Excited States
TeraChem can reach electronically excited states through several complementary routes. Which one is appropriate depends on the character of the states of interest (valence vs. charge-transfer vs. core-excited), whether they have significant multireference or double-excitation character, and what properties (gradients, nonadiabatic couplings) you need.
Choosing a method
| Method | Reference | Best for | Where it is documented |
|---|---|---|---|
| CIS / TDA-DFT / TDDFT | HF or DFT | General-purpose, singly-excited valence states; large systems; gradients, NACs, photochemistry | CIS / TDDFT (this section) |
| hh-TDA-DFT | \(N+2\) DFT | Photochemistry with low-lying \(\pi\pi^\ast\)/\(n\pi^\ast\) states; consistent ground/excited description through conical intersections | hh-TDA |
| EOM-CCSD | CCSD | High-accuracy excitation energies and ionization potentials for single-reference systems | EOM-CCSD |
| CASSCF / FOMO-CASCI | multireference | Strong static correlation, bond breaking, near-degeneracies; state-averaged excited states | CASSCF |
| Δ-SCF / MOM | HF or DFT | Core-excited / core-ionized states (XPS/XAS) and single-determinant excited states reached by a non-aufbau SCF | MOM / Δ-SCF (this section) |
The bulk of TeraChem's excited-state functionality is built on linear response from a single-determinant SCF reference — this is the CIS / TDA-DFT / TDDFT family covered on the CIS / TDDFT page. The same machinery supports restricted, unrestricted, and restricted-open-shell references, spin-flip excitations, excited-state gradients, nonadiabatic couplings, and a wide range of excited-state properties.
Related capabilities
- Diabatization — transforming the adiabatic states into smoothly varying quasidiabatic states and extracting diabatic couplings — is described on the Diabatization page.
- Conical intersections / minimum-energy crossing points are optimized with the dedicated CIOpt driver — see Optimization → CIOpt.
- Excited-state dynamics (Born–Oppenheimer AIMD on an excited state) is
set up by combining an excited-state method with
run md— see AIMD. - Polaritonic / cavity-coupled excited states are described in the Polaritons section.