[Thermo_pw-forum] Elastic constants at finite pressures using mur_lc_elastic_constants

[Andrea Dal Corso]

This is a summary of what I obtain with your input.
It seems ok to me
   ----------------------------------------------------------------------------
      At pressure     1375.500000 kbar
      The equilibrium lattice constant is                6.9974 a.u.
      The bulk modulus is                             6552.925  kbar
      The pressure derivative of the bulk modulus is     3.238
      The enthalpy at the minimum is                 -33.363144856 Ry
   ----------------------------------------------------------------------------

The geometry to which the strain is applied to get the elastic constants

      bravais-lattice index     =            0
      lattice parameter (alat)  =       6.9974  a.u.
      unit-cell volume          =      85.0110 (a.u.)^3
      number of atoms/cell      =            2
      number of atomic types    =            2
      number of electrons       =         8.00
      number of Kohn-Sham states=            4
      kinetic-energy cutoff     =      44.0000  Ry
      charge density cutoff     =     176.0000  Ry

seems the one that corresponds to the high pressure that you specify.
And also the final elastic constants seems reasonable:

      Elastic constants C_ij (kbar)
     i j=        1           2           3           4           5           6
     1 14278.72847  2735.40645  2735.40645     0.00000     0.00000     0.00000
     2  2735.40645 14278.72847  2735.40645     0.00000     0.00000     0.00000
     3  2735.40645  2735.40645 14278.72847     0.00000     0.00000     0.00000
     4     0.00000     0.00000     0.00000  2085.66084     0.00000     0.00000
     5     0.00000     0.00000     0.00000     0.00000  2085.66084     0.00000
     6     0.00000     0.00000     0.00000     0.00000     0.00000  2085.66084

If you do not obtain these numbers, please update your version of thermo_pw.

If you obtain these numbers but think they are wrong, please explain what it
wrong.

Also note that to get more accurate values of C_44 you might need to
decrease force_thr  in the pw.x input.


Andrea
Quoting Sarah Arveson <sarah.arveson at gmail.com>:

> Hi all,
>
> I am struggling to calculate the elastic constants at finite  
> pressures and wonder if I am missing a crucial flag. The pressure is  
> being read by the program, as the densities are correctly updated in  
> the output file, but for any given pressure, only the equilibrium 0  
> pressure tensors are printed. (Resulting in incorrect sound speeds,  
> etc.) Any advice would be greatly appreciated!
>
> Here are my inputs for an example of B1 MgO at a pressure of 1375.5  
> kbar corresponding to a previously calculated corresponding lattice  
> parameter of 7 a.u.:
>
>  ********* thermo_control *********
>  &INPUT_THERMO
>   what='mur_lc_elastic_constants',
>   pressure=1375.5
>  /
>
>  ********* pw.mgo.scf.in *********
>  &CONTROL
>     calculation = 'scf'
>     restart_mode='from_scratch',
>     prefix='MgO',
>     tstress = .TRUE.,
>
>     ! Note: MgO B1 (rocksalt) structure
>
>      pseudo_dir = '../../pseudo',
>      outdir     = 'output',
>  /
>
>  &SYSTEM
>     ibrav = 2,
>     celldm(1) = 7,
>     nat =  2,
>     ntyp = 2,
>     ecutwfc = 44.0,
>  /
>
>  &ELECTRONS
>     conv_thr = 1.0d-8
>  /
>
> ATOMIC_SPECIES
>    Mg  24.3050  Mg.pz-n-vbc.UPF
>    O   15.9994  O.pz-rrkjus.UPF
>
> ATOMIC_POSITIONS alat
>    Mg 0.0 0.0 0.0
>    O  0.5 0.5 0.5
>
> K_POINTS automatic
>    4 4 4  1 1 1
>
> ******************
> Thanks in advance,
> Sarah
> -------------------------------------------------
> Sarah M. Arveson
> PhD Candidate, Yale University
> Dept. of Earth and Planetary Sciences
> sarah.arveson at gmail.com