Simple Delta SCF
Electronic excitations of the Azobenzene molecule
In this example we calculate the first two electronic excited states of E-Azobenzene in a supercell.
The required files are azo.cell and azo.param:
azo.param
task: SinglePoint
reuse: default
calculate_deltascf : true
deltascf_method : simple
deltascf_smearing : 0.01
#band occ spin from_band to_band
%block deltascf_constraints
34 0.5000 1 34 34
35 0.5000 1 35 35
%endblock deltascf_constraints
spin_polarized : False
cut_off_energy : 350.0
elec_energy_tol : 1e-07
fix_occupancy : False
iprint : 1
max_scf_cycles : 200
metals_method : dm
mixing_scheme : Pulay
nextra_bands : 10
num_dump_cycles : 0
opt_strategy_bias : 3
smearing_scheme : Gaussian
smearing_width : 0.1
xc_functional : PBE
azo.cell
%BLOCK LATTICE_CART
10.0000000 0.0000000000 0.0000000000
0.0000000000 20.0000000 0.0000000000
0.0000000000 0.0000000000 10.0000000000
%ENDBLOCK LATTICE_CART
%BLOCK POSITIONS_ABS
C -6.72081 -1.66625 0.00000
C -6.64967 -0.26964 0.00000
C -5.40647 0.36858 -0.00000
C -4.23175 -0.38857 -0.00000
C -4.29745 -1.78579 -0.00000
C -5.54882 -2.43430 -0.00000
H -7.68820 -2.15296 0.00000
H -7.55879 0.31772 0.00000
H -5.35348 1.44963 -0.00000
H -3.26966 0.10734 -0.00000
H -3.37789 -2.35693 -0.00000
N -5.65342 -3.85046 -0.00000
N -4.64259 -4.58194 -0.00000
C -4.75058 -5.99808 -0.00000
C -6.00434 -6.64214 -0.00000
C -6.07567 -8.03881 -0.00000
C -4.90409 -8.80053 -0.00000
C -3.65828 -8.16721 -0.00000
C -3.58139 -6.77065 -0.00000
H -2.61200 -6.28795 -0.00000
H -6.92178 -6.06761 -0.00000
H -7.03986 -8.53061 -0.00000
H -4.96168 -9.88134 -0.00000
H -2.75170 -8.75849 -0.00000
%ENDBLOCK POSITIONS_ABS
FIX_ALL_CELL : True
KPOINTS_MP_GRID : 1 1 1
We start by calculating the total DFT ground state energy as
Now we reuse the calculated wavefunctions and switch to the DeltaSCF calculation (param file above)
There are 68 valence electrons. Therefore, for this non-spin-polarized system the HOMO orbital is orbital no. 34. The LUMO is orbital no. 35.
The first two excited states of azobenzene are known to be S1(n->pi*) and S2(pi->pi*) transitions between the HOMO and LUMO and the HOMO-1 and the LUMO.
The corresponding constraint sequence in azo.deltascf for the S1 excitation is
#band occ spin from_band to_band
%block deltascf_constraints
34 0.5000 1 34 34
35 0.5000 1 35 35
%endblock deltascf_constraints
- WARNING
-
When running non-spin-polarized calculations, orbital occupations range from 0 to 1. !!When running a spin-polarized calculation, they also range from 0 to 1, although they contain 0 to 2 electrons. In this (non-spin-polarized) case, if we want to transfer an electron from the HOMO(34) to LUMO(35) we need to enforce the occupation of both to be 0.50.
Feel free to increase the print level with iprint to study the output in more detail.
The corresponding total energy is
This corresponds to an S1 excitation energy of 1.96 eV.
The constraint sequence for an S2 excitation is
#band occ spin from_band to_band
%block deltascf_constraints
33 0.5000 1 33 33
35 0.5000 1 35 35
%endblock deltascf_constraints
The resulting final energy is
The corresponding S2 excitation energy is 2.90 eV.