# BSE

Parameters for a calculation solving the Bethe-Salpeter equation (BSE) for electronic excitations. The full BSE $$\left( \begin{array}{cc}A & B\\B & A\end{array} \right)$$ $$\left( \begin{array}{cc}\mathbf{X}^{(n)}\\\mathbf{Y}^{(n)}\end{array} \right) = \Omega^{(n)}\left(\begin{array}{cc}1&0\\0&-1\end{array}\right)$$ $$\left(\begin{array}{cc}\mathbf{X}^{(n)}\\\mathbf{Y}^{(n)}\end{array}\right)$$ enables of electronic excitation energies $$\Omega^{(n)}$$. The BSE can be solved by diagonalizing the full ABBA-matrix or by setting B=0, i.e. within the Tamm-Dancoff approximation (TDA). Preliminary reference: Eq. (35) in PRB 92, 045209 (2015); http://dx.doi.org/10.1103/PhysRevB.92.045209 [Edit on GitHub]

Subsections

## Keyword descriptions

SECTION_PARAMETERS: logical = F

Lone keyword: T

Usage: &BSE .TRUE.

Activates BSE calculations. [Edit on GitHub]

BSE_APPROX: enum = TDA

Usage: &BSE_APPROX TDA

Valid values:

• TDA The TDA is applied, i.e. B=0.

• ABBA The ABBA-matrix is diagonalized, i.e. the TDA is not applied.

• BOTH The BSE is solved within the TDA (B=0) as well as for the full ABBA-matrix.

Level of approximation applied to BSE calculations. Choose between Tamm Dancoff approximation (TDA) and diagonalization of the full ABBA-matrix. [Edit on GitHub]

BSE_DEBUG_PRINT: logical = F

Lone keyword: T

Usage: &BSE_DEBUG_PRINT .TRUE.

Activates debug print statements in the BSE calculation. [Edit on GitHub]

BSE_DIAG_METHOD: enum = FULLDIAG

Usage: &BSE_DIAG_METHOD FULLDIAG

Valid values:

• FULLDIAG Fully diagonalizes the BSE matrices within the chosen level of approximation.

• ITERDIAG Iterative diagonalization has not been implemented yet.

Method for BSE calculations. Choose between full or iterative diagonalization. [Edit on GitHub]

ENERGY_CUTOFF_OCC: real = -2.72113839E+001 [eV]

Usage: ENERGY_CUTOFF_OCC 10.0

Removing all orbitals with indices i,j from A_ia,jb and B_ia,jb with energy difference to HOMO level larger than the given energy cutoff, i.e. $$\epsilon_{\mathrm{HOMO}}-\epsilon_a>\mathtt{ENERGY\_CUTOFF\_OCC}$$. Can be used to accelerate runtime and reduce memory consumption. [Edit on GitHub]

ENERGY_CUTOFF_VIRT: real = -2.72113839E+001 [eV]

Usage: ENERGY_CUTOFF_VIRT 10.0

Removing all orbitals with indices a,b from A_ia,jb and B_ia,jb with energy difference to LUMO level larger than the given energy cutoff, i.e. $$\epsilon_i-\epsilon_{\mathrm{LUMO}}>\mathtt{ENERGY\_CUTOFF\_VIRT}$$. Can be used to accelerate runtime and reduce memory consumption. [Edit on GitHub]

EPS_X: real = 1.00000000E-001

Usage: EPS_X 0.1

Threshold for printing contributions of singleparticle transitions, i.e. elements of the eigenvector $$X_{ia}^{(n)}$$. [Edit on GitHub]

NUM_PRINT_EXC: integer = 10

Usage: NUM_PRINT_EXC 10

Number of printed excitation energies. Does not affect computation time. [Edit on GitHub]

SPIN_CONFIG: enum = SINGLET

Usage: SPIN_CONFIG TRIPLET

Valid values:

• SINGLET Computes singlet excitations.

• TRIPLET Computes triplet excitations.

Choose between calculation of singlet or triplet excitation (cf. given Reference above). [Edit on GitHub]