[Thermo_pw-forum] Significant difference between the approximate Debye temperature and Debye temperature

[Boziki Ariadni]

Dear all,


I am using thermo_pw in order to calculate the elastic properties as well as the Debye temperature of a series of systems.

For the majority of the calculations I get approximate Debye temperatures and Debye temperatures that differ by 1 K. However for some of the systems, such difference is significant (up to 53 K for example). Would it be possible to give me any hint about the reasons, this could happen. I am thinking if an increase of K-points leads to better results.


In addition, for only one of the systems that I am studying the approximate Debye temperature is not available, since according to the output the system is unstable for shear deformations. Is there any way to improve that, too?


Below I attach the main part of the inputs with the parts of the outputs that describe the elastic properties and Debye temperatures.



***********************************************************

Input:

&CONTROL
                   calculation = "scf"
!                    wf_collect = .true.
                         title = 'scf',
                        prefix = 'scf',
                  restart_mode = 'from_scratch',
                    pseudo_dir = '/PP/'
                        outdir = './'
                         nstep = 1000,
                       tstress = .true.
                       tprnfor = .true.
/


&SYSTEM
                        ibrav = 14,
                          nat = 146
                         ntyp = 5,
                      ecutwfc = 40 ,
                      ecutrho = 280 ,
                    celldm(1) = 34.24774975539434046984
                    celldm(2) = 0.49825775128247714384
                    celldm(3) = 1.35163407203943248213
                    celldm(4) = 0.0
                    celldm(5) = 0.0
                    celldm(6) = -0.42
/

&ELECTRONS
!electron_maxstep = 500,
conv_thr = 1.d-8
diagonalization = 'cg'
/

&IONS
/

&CELL
/

K_POINTS automatic
2 2 1 1 1 1

ATOMIC_SPECIES
    Pb  207.2    Pb.pbe-dn-rrkjus_psl.0.2.2.UPF
    I   126.904  I.pbe-n-rrkjus_psl.0.2.UPF
    C   12.0107  C.pbe-rrkjus.UPF
    N   14.0067  N.pbe-rrkjus.UPF
    H   1.00784  H.pbe-rrkjus.UPF

ATOMIC_POSITIONS (crystal)
Pb       0.992789319   0.992744559   0.978309809
Pb       0.991367642   0.989878647   0.146751474
Pb       0.506306111   0.007712595   0.976472657
Pb       0.504988223   0.004566493   0.144514039
....
....
....

Output:

     Voigt approximation:
     Bulk modulus  B =     57.08571 kbar
     Young modulus E =     62.26770 kbar
     Shear modulus G =     23.61838 kbar
     Poisson Ratio n =      0.31820

     Reuss approximation:
     Bulk modulus  B =     82.98611 kbar
     Young modulus E =     11.01452 kbar
     Shear modulus G =      3.72646 kbar
     Poisson Ratio n =      0.47788

     Voigt-Reuss-Hill average of the two approximations:
     Bulk modulus  B =     70.03591 kbar
     Young modulus E =     36.64111 kbar
     Shear modulus G =     13.67242 kbar
     Poisson Ratio n =      0.33996

     Voigt-Reuss-Hill average; sound velocities:

     Compressional V_P =     1594.022 m/s
     Bulk          V_B =     1419.902 m/s
     Shear         V_G =      627.366 m/s

     The approximate Debye temperature is       76.616 K

                    ----------------------------------------

     Average Debye sound velocity =      229.337 m/s

     Debye temperature =       23.377 K

********************************************************************
Input:

&CONTROL
                   calculation = "scf"
!                    wf_collect = .true.
                         title = 'scf for gap',
                        prefix = 'scf',
                  restart_mode = 'from_scratch',
                    pseudo_dir = '/home/boziki/PP-new/PP/'
                        outdir = './'
                         nstep = 1000,
                       tstress = .true.
                       tprnfor = .true.
/


&SYSTEM
                        ibrav = 14,
                          nat = 144
                         ntyp = 5,
                      ecutwfc = 40 ,
                      ecutrho = 280 ,
                    celldm(1) = 34.59613963864875075156
                    celldm(2) = 0.49999972688773829727
                    celldm(3) = 1.33737830800399180881
                    celldm(4) = 0.0
                    celldm(5) = 0.0
                    celldm(6) = -0.45
/

&ELECTRONS
!electron_maxstep = 500,
conv_thr = 1.d-8
diagonalization = 'cg'
/

&IONS
/

&CELL
/

K_POINTS automatic
2 2 1 1 1 1

ATOMIC_SPECIES
    Pb   1  Pb.pbe-dn-rrkjus_psl.0.2.2.UPF
    I    1  I.pbe-n-rrkjus_psl.0.2.UPF
    C    1  C.pbe-rrkjus.UPF
    N    1  N.pbe-rrkjus.UPF
    H    1  H.pbe-rrkjus.UPF

ATOMIC_POSITIONS (crystal)
Pb       0.000285166   0.999427499   0.984407789
Pb       0.999692114   0.000611651   0.151070638
Pb       0.500287344   0.999428351   0.984408394
...
...
...


Output:

 Voigt approximation:
     Bulk modulus  B =     79.06975 kbar
     Young modulus E =     61.02940 kbar
     Shear modulus G =     22.25142 kbar
     Poisson Ratio n =      0.37136

     Reuss approximation:
     Bulk modulus  B =     91.94692 kbar
     Young modulus E =  -1202.99533 kbar
     Shear modulus G =   -163.42392 kbar
     Poisson Ratio n =      2.68060

     Voigt-Reuss-Hill average of the two approximations:
     Bulk modulus  B =     85.50834 kbar
     Young modulus E =   -570.98297 kbar
     Shear modulus G =    -70.58625 kbar
     Poisson Ratio n =      3.04458

     Voigt-Reuss-Hill average; sound velocities:

     The system is unstable for compressional deformations
     Bulk          V_B =    11446.859 m/s
     The system is unstable for shear deformations

     The approximate Debye temperature not available

                    ----------------------------------------

     Average Debye sound velocity =     1679.234 m/s

     Debye temperature =      169.977 K



Thank you very much in advance for your time.


Sincerely yours


Ariadni Boziki

EPFL

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