[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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