BSE for Molecules
In this tutorial we will show the reader how to perform the Bethe-Salpeter equation (BSE) calculation on molecules using FHI-aims to study the neutral excitations. The implementation of the BSE in FHI-aims based on NAO are described by Liu, et al. 1
Background
The physical process described by the BSE equation is the charge neutral optical excitation as shown in this figure.
Figure from Golze et al. 2
As shown, the optical excitation involves the interaction between a hole and a excited electron. To effectively describe such phenomenon, the electron-hole vector space, which is the combination of a occupied orbital and an unoccupied orbital, is a natural choice to expand the BSE equation. In our implementation, the BSE equation is written as
where, \(X_s\) and \(Y_s\) are the eigenvectors within the electron-hole vector space. With the Tamm-Dancoff approximation3, we ignore the coupling matrix \(B\), thus a reduced matrix can be solved. Let us look at the elements in \(A\) matrix.
where \(|i a)\) and \(|j b)\) are the electron-hole vector space prepared from the GW quasi-particle wavefunctions. \(\varepsilon_{i}\) is the GW quasiparticle energies. \(V\) and \(W\) are the Coulomb and screened Coulomb interactions.
The eigenvalues of the BSE matrix are the calculated optical excitation energies.
Exercise
In this BSE exercise, you will perform a BSE calculation based on \(G_0W_0\) calculation which is started from PBE exchange-correlation functional DFT calculation (BSE@\(G_0W_0\)@PBE). Tier 2 + aug2 basis set for ethylene C\(_2\)H\(_4\) is used. The quantities of interest in this case are the BSE eigenvalues. You can proceed as follows:
- Generate the
geometry.in
file for ethylene from the experimental data (extracted from here):
atom 0.0000 0.0000 0.6695 C
atom 0.0000 0.0000 -0.6695 C
atom 0.0000 0.9289 1.2321 H
atom 0.0000 -0.9289 1.2321 H
atom 0.0000 0.9289 -1.2321 H
atom 0.0000 -0.9289 -1.2321 H
- Here are the commands for the BSE calculations in the
control.in
file. As can be seen, in general, we separate the commands into three parts: The DFT part, the GW part, and the BSE part. There is also the basis set information for different species. As recommended in our paper,1 we use Tier2 + aug2 as a relatively converged basis sets for molecular BSE calculation. We can find these basis set information in the folderAIMS_DIR/species_defaults/non-standard/Tier2_aug2
.
#########################################
xc pbe
spin none
relativistic atomic_zora scalar
calculate_all_eigenstates # Make sure all the unoccupied orbitals are calculated
#########################################
#########################################
qpe_calc gw # G0W0
anacon_type 1 # PADE
n_anacon_par 16
#########################################
neutral_excitation bse
read_write_qpe rw
bse_s_t singlet # singlet or triplet excitation
#########################################
- Run the calculation with the FHI-aims executable.
mpirun -n 4 aims.x | tee output
- Read the first excitation energy from the output. Given the best theoretical estimation of the first excitation energy to be 7.8 eV.4, how good is the result from BSE?
Here is an example output, where the
6.1698 eV
is the first excitation state.
| 1. Singlet: 6.1698 eV [ 0.2267 Ha] - Oscillator Strength: 0.0527
| Transition Moments X: -0.2811 Y: 0.0000 Z: 0.0000
| Dominant single-particle level transitions in this excitation
| eigenvector: [occupied ==> virtual, eigenvector element]
| 8==> 10 -0.6261
| 8==> 15 0.2002
| 8==> 19 0.2304
- Optional: Start the calculation with the DFT calculation with PBE0 functional. i.e. BSE@\(G_0W_0\)@PBE0. Play around with the hybrid mixing parameter in PBE0. We expect the hybrid mixing parameter to have an influence on the BSE first excitation energy. Here is a reference on this topic 5.
To perform a BSE@\(G_0W_0\)@PBE0 calculation, we need to modify the
xc pbe
line incontrol.in
to these two lines.
xc pbe0
hybrid_xc_coeff 0.25
- For convenience, here we provide the full
control.in
file wherexc
part can be modified accordingly to perform above exercises.
#########################################
xc pbe
spin none
relativistic atomic_zora scalar
calculate_all_eigenstates
#########################################
#########################################
qpe_calc gw
anacon_type 1
n_anacon_par 16
#########################################
neutral_excitation bse
read_write_qpe rw
bse_s_t singlet
#########################################
################################################################################
#
# FHI-aims code project
# VB, Fritz-Haber Institut, 2007
#
# Suggested "safe" defaults for H atom (to be pasted into control.in file)
#
################################################################################
species H
# global species definitions
nucleus 1
mass 1.00794
#
l_hartree 8
#
cut_pot 4.0 2.0 1.0
basis_dep_cutoff 0.d0
#
radial_base 24 7.0
radial_multiplier 6
angular_grids specified
division 0.1930 50
division 0.3175 110
division 0.4293 194
division 0.5066 302
division 0.5626 434
division 0.5922 590
division 0.6227 974
division 0.6868 1202
outer_grid 1202
################################################################################
#
# Definition of "minimal" basis
#
################################################################################
# valence basis states
valence 1 s 1.
