[Pw_forum] how to obtain a optimized lattice constant
Nichols A. Romero
naromero at gmail.com
Thu Mar 29 15:16:01 CEST 2007
Dear Niu,
Have you searched the mail postings for a similar topic?
Go to
www.pwscf.org
On the right hand panel, click on "Search Archive" under USERS FORUM. Search
for
"pw_forum lattice constant"
Do not know too much about phonon calculations, but in general they are much
more
delicate than a total energy calculations + structural minimization.
Please note that the stress tensor converges much slower than than the total
energy.
For example, for a norm-conserving pseudopotential. If your energy is
converged at 40 Ry,
the stress tensor may not converged (+/- 3 kbar) until you get to 80 Ry.
On 3/29/07, li niu <niuli1978 at yahoo.com.cn> wrote:
>
> Dear all,
> I have several questions on how to obtain a optimized lattice constant.
> I took the diamond as an example,
> when: C.blyp-mt.UPF
> ecutrho = 4*ecutwfc
> a = 6.74 (experimental value)
> K_POINTS AUTOMATIC 8 8 8 1 1 1
> 1.I did several scf and phonon calculations with different ecutwfc from 40
> to 80 Ry and found a convergence of the calculated frequencies at 55 Ry (or
> 60).[Is it right?]
> ====================================================
> ecutwfc total energy P= frequency( gamma
> point)
>
> --------------------------------------------------------------------------------------------------
> 40 Ry -22.54608438 Ry -324.91 1364.516269 [cm-1]
>
> ---------------------------------------------------------------------------------------------------
> 45 -22.58756098 P= -198.59 1356.978995 [cm-1]
>
> ---------------------------------------------------------------------------------------------
> 46 -22.59358560 Ry P= -177.09 1350.238444 [cm-1]
>
> -------------------------------------------------------------------------------------------------
> 48 -22.60333041 P= -139.82 1331.376654 (-11)
>
> ---------------------------------------------------------------------------------------------
> 49 -22.60713824 Ry P= -124.22 1320.862623 (-9)
>
> -------------------------------------------------------------------------------------------------
> 50 -22.61073198 -108.78 1311.304111 (9)
>
> -------------------------------------------------------------------------------------------------
> 51 -22.61399076 Ry P= -94.25 1302.308894 [cm-1]
> (8)
>
> -------------------------------------------------------------------------------------------------
> 52 -22.61688998 Ry P= -80.83 1294.840278 [cm-1]
> (4)
>
> -----------------------------------------------------------------------------------------------------
> 53 -22.61953601 Ry P= -67.95 1290.058731 [cm-1]
> (5)
>
> -----------------------------------------------------------------------------------------------------
> 54 -22.62200281 P= -55.48 1285.954996 [cm-1]
> (2)
>
> ------------------------------------------------------------------------------------------------------
> 55 -22.62410759 Ry P= -44.35 1283.736877 [cm-1]
> (1)
>
> -------------------------------------------------------------------------------------------------------
> 56 -22.62623466 P= -32.75 1282.451118 [cm-1]
> (0)
> -------------------------------------------------------------------------
> 57 -22.62815534 Ry P= -21.82 1282.144868 [cm-1]
> (0)
> -------------------------------------------------------------------------
> 58 -22.62986744 Ry P= -11.70 1282.755051 [cm-1]
> (0)
> -------------------------------------------------------------------------
> 59 -22.63154997 Ry P= -1.37 1284.145461 [cm-1]
> (-2)
>
> ---------------------------------------------------------------------------
> 60 -22.63297919 Ry P= 7.72 1285.831946 [cm-1]
> (-1)
>
> ----------------------------------------------------------------------------------------------
> 62 -22.63565052 Ry P= 25.81 1289.073779 [cm-1]
> (-4)
>
> ----------------------------------------------------------------------------------------------------
> 65 warning: symmetry operation # 5 not allowed.
> -22.63882875 Ry P= 49.83 1290.953300 [cm-1]
>
> -----------------------------------------------------------------------------------------
> 70 warning: symmetry operation # 5 not allowed.
