EPW Forum

Hello everyone,

I'm trying to compute the superconducting critical temperature (Tc) for a material using EPW, but I’m encountering an issue with the electron-phonon coupling constant (λ) being much larger than expected.

In the original publication, λ is reported to be around 0.2, with Tc > 65 K. However, my calculations are yielding anomalously large values of λ — far from the reported range.

Here’s what I’ve tried so far:

Wannierization accuracy:
Initially, I suspected a poor Wannierization. I have tested both manual projections and the SCDM method, but the coupling constant remains unusually high.

q-grid density:
I now suspect the sparse q-point mesh might be the source of the error, but I'm not entirely sure how critical that is or if there are other contributing factors I'm missing.

Attached:
Input files for both ph.x and epw.x

Has anyone encountered similar discrepancies?
What’s the best way to confirm whether it’s a q-grid issue, interpolation artifact, or something else (e.g., ep_coupling cutoff, smearing, etc.)?

Any insights or suggestions are most welcome!

Thanks in advance.


&inputph
prefix = 'Mg2IrH6',
fildyn = 'Mg2IrH6.dyn',
amass(1) = 24.305, ! Mg mass
amass(2) = 1.00794, ! H mass
amass(3) = 192.217, ! Ir mass
outdir = './',
ldisp = .true.,
trans = .true.,
fildvscf = 'dvscf',
fildrho = 'drho',
nq1 = 4,
nq2 = 4,
nq3 = 4,
nk1 = 18,
nk2 = 18,
nk3 = 18,
tr2_ph = 1.0d-16,
!alpha_mix(1) = 0.5\,
verbosity = 'high'
/

'--
&inputepw
prefix = 'Mg2IrH6',
outdir = './'

ep_coupling = .true.
elph = .true.
epbwrite = .true.
epbread = .false.

epwwrite = .true.
epwread = .false.

etf_mem = 1


nbndsub = 23,

wannierize = .true.
num_iter = 100

scdm_proj = .true.
scdm_entanglement = 'erfc'
auto_projections = .true.
scdm_mu = 6.59389 !-4.948
scdm_sigma = 2.62317 !6.804

iverbosity = 2

!eps_acustic = 1.0 ! Lowest boundary for the phonon frequency
ephwrite = .true. ! Writes .ephmat files used when Eliasberg = .true.

fsthick = 0.1 ! eV
degaussw = 0.01 ! eV
nsmear = 1
delta_smear = 0.04 ! eV

degaussq = 0.5 ! meV
nqstep = 500


eliashberg = .true.

laniso = .true.
limag = .true.
lpade = .true.

conv_thr_iaxis = 1.0d-4

wscut = 1.0 ! eV Upper limit over frequency integration/summation in the Elisashberg eq

nstemp = 1 ! Nr. of temps $
temps = 20 ! K provide list of temperetures OR (nstemp and temps = tempsmin tempsmax for even space mode) $

nsiter = 500

muc = 0.1

dvscf_dir = '../phonons/save'

nk1 = 12
nk2 = 12
nk3 = 12

nq1 = 4
nq2 = 4
nq3 = 4

mp_mesh_k = .true.
nkf1 = 36
nkf2 = 36
nkf3 = 36

nqf1 = 12
nqf2 = 12
nqf3 = 12
/

Dear Mab,

I have a few points I would like to clarify regarding your setup:

Have you compared the phonon results using a 4×4×4 q-grid with those reported in the literature?

The current 12×12×12 q-grid appears to be quite coarse. I recommend increasing the q-grid density, as too small a grid can sometimes lead to unconverged results and an overestimated electron-phonon coupling constant. Please perform a convergence test to verify this.

Additionally, please check whether a fsthick value of 0.1 eV is sufficient for your system. You might also consider increasing the degaussw parameter to see if it improves the results.

If you can share your input and output test files, I would be happy to take a closer look and offer more specific suggestions.

Best regards,
Shashi