[Pw_forum] vc-relax of graphite

[Jin Zhang]

Dear Li,

Normally to otain a optimized lattice constant, you do not need to perform
vc-relax. Instead,
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.

Best,
Jin Zhang

On 3/21/07, li niu <niuli1978 at yahoo.com.cn> wrote:
>
> dear Paolo,
>
> Thanks for your help.
> 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."
> So I tried to optimize lattice constant for graphite but always met
> errors.
> You said the good old way: fix c to the experimental value and only
> optimize a.
> How to excecute in the code?
>
> Thanks in advance!
>
> Niu Li
> ==================================================
> My input as follows:
> cat > graphite.vcrx.in << EOF
> &CONTROL
>   calculation  = "vc-relax",
>   prefix       = "graphite",
>   pseudo_dir   = "$PSEUDO_DIR",
>   outdir       = "$TMP_DIR",
>   tstress = .true. ,
>   tprnfor = .true. ,
>                        nstep = 65 ,
>                etot_conv_thr = 1.0E-4 ,
>                forc_conv_thr = 1.0D-3 ,
>                           dt = 100 ,
> /
> &SYSTEM
>     ibrav = 4,
>     celldm(1) =4.6336,
>     celldm(3)=2.7243,
>     nat= 4, ntyp= 1,
>     ecutwfc =40
> /
> &ELECTRONS
>    conv_thr    = 1.D-9,
>   mixing_beta = 0.7D0,
> /
> &IONS
>   pot_extrapolation = "second_order",
>   wfc_extrapolation = "second_order",
> /
> &CELL
>                cell_dynamics = 'damp-w',
>                        press = 0.0 ,
>                        wmass = 0.010 ,
>  /
> ATOMIC_SPECIES
>  C   12.0107  C.pz-vbc.UPF
> ATOMIC_POSITIONS {crystal}
>  C  0.0000000 0.0000000 0.0000000
>  C  0.0000000 0.0000000 0.5000000
>  C  0.6666667 0.3333333 0.0000000
>  C  0.3333333 0.6666667 0.5000000
> K_POINTS automatic
> 16 16 6 1 1 1
> EOF
> $ECHO " optimizing the lattice constant for bulk graphite...\c"
> $PW_COMMAND < graphite.vcrx.in > graphite.vcrx.out
> $ECHO " done"
>
>
>
>
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-- 
MSc
Dept. of Physics, Peking University
Beijing, 100871, P.R.China
Phone: 86-10-62768590
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