[Pw_forum] how to obtain a optimized lattice constant
li niu
niuli1978 at yahoo.com.cn
Thu Mar 29 09:02:20 CEST 2007
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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