  Using the strain-dependent effective Hamiltonian and the geometric phase,
Droth et al. [Phys. Rev. B 94, 075404 (2016)] obtain an analytical expression
for the electronic contribution to the piezoelectricity of planar hexagonal
boron nitride (h -BN). Their analytical results of piezoelectric constants for
h -BN are invalid because of the mistakes in constructing the adiabatic process
of the piezoelectricity. In this comment, we reconstruct a proper adiabatic
process for piezoelectricity and formulate a general Berry phase expression for
the piezoelectric coefficients of two-dimensional piezoelectric crystals by
means of the modern theory of polarization. The corrected analytical results of
the piezoelectric constants are in complete consistency with the
first-principles calculations and hence manifest the validity and generality of
the Berry phase expression of piezoelectric coefficients.
