<div>Dear Li,</div>
<div> </div>
<div>Normally to otain a optimized lattice constant, you do not need to perform vc-relax. Instead,</div>
<div>doing several single scf calculation with different a (around experiment value, not too far away) but a fixed c, then fit the aquired data to get zero-pressure a. Hope this is what you want.<br><br>Best,</div>
<div>Jin Zhang</div>
<div> </div>
<div><span class="gmail_quote">On 3/21/07, <b class="gmail_sendername">li niu</b> <<a href="mailto:niuli1978@yahoo.com.cn">niuli1978@yahoo.com.cn</a>> wrote:</span>
<blockquote class="gmail_quote" style="PADDING-LEFT: 1ex; MARGIN: 0px 0px 0px 0.8ex; BORDER-LEFT: #ccc 1px solid">
<div>dear Paolo,</div>
<div><br>Thanks for your help.<br>In the paper that "The phonon dispersion of graphite revised [Solid State Communication 131 (2004) 141-152] ", there is a sentence that"For a meaningful comparison of phonon frequencies obtained by different calculations( using different pseudopotentials, basis-sets,parameteriations of the exchange-correlation functional), each calculation should be performed at the respective optimized lattice constant."
<br>So I tried to optimize lattice constant for graphite but always met errors.<br>You said the good old way: fix c to the experimental value and only optimize a.<br>How to excecute in the code?</div>
<div><br>Thanks in advance!</div>
<div> </div>
<div>Niu Li</div>
<div>==================================================</div>
<div>My input as follows:<br>cat > <a onclick="return top.js.OpenExtLink(window,event,this)" href="http://graphite.vcrx.in/" target="_blank">graphite.vcrx.in</a> << EOF<br>&CONTROL<br>  calculation  = "vc-relax",
<br>  prefix       = "graphite",<br>  pseudo_dir   = "$PSEUDO_DIR",<br>  outdir       = "$TMP_DIR",<br>  tstress = .true. ,<br>  tprnfor = .true. ,<br>                       nstep = 65 ,<br>               etot_conv_thr = 
1.0E-4 ,<br>               forc_conv_thr = 1.0D-3 ,<br>                          dt = 100 ,<br>/<br>&SYSTEM<br>    ibrav = 4, <br>    celldm(1) =4.6336, <br>    celldm(3)=2.7243,<br>    nat= 4, ntyp= 1,<br>    ecutwfc =40  
<br>/<br>&ELECTRONS<br>   conv_thr    = 1.D-9,<br>  mixing_beta = 0.7D0,<br>/<br>&IONS<br>  pot_extrapolation = "second_order",<br>  wfc_extrapolation = "second_order",<br>/<br>&CELL<br>               cell_dynamics = 'damp-w',
<br>                       press = 0.0 ,<br>                       wmass = 0.010 ,<br> /<br>ATOMIC_SPECIES<br> C   12.0107  C.pz-vbc.UPF</div>
<div>ATOMIC_POSITIONS {crystal}<br> C  0.0000000 0.0000000 0.0000000 <br> C  0.0000000 0.0000000 0.5000000<br> C  0.6666667 0.3333333 0.0000000<br> C  0.3333333 0.6666667 0.5000000<br>K_POINTS automatic<br>16 16 6 1 1 1</div>

<div>EOF<br>$ECHO " optimizing the lattice constant for bulk graphite...\c"<br>$PW_COMMAND < <a onclick="return top.js.OpenExtLink(window,event,this)" href="http://graphite.vcrx.in/" target="_blank">graphite.vcrx.in
</a> > graphite.vcrx.out<br>$ECHO " done"<br><br><br> </div>
<p>
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<p></p></p></blockquote></div><br><br clear="all"><br>-- <br>MSc<br>Dept. of Physics, Peking University<br>Beijing, 100871, P.R.China<br>Phone: 86-10-62768590