[Pw_forum] heat of formation for Mg3N2

[Xiangmei Duan]

Dear all,

I've opotimized \alpha-Mg3N2 with cubic anti-bixbyite structure (J. Mater. 
Chem., 12, 2475, 2002)using espresso-2.1.5,
and got the reasonable lattice constant 9.896 Ang. (LDA) (exp: 9.953 Ang).
When I calculated the heat of formation (in terms of the bulk Mg and N2),
I was expected to get an absolute value larger than the experimental one,
for the well known overbinding of LDA. In fact, I got -2.91 eV (exp. -4.80 
eV).
(we tested with LDA calculation using other code like Dmol^3, and got 
-5.38 eV with the lattice parameter 9.942 A)

For N2, the calculated bond length is 1.103 AA, and the binding energy is 
11.11 eV, and the exp. values 1.098 A and  9.91 eV

For Mg bulk, the lattice constants are 3.1317 A (a) and 5.21 (c). 
(comparing with 3.21 and 5.21)

Thanks in advance for your comments.

Best wishes,
Xiangmei

Here is the input for Mg3N2:
-----------------------------------

  &control
     calculation = 'relax'
     restart_mode='from_scratch',
     prefix='Mg3N2',
     tstress = .true.
     tprnfor = .true.
     pseudo_dir = '/home/562/xxd562/pw_potential/',
     outdir='/short/g46/Mg3N2/'
  /
  &system
     ibrav=  0, celldm(1) =0.0,
     nat=  80, ntyp= 2, ecutwfc =60.0,
  /
  &electrons
     mixing_beta = 0.5
     conv_thr =  1.0d-6
  /
  &ions
  /
ATOMIC_SPECIES
  N   14.0067   N.pz-rrkjus.UPF  (!!! I tried N.pz-vbc.UPF as well)
  Mg  24.3050   Mg.pz-n-vbc.UPF
ATOMIC_POSITIONS {bohr}
Mg      -2.859307108   2.203853843   7.281662824
Mg       2.854200083   7.144760834   7.281662591
Mg      -2.857214509  -2.207201820   2.065435928
Mg       2.852106059  -7.148768053   2.064960719
Mg       7.281662824  -2.859307108   2.203853843
Mg      -7.284950160  -6.496093980   2.204067017
Mg       7.281662591   2.854200083   7.144760834
Mg      -7.284813079   6.493789879   7.144043305
Mg       2.203853843   7.281662824  -2.859307108
Mg       2.204067017  -7.284950160  -6.496093980
Mg      -2.206521406  -2.069113782  -6.497311905
Mg       7.144043305  -7.284813079   6.493789879
Mg       2.854906875  -2.208126838  -7.284612512
Mg      -2.857632220  -7.147335003  -7.285149136
Mg       2.853458071   2.203185880  -2.069725394
Mg      -2.856132703   7.144985498  -2.069587582
Mg      -7.284612512   2.854906875  -2.208126838
Mg       7.281876795   6.491954164  -2.206999931
Mg      -7.285149136  -2.857632220  -7.147335003
Mg       7.282657619  -6.496621551  -7.148989844
Mg      -2.208126838  -7.284612512   2.854906875
Mg      -2.206999931   7.281876795   6.491954164
Mg       2.203039138   2.065404974   6.494363489
Mg      -7.148989844   7.282657619  -6.496621551
Mg       6.495220533  -7.149251987  -2.068929708
Mg      -6.497311905  -2.206521406  -2.069113782
Mg       6.493789879   7.144043305  -7.284813079
Mg      -6.496093980   2.204067017  -7.284950160
Mg      -2.068929708   6.495220533  -7.149251987
Mg       2.064960719   2.852106059  -7.148768053
Mg      -2.069113782  -6.497311905  -2.206521406
Mg       2.065435928  -2.857214509  -2.207201820
Mg      -7.149251987  -2.068929708   6.495220533
Mg      -7.148768053   2.064960719   2.852106059
Mg       7.144760834   7.281662591   2.854200083
Mg      -2.207201820   2.065435928  -2.857214509
Mg      -6.498845599   7.145349533   2.064874914
Mg       6.494363489   2.203039138   2.065404974
Mg      -6.496621551  -7.148989844   7.282657619
Mg       6.491954164  -2.206999931   7.281876795
Mg       2.064874914  -6.498845599   7.145349533
Mg      -2.069587582  -2.856132703   7.144985498
Mg       2.065404974   6.494363489   2.203039138
Mg      -2.069725394   2.853458071   2.203185880
Mg       7.145349533   2.064874914  -6.498845599
Mg       7.144985498  -2.069587582  -2.856132703
Mg      -7.147335003  -7.285149136  -2.857632220
Mg       2.203185880  -2.069725394   2.853458071
N        4.674971919   4.674971919   4.674971919
N       -4.679097645   4.673994784   4.673250945
N        4.673250945  -4.679097645   4.673994784
N       -4.677243850  -4.676660785   4.675574921
N        4.675574921  -4.677243850  -4.676660785
N       -4.678385311  -4.678385311  -4.678385311
N       -4.676660785   4.675574921  -4.677243850
N        4.673994784   4.673250945  -4.679097645
N        9.349058284  -4.678339930   8.762003402
N        9.349380983   4.674933400   0.581867352
N        8.762003402   9.349058284  -4.678339930
N       -8.765957447  -0.002109422  -4.677660355
N       -4.678339930   8.762003402   9.349058284
N       -4.677660355  -8.765957447  -0.002109422
N        4.674355140  -0.586753626  -0.002368775
N        9.348593427   4.673979601  -8.766153991
N        9.348905103  -4.677360759  -0.586128762
N       -8.766153991   9.348593427   4.673979601
N        8.761436635  -0.001647603   4.674640396
N        4.673979601  -8.766153991   9.348593427
N        4.674640396   8.761436635  -0.001647603
N       -4.677968914   0.582078512  -0.001873839
N       -0.002368775   4.674355140  -0.586753626
N       -0.002109422  -4.677660355  -8.765957447
N       -0.586753626  -0.002368775   4.674355140
N        0.581867352   9.349380983   4.674933400
N        4.674933400   0.581867352   9.349380983
N       -0.001873839  -4.677968914   0.582078512
N       -0.001647603   4.674640396   8.761436635
N        0.582078512  -0.001873839  -4.677968914
N       -0.586128762   9.348905103  -4.677360759
N       -4.677360759  -0.586128762   9.348905103
K_POINTS {automatic}
2 2 2 1 1 1  (!!! with the opotimized coordinates, I used the k-mesh 4 4 4 
0 0 0 to do 'scf')
CELL_PARAMETERS
            18.70000000000000    0.00000000000000    0.00000000000000
               0.00000000000000  18.70000000000000    0.00000000000000
               0.00000000000000    0.00000000000000  18.70000000000000
EOF
--------------------