MoS2 spectrum : strange result

[Flex]

I am computing the spectrum of monolayer MoS2, including spin-orbit effects, on top of correctly converged GW corrections. The result is nice, but not exactly as expected. The spectrum has the right shape, but the exciton peaks seem too broad, compared to theoretical and experimental results from the literature. (for example, PHYSICAL REVIEW B 93, 155435 (2016) and PHYSICAL REVIEW B 86, 115409 (2012))

Here is the input :

Code: Select all

#                                                           
#  __  __   ________   ___ __ __    _______   ______        
# /_/\/_/\ /_______/\ /__//_//_/\ /_______/\ /_____/\       
# \ \ \ \ \\::: _  \ \\::\| \| \ \\::: _  \ \\:::_ \ \      
#  \:\_\ \ \\::(_)  \ \\:.      \ \\::(_)  \/_\:\ \ \ \     
#   \::::_\/ \:: __  \ \\:.\-/\  \ \\::  _  \ \\:\ \ \ \    
#     \::\ \  \:.\ \  \ \\. \  \  \ \\::(_)  \ \\:\_\ \ \   
#      \__\/   \__\/\__\/ \__\/ \__\/ \_______\/ \_____\/   
#                                                           
#                                                           
#             GPL Version 4.1.2 Revision 120                
#                        MPI Build                          
#                http://www.yambo-code.org                  
#
optics                       # [R OPT] Optics
em1s                         # [R Xs] Static Inverse Dielectric Matrix
bss                          # [R BSS] Bethe Salpeter Equation solver
bse                          # [R BSE] Bethe Salpeter Equation.
bsk                          # [R BSK] Bethe Salpeter Equation kernel
#em1d                         # [R Xd] Dynamical Inverse Dielectric Matrix
#ppa                          # [R Xp] Plasmon Pole Approximation
Chimod= "hartree"            # [X] IP/Hartree/ALDA/LRC/BSfxc
BSEmod= "retarded"           # [BSE] resonant/retarded/coupling
BSKmod= "SEX"                # [BSE] IP/Hartree/HF/ALDA/SEX
BSSmod= "d"                  # [BSS] (h)aydock/(d)iagonalization/(i)nversion/(t)ddft`
BSENGexx= 38000        mHa    # [BSK] Exchange components
BSENGBlk= 8000            mHa    # [BSK] Screened interaction block size

NLogCPUs=0                   # [PARALLEL] Live-timing CPU`s (0 for all)
X_all_q_CPU= "1 2 68 1"              # [PARALLEL] CPUs for each role
X_all_q_ROLEs= "q k c v"            # [PARALLEL] CPUs roles (q,k,c,v)
X_all_q_nCPU_LinAlg_INV= 1   # [PARALLEL] CPUs for Linear Algebra
SE_CPU= "1 2 68"                   # [PARALLEL] CPUs for each role
SE_ROLEs= "q qp b"                 # [PARALLEL] CPUs roles (q,qp,b)

BS_CPU= "136 1 1"             # [PARALLEL] CPUs for each role
BS_ROLEs= "k eh t"           # [PARALLEL] CPUs roles (k,eh,t)
BS_nCPU_invert= 1            # [PARALLEL] CPUs for matrix inversion
BS_nCPU_diago= 1             # [PARALLEL] CPUs for matrix diagonalization

#WehCpl                      # [BSK] eh interaction included also in coupling

RandQpts=1000000                   # [RIM] Number of random q-points in the BZ
RandGvec= 1000            mHa    # [RIM] Coulomb interaction RS components
CUTGeo= "box Z"               # [CUT] Coulomb Cutoff geometry: box/cylinder/sphere X/Y/Z/XY..
% CUTBox
 0.00     | 0.00     | 25.00     |        # [CUT] [au] Box sides
%

KfnQPdb= "E < SAVE/ndb.QP"   # [EXTQP BSK BSS] Database

% BEnRange
  0.00000 | 10.00000 | eV    # [BSS] Energy range
%
% BDmRange
  0.10000 |  0.10000 | eV    # [BSS] Damping range
%
BEnSteps= 1000                # [BSS] Energy steps
% BLongDir
 1.000000 | 0.000000 | 0.000000 |        # [BSS] [cc] Electric Field
%
% BSEBands
    25 |  28 |               # [BSK] Bands range !plus après
%

% BndsRnXs
    1 |  200 |               # [Xs] Polarization function bands
%
NGsBlkXs=8000            mHa    # [Xs] Response block size
% DmRngeXs
  0.10000 |  0.10000 | eV    # [Xs] Damping range
%
% LongDrXs
 1.000000 | 0.000000 | 0.000000 |        # [Xs] [cc] Electric Field
%

Is there some kind of broadening parameter I should set somewhere ? Is the electric field in the right direction ? Do I need more bands around the gap to converge ?

BTW, I'm sorry for requesting that, but I also have a problem with the YPP utility that I posted on that part of the forum, and I haven´t had any answer yet.

Thanks in advance

[martinspenke]

Dear Thierry,

You need to increase the number of transition bands for convergence.
% BSEBands
25 | 28 | # [BSK] Bands range !plus après
%
This is not that much. Try 8 occupied and 8 unoccupied.
Further, for MoS2 it is important to include semi-core electrons.

YPP had a problem with spin-orbit coupling if I remember correctly. Probably this is solved in the newer version of yambo (4.1.4). Just check it.
https://github.com/yambo-code/yambo

Best,
Martin