahali ha scritto:</div><br class="Apple-interchange-newline"><blockquote type="cite">Dear Quantum Espresso Users<br><br>I vc-relaxed a (1*1) slab of graphite
surface with 3 layers; It takes 20 minutes with parallel running by 4
CPUs. Then I used the exact optimized cell parameters (obtained from
vc-relaxed calculation) to make a (2*2) slab of graphite with 3 layers
and I expected to see the results in a few minutes. But amazingly it
took 17 hours to complete. 48 steps were done in the calculation for
vc-relaxing the cell which have the parameters that had been optimized
before. The cell parameters only change a very bit in the current
vc-relaxing the (2*2) slab. I appreciate if one explain the physical
procedure of vc-relaxing and the reason of the time needed for the
computation.<br>
<br>input file:<br><br>CONTROL<br> calculation = "vc-relax",<br>
pseudo_dir = "/home/koa/soft/qe4.2/<div id=":tc" class="ii gt">espresso-4.2/pseudo",<br>
outdir = "/home/koa/tmp",<br> etot_conv_thr= 1.0D-4,<br>
forc_conv_thr= 1.0D-3,<br> dt=80,<br> /<br>&SYSTEM<br>
ibrav = 4,<br> a = 2.4579,<br> b = 2.4579,<br>
c = 16.3069,<br> cosab = -0.5,<br> cosac = 1.0,<br>
cosbc = 1.0,<br>
nat = 6,<br> ntyp = 1,<br> ecutwfc = 40.D0,<br>
ecutrho = 480.D0,<br> occupations = 'smearing'<br> smearing ='mp', <br>
degauss = 0.03,<br> nspin = 2,<br> starting_magnetization(1)= 0.003,<br>
london=.true.,<br> /<br>&ELECTRONS<br> conv_thr = 1.D-6,<br>
mixing_beta = 0.7D0,<br> diagonalization = "david",<br>/<br>&IONS<br> ion_dynamics="cg"<br>/<br>&CELL<br>cell_dynamics
= 'damp-w',<br>
press = 0.0,<br>/<br>ATOMIC_SPECIES<br>C 12.0107 C.pbe-rrkjus.UPF<br>ATOMIC_POSITIONS
{angstrom}<br> C 0.00000000 0.00000000
0.00000000 1 1 0 <br> C 0.00000000 1.41908472
0.00000000<br>
C 0.00000000 0.00000000 3.15347111<br> C
11.22896342 0.70954236 3.15347111<br> C
0.00000000 0.00000000 6.30694222<br> C
0.00000000 1.41908472 6.30694222<br>
K_POINTS {automatic}<br>4 4 1 1 1 1 <br><br>Sincerely Yours<br>Masoud Nahali<br>SUT<br><br>
</div>
_______________________________________________<br>Pw_forum mailing list<br><a href="mailto:Pw_forum@pwscf.org">Pw_forum@pwscf.org</a><br>http://www.democritos.it/mailman/listinfo/pw_forum<br></blockquote></div><br><div>
<span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-size: medium; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; "><div><span class="Apple-style-span" style="color: rgb(126, 126, 126); font-size: 16px; font-style: italic; "><br class="Apple-interchange-newline">§ Gabriele Sclauzero, EPFL SB ITP CSEA</span></div><div><font class="Apple-style-span" color="#7E7E7E"><i> PH H2 462, Station 3, CH-1015 Lausanne</i></font></div></span>
</div>
<br></div></body></html>