<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 > graphite.vcrx.in << 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 < graphite.vcrx.in > graphite.vcrx.out<BR>$ECHO " done"<BR><BR><BR></div><p>
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