# [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.
>
>
> 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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