> -22.64202864 P= 78.96 1295.485613 [cm-1]
>
> ---------------------------------------------------------------------------------------------
> 80 warning: symmetry operation # 5 not allowed.
> -22.64415626 Ry P= 101.58 1293.336207 [cm-1]
> ======================================================
> 2. Then I did several calculations with different a (around experiment
> value)
> when ecutwfc equal to 60 Ry. The results are follows.
>
> ecutwfc = 60
>
> -------------------------------------------------------------------------------------------------------
> a total energy
> frequency(experimental:1332)
> 6.74 -22.63297919 Ry P= 7.72 1285.831946 [cm-1]
> 6.75 -22.63324360 Ry P= -11.28 1279.754275 [cm-1]
> 6.76 -22.63347229 Ry P= -29.78 1274.313572 [cm-1]
> 6.80 -22.63386503 Ry P= -101.36 1252.534320 [cm-1]
> 6.85 -22.63347988 Ry P= -183.28 1226.237110 [cm-1]
>
> -----------------------------------------------------------------------------------------------------
> 3. The potential energy surface is then fitted by the murnaghan equation (
> ev.x),
> and its mininum provides the theoetical precictions for the ground state
> equilibrium
> lattice constant.
>
> --------------------------------------------------------------------------------------------------------
> # equation of state: murnaghan. chisq = 0.1134D-10
> # a0 = 6.80 k0 = 3746 kbar, dk0 = 14.50 d2k0 = 0.000 emin = -
> 22.63387
>
> --------------------------------------------------------------------------------------------------------
> 4. In the end, I obtained optimized lattice constant is 6.8 when the
> energy cutoff is 60 Ry. In this case the phonon frequency of gamma piont
> equals to 1252 cm-1 but experimental is 1332 cm-1 and the stress-tensor is
> -101 kbar but not is 0.
>
> My questions are:
> (1)Is it right to choose the energy cutoff like this?
> (2) Why the stress-tensor is so large?
> (3) How should I do to obtain the correct optimized parameters?
>
> I'm puzzled by these questions all the times so I hope that you can help
> me.
>
> Thanks in advance!
>
> Bset!
> Niu Liu
>
> ========================================================
> # self-consistent calculation
> cat >diamond.scf.in << EOF
> &control
> calculation='scf'
> restart_mode='from_scratch',
> prefix='diamond',
> pseudo_dir = '$PSEUDO_DIR/',
> outdir='$TMP_DIR/'
> tstress = .true. ,
> tprnfor = .true. ,
> /
> &system
> ibrav= 2, celldm(1) =6.74, nat= 2, ntyp= 1,
> ecutwfc =60,
> /
> &electrons
> conv_thr = 1.0d-9
> mixing_beta = 0.7
> /
> ATOMIC_SPECIES
> C 12.0107 C.blyp-mt.UPF
> ATOMIC_POSITIONS
> C 0.00 0.00 0.00
> C 0.25 0.25 0.25
> K_POINTS AUTOMATIC
> 8 8 8 1 1 1
> EOF
> $ECHO " running the scf calculation...\c"
> $PW_COMMAND < diamond.scf.in > diamond.scf.out
> $ECHO " done"
> # phonon calculation at G
> cat >diamond.phG.in << EOF
> phonons of C at Gamma
> &inputph
> tr2_ph=1.0d-14,
> epsil=.true.,
> prefix='diamond',
> fildyn='diamond_g.dyn',
> amass(1)=12.0107,
> outdir='$TMP_DIR/'
> /
> 0.0 0.0 0.0
> EOF
> $ECHO " running the phonon calculation at Gamma...\c"
> $PH_COMMAND < diamond.phG.in > diamond.phG.out
> $ECHO " done"
> $ECHO
> $ECHO "$EXAMPLE_DIR: done"
> ================================================
>
> ------------------------------
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>
--
Nichols A. Romero, Ph.D.
1613 Denise Dr. Apt. D
Forest Hill, MD 21050
443-567-8328 (C)
410-306-0709 (O)
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