When running TD calculations the resulting excited states have contributions from several transitions (e.g. from HOMO-1 to LUMO and and from HOMO-2 to LUMO). Why is that? Theoretically If I have a photon of specific energy I will have only one transition between the orbitals that match this specific energy.
Orbitals are just a mathematical construct (very convenient though). Transitions are not between orbitals, they are between states, which frequently can not be described by single Slater determinant (constructed of orbitals), hence multiple contributions mean that you need several "excited" determinants to describe this state.
Orbitals are just a mathematical construct (very convenient though). Transitions are not between orbitals, they are between states, which frequently can not be described by single Slater determinant (constructed of orbitals), hence multiple contributions mean that you need several "excited" determinants to describe this state.
excited electronic states are considered as multi-reference (contribution of the ground electronic state and single ionized, double ionized, triple...states) so the resulting is multiconfigurationnal states
Thank you very much for your answers Oleg and Said.
So the energy that is given for each excited state in the log file when you are running a TD calculation what is it? is referred to what? Is it a mean value of the energy required for the contributing transitions to take place?! Or is just the energy difference between my ground state and excited state?
So far I thought that this is the energy needed for exciting an electron from one orbital to another. Also, I have been told that the energies of the unoccupied orbitals calculated are not correct and if I want for example the energy of the LUMO I have to take the energy for the excitation with major contribution HOMO -> LUMO and add it to the energy of the HOMO. But since the transitions are between states or multireference this is cant be correct no?
Resulting energy is energy difference between ground and excited state. HOMO-LUMO gap is just (generaly very poor) a way to somehow estimate energy of first excited state.
- Badges
- Science topic