# Glossary¶

Adiabatic: Electronic excitation not coupled with nuclear motion

Non-adiabatic: Electronic excitation coupled with nuclear motion

Rydberg States: Excitations in which the electrons is excited so high up that rather than being a part of the molecule, it behaves more like an electron orbiting a nucleus, in a S/P/D like orbital.

Kasha’s Rule: Due to how close in energy excited states will be to each other, photonic emission will only happen in appreciable yield from the lowest excited to the ground state. Another way to think about this is that the wavelength of an emitted photon is going to be independent of the photon that excited the molecule.

Stokes shift: The difference between an adiabatic absorbed and adiabatic emitted photon for a particular set of $$\ce{\Psi^0_{minima}->[h\nu]\Psi^*\to\Psi^*_{minima}->[h\nu]\Psi^0}$$

Charge-Transfer State: “A state, related to the ground state by a charge transfer transition”

Charge-Transfer Transition: The movement of an electron from one part of a molecule/complex to another

Quantum Yield ($$\Phi$$): The ratio of photons emitted to the ratio of photons absorbed (range of 0 to 1).

Internal conversion: vibronic/electronic transitions that release energy in the form of heat instead of light

Intersystem Conversion: The relaxation process of an excited singlet state to the more stable excited triplet state $$S_1\to T_1$$

Real-time TDDFT: considers the actual time porpogation of the electron density by simulating it over tens of femtoseconds. The absorption spectrum can be obtained from the response of the dipole moment in response to an electric perturbation in this simulation

Linear-response TDDFT: This approach considers the instantaneous response through perturbatiuon of the ground state density at time $$=t_0$$.

Herzberg-Teller effect: It looks like this is some form of coupling between ground and excited states that can allow FC forbidden transitions to occur. In talking to someone more versed in physics than myself, it sounds like this is a term that’s used to account for non-adiabatic (diagonal) transitions, which is why it might allow for certain forbidden FC vertical excitations to happen.

0-0 transition: The non-adiabatic energy difference between the enthalpy of the ground and excited state ($$\Delta (E^{ES}+ZPVE^{ES})-(E^{GS}+ZPVE^{GS})$$)

CASSCF VS CASCI: This one pops up a lot, but is simply that if CASCI creates a multireference wavefunction based on many determinants of the reference wavefunction/theory, with different orbital occupations, CASSCF does so, AND optimises the orbitals for the linear combination of those states accordingly. Excited states seem to be added in separately as an MRCI on top of CASSCF, so that each of the states has their own set of partial orbital occupations.

Size intensivity: is similar to size extensivity, except that rather than looking at the difference in the energy of multiple isolated systems, it considers whether the excitation energy of one system ($$A\to A^*$$) will be effected by a noninteracting system ($$B)$$. Where a method could be considered size extensive if:

$E(A) + E(B) = E(A\cup B)$

A method can be considered size intensive if:

$E(A^*)-E(A)=E(A^*\cup B)-E(A\cup B)$

SCRF: Solvation that acts within the SCF. It is converged as a part of the reaction field instead of being applied as a correction.

State Specific solvation: When the SCRF procedure is performed, it has to be performed on the same electronic configuration as is being formed in the SCF procedure. This is usually the groun state. State specific solvation considers a specific state other than the ground state.
Equilibrium solvation: When considering solvation of excited states, since the electronic excitation happens very quickly compared to the nuclear relaxation, the solvent is considered to be in a non-equilibrium if the ground state solvation is used for the excited states. Equilibrium solvation recomputes the solvent field for each root, as though the solvent has has time to relax around the excited state.