LTRIPLET
LTRIPLET = .TRUE. | .FALSE.
| Default: LTRIPLET | = .FALSE. |
Description: LTRIPLET selects a triplet ansatz for Bethe-Salpether-Equation (BSE) calculations.
For a usual BSE calculation (LHARTREE=.TRUE.; LTRIPLET = .FALSE.), the excited state corresponds to a singlet if the ground state is not spin-polarized (ISPIN=1). That is because the ansatz for the BSE calculation involves a hole and an electron pair prepared within each spin channel. For instance, an electron with up spin is removed from the ground-state determinant and placed with the same spin into a previously unoccupied orbital. If there are no seperate spin channels this ansatz results in a net spin zero, which corresponds to a singlet state.
If LTRIPLET=.TRUE., VASP assumes that an up electron is removed from the ground-state determinant and places a down electron in a previously unoccupied orbital. This ansatz corresponds to a triplet state as it has a net spin one. Without spin-orbit coupling (LSORBIT = .FALSE.), many (or maybe even all) of the triplet solutions will have zero transition probabilities. These correspond to dark, potentially long-lived triplet excitons. To obtain non-zero transition probabilities for the triplets, spin-orbit coupling should be included by setting LSORBIT = .TRUE. in the INCAR file throughout the calculation, i.e., also in the ground-state calculation.
For a BSE calculation (LHARTREE=.TRUE.; LTRIPLET = .FALSE.) based on a spin-polarized ground-state calculation (ISPIN=2), VASP yields one set of singlet solutions and one set of triplet solutions. With LTRIPLET=.TRUE., VASP will obtain the remaining two sets of triplet solutions.
| Warning: LTRIPLET=.TRUE. has not been extensively tested in combination with spin-polarized ground states (ISPIN=2). |
| Warning: Not sure whether the BSEFATBAND approach can be combined with the LTRIPLET tag. |
Related tags and articles
LHARTREE, ISPIN, LSORBIT, BSE calculations