In order to obtain accurate results, the crystal structure should first be relaxed using "tight" calculation settings including high force and energy convergence criteria. Note, that this can often be expensive for very large structures.
VASP settings for tight convergence
ADDGRID = True EDIFF = 1E-8 EDIFFG = -5E-4 PREC = Accurate NSW = 100 ISIF = 3 NELMIN = 5
Dense uniform band structure and wave function coefficients¶
AMSET should be run on a
vasprun.xml file from a "dense" uniform band structure
calculation. Typically a k-point mesh density at least twice that needed to converge
the total energy will be necessary to converge transport properties. Note this refers
to the initial DFT mesh before Fourier interpolation. In order to obtain accurate band
gaps often a hybrid DFT functional such as HSE06 is required.
Wave function coefficients are required to calculate wave function overlaps.
This requires the
WAVECAR file to be written by VASP (achieved by setting
LWAVE = True). Wave function coefficients can then be extracted using the
amset wave command. Coefficients are stored in the
VASP settings for uniform calculations
ADDGRID = True EDIFF = 1E-8 PREC = Accurate NSW = 1 LWAVE = True
Elastic constants can be calculated using finite differences in VASP. It is very important to first relax the structure using tight convergence settings, as described in the structural relaxation section. Details on the finite difference approach in VASP can be found on the IBRION documentation page.
VASP settings for elastic constants
ADDGRID = True EDIFF = 1E-8 PREC = Accurate NSW = 1 IBRION = 6 ISIF = 3
The absolute deformation potential describes the change in energy of the bands with change in volume and is calculated as where is the uniform stress tensor. The deformation potential should be averaged over contraction (–0.5 %) and expansion (+0.5 %) of the lattice and calculated separately for each component of the strain tensor. To account for shifts in the average electrostatic potential between deformed cells, the eigenvalues are aligned to the average energy level of the core states.
AMSET includes a tool to assist with the calculation of the deformation potentials.
The initial input is a "tight" optimised structure as described in the
structural relaxation section. Deformed structures are
generated using the
amset deform create command, which will generate a list of
POSCARs each corresponding to a component of the strain tensor. Note that symmetry is
automatically used to reduce the number of calculations needed. A single point
calculation (no relaxation, i.e.,
NSW = 0) should be performed for each deformed
POSCAR as well as the undeformed structure.
VASP settings for deformation calculations
ADDGRID = True EDIFF = 1E-8 PREC = Accurate NSW = 1 ICORELEVEL = 1 # needed to write the core levels to OUTCAR
The deformation potentials can be calculated using the
amset deform read command.
This requires the paths to the undeformed and deformation calculations as inputs.
The undeformed folder should be specified first, followed by the deformation folders.
amset deform read undeformed def-1 def-2 def-3
This will write the deformations potentials to a
deformation.h5 file in the current
directory. You can specify to use this file when calculating scattering rates by
deformation_potential option to
See the settings page for more details.
Dielectric constants, piezoelectric constants and polar-phonon frequency¶
Static and high-frequency dielectric constants, piezoelectric constants, and the "effective polar phonon frequency" can be obtained using density functional perturbation theory (DFPT). It is very important to first relax the structure using tight convergence settings, as described in the structural relaxation section. Details on DFPT in VASP can be found on the IBRION and LEPSILON documentation pages.
VASP settings for dielectric constants and phonon frequency
ADDGRID = True EDIFF = 1E-8 PREC = Accurate NSW = 1 IBRION = 8 LEPSILON = True
Note, DFPT cannot be used with hybrid exchange-correlation functionals. In these
cases the LCALCEPS flag should be
used in combination with
IBRION = 6.
The dielectric constants and polar phonon frequency can be extracted from the VASP outputs using the command:
vasprun.xmlfile output from the DFPT calculation.
The effective phonon frequency is determined from the phonon frequencies (where is a phonon branch and is a phonon wave vector) and eigenvectors (where is an atom in the unit cell). In order to capture scattering from the full phonon band structure in a single phonon frequency, each phonon mode is weighted by the dipole moment it produces according to
where is the Born effective charge. This naturally suppresses the contributions from transverse-optical and acoustic modes in the same manner as the more general formalism for computing Frölich based electron-phonon coupling.
The weight is calculated only for -point phonon frequencies and averaged over the full unit sphere to capture both the polar divergence at and any anisotropy in the dipole moments. The effective phonon frequency is calculated as the weighted sum over all -point phonon modes according to