EPW Forum

Dear developer,
I have two questions about the polaron calculation.
First, I use EPW to calculate the hole polaron. My system has soft phonon modes, and the phonon dispersion exhibits imaginary frequency. Does the EPW work well with these condition?
Second, the EPW ignores the imaginary frequency and obtains the polaron wavefunction, which is localized at Sb ion and its nearest Cl ion. While the formation energy of the hole polaron is -7.6 eV. I don't know why the value is so curious.
The inputs file of EPW are list below.
**********************************************
&inputepw
prefix = 'Cs'
outdir = './'
elph = .true.
epwwrite = .true.
lpolar = .true.
nbndsub = 1
dvscf_dir = './save/'
etf_mem = 0
bands_skipped = 'exclude_bands = 1:46,48-57'
wannierize = .true.
num_iter = 500
iprint = 2
proj(1) = 'Sb:s'
wannier_plot = .true.
wannier_plot_supercell = 6 6 6
nk1 = 6
nk2 = 6
nk3 = 6
nq1 = 2
nq2 = 2
nq3 = 2
band_plot = .true.
filkf = './kpoints'
filqf = './kpoints'
/
*******************************************
&inputepw
prefix = 'Cs'
outdir = './'
elph = .true.
epwread = .true.
lpolar = .true.
nbndsub = 1
dvscf_dir = './save/'
etf_mem = 0
wannierize = .false.
plrn = .true.
restart_plrn = .false.
type_plrn = 1
init_plrn = 1
init_sigma_plrn = 2.0
niter_plrn = 500
conv_thr_plrn = 5E-4
ethrdg_plrn = 1E-5
adapt_ethrdg_plrn = .true.
nk1 = 6
nk2 = 6
nk3 = 6
nq1 = 2
nq2 = 2
nq3 = 2
nkf1 = 4
nkf2 = 4
nkf3 = 4
nqf1 = 4
nqf2 = 4
nqf3 = 4
/

Dear jack,

The current implementation in the polaron module is based on the theory described in this reference: https://doi.org/10.1103/PhysRevB.99.235139

As you can see in Eq. (18), the theory assumes that the atomic configuration in the initial configuration (no extra added/removed electron) is such that the energy surface is at a minimum and can be well approximated by a harmonic expansion. If your system has imaginary phonon frequencies, that is not the case.

Thus, you can only proceed with caution in these cases, and that is why the code prints the warning message. Only in systems where these soft phonons do not play a role, the solutions of the polaron equations *might* be somewhat meaningful; but, in general, are not to be trusted when the warning message appears.

I would recommend finding the ground state configuration of your system which is dynamically stable (no imaginary frequencies), before proceeding with further polaron calculations.

Best,
Jon Lafuente-Bartolome

Dear Jon,
The hole polaron of this system may be composed of two types of atoms. Should the 'proj' be set as proj(1)='atom1:' and proj(2)='atom2:'?
This structure exhibits Cu ions. I note that the EPW is failing with transition metals from the forum. Can I calculate the polaron localized at Cu ions?
Best.