[QE-users] Error in piezoelectric coefficients calculation (Berry phase)

[Zimmi Singh]

Dear Users,

I am attempting to calculate the piezoelectric coefficients of 2D MoS2
using the Berry Phase method on a strained structure (0.5% in the armchair
direction).

I faced two issues during the process.
1) In the nscf calculations, I added the 'nbnd' tag in the input file (
nscf1.in). However, when examining the output file (nscf1.out), the
polarization value was displayed as NaN (see below). I have attached the
output file for your reference.

                            VALUES OF POLARIZATION
                             ~~~~~~~~~~~~~~~~~~~~~~

        The calculation of phases done along the direction of vector 2
        of the reciprocal lattice gives the following contribution to
        the polarization vector (in different units, and being Omega
        the volume of the unit cell):

           P =         NaN  (mod  10.4913000)  (e/Omega).bohr

           P =         NaN  (mod   0.0037363)  e/bohr^2

           P =         NaN  (mod   0.2136113)  C/m^2

        The polarization direction is:  (-0.00000 , 1.00000 , 0.00000 )

However, after removing the 'nbnd' tag in the nscf input file (nscf.in), I
successfully obtained the polarization values (see below), and also
attached the output file (nscf.out)).

                             VALUES OF POLARIZATION
                             ~~~~~~~~~~~~~~~~~~~~~~

        The calculation of phases done along the direction of vector 2
        of the reciprocal lattice gives the following contribution to
        the polarization vector (in different units, and being Omega
        the volume of the unit cell):

           P =  14.0341794  (mod  10.4913000)  (e/Omega).bohr

           P =   0.0049981  (mod   0.0037363)  e/bohr^2

           P =   0.2857471  (mod   0.2136113)  C/m^2

        The polarization direction is:  (-0.00000 , 1.00000 , 0.00000 )


2) At a strain of 0.5% in the armchair direction, the calculated
polarization vector is P = 0.2857471 C/m². To convert it to C/m, I
multiplied it by the armchair lattice parameter (10.5 angstroms) and
obtained the piezoelectric coefficient e22 = 60006.89 pC/m. However, the
published value for e22 is 362 pC/m (10.1021/acsnano.5b03394).

I have scavenged through the mailing list but found no tutorial on this
topic.

-- 
Best Regards
Zimmi Singh
*Ph.D. Candidate*


*Department of Metallurgical and Materials Engineering Indian Institute of
Technology, Kharagpur Kharagpur, India*
*Mob No. 9935804727*
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