# ion occupancy
ion_occ 1 s 0.5
################################################################################
#
# Suggested additional basis functions. For production calculations,
# uncomment them one after another (the most important basis functions are
# listed first).
#
# Basis constructed for dimers: 0.5 A, 0.7 A, 1.0 A, 1.5 A, 2.5 A
#
################################################################################
# "First tier" - improvements: -1014.90 meV to -62.69 meV
hydro 2 s 2.1
hydro 2 p 3.5
# "Second tier" - improvements: -12.89 meV to -1.83 meV
hydro 1 s 0.85
hydro 2 p 3.7
hydro 2 s 1.2
hydro 3 d 7
# "Third tier" - improvements: -0.25 meV to -0.12 meV
# hydro 4 f 11.2
# hydro 3 p 4.8
# hydro 4 d 9
# hydro 3 s 3.2
###############################################################################
#
# Gaussian augmentation functions.
# Obtained by diff cc-pV5Z aug-cc-pV5Z
#
###############################################################################
gaussian 0 1 0.0207000
gaussian 1 1 0.0744000
################################################################################
#
# FHI-aims code project
# VB, Fritz-Haber Institut, 2007
#
# Suggested "safe" defaults for C atom (to be pasted into control.in file)
#
################################################################################
species C
# global species definitions
nucleus 6
mass 12.0107
#
l_hartree 8
#
cut_pot 4.0 2.0 1.0
basis_dep_cutoff 0.d0
#
radial_base 34 7.0
radial_multiplier 6
angular_grids specified
division 0.2187 50
division 0.4416 110
division 0.6335 194
division 0.7727 302
division 0.8772 434
division 0.9334 590
division 0.9924 770
division 1.0230 974
division 1.5020 1202
outer_grid 1202
################################################################################
#
# Definition of "minimal" basis
#
################################################################################
# valence basis states
valence 2 s 2.
valence 2 p 2.
# ion occupancy
ion_occ 2 s 1.
ion_occ 2 p 1.
################################################################################
#
# Suggested additional basis functions. For production calculations,
# uncomment them one after another (the most important basis functions are
# listed first).
#
# Constructed for dimers: 1.0 A, 1.25 A, 1.5 A, 2.0 A, 3.0 A
#
################################################################################
# "First tier" - improvements: -1214.57 meV to -155.61 meV
hydro 2 p 1.7
hydro 3 d 6
hydro 2 s 4.9
# "Second tier" - improvements: -67.75 meV to -5.23 meV
hydro 4 f 9.8
hydro 3 p 5.2
hydro 3 s 4.3
hydro 5 g 14.4
hydro 3 d 6.2
# "Third tier" - improvements: -2.43 meV to -0.60 meV
# hydro 2 p 5.6
# hydro 2 s 1.4
# hydro 3 d 4.9
# hydro 4 f 11.2
# "Fourth tier" - improvements: -0.39 meV to -0.18 meV
# hydro 2 p 2.1
# hydro 5 g 16.4
# hydro 4 d 13.2
# hydro 3 s 13.6
# hydro 4 f 17.6
# Further basis functions - improvements: -0.08 meV and below
# hydro 3 s 2
# hydro 3 p 6
# hydro 4 d 20
###############################################################################
#
# Gaussian augmentation functions.
# Obtained by diff cc-pV5Z aug-cc-pV5Z
#
###############################################################################
gaussian 0 1 0.0394000
gaussian 1 1 0.0272000
-
Liu, Chi, et al. "All-electron ab initio Bethe-Salpeter equation approach to neutral excitations in molecules with numeric atom-centered orbitals." The Journal of chemical physics 152.4 (2020): 044105. ↩↩
-
Golze, Dorothea, Marc Dvorak, and Patrick Rinke. "The GW compendium: A practical guide to theoretical photoemission spectroscopy." Frontiers in chemistry 7 (2019): 377. ↩
-
Benedict, Lorin X., Eric L. Shirley, and Robert B. Bohn. "Optical absorption of insulators and the electron-hole interaction: An ab initio calculation." Physical review letters 80.20 (1998): 4514. ↩
-
Schreiber, Marko, et al. "Benchmarks for electronically excited states: CASPT2, CC2, CCSD, and CC3." The Journal of chemical physics 128.13 (2008): 134110. ↩
-
Bruneval, Fabien, Samia M. Hamed, and Jeffrey B. Neaton. "A systematic benchmark of the ab initio Bethe-Salpeter equation approach for low-lying optical excitations of small organic molecules." The Journal of chemical physics 142.24 (2015): 244101. ↩