As described in Section 12.2 (and especially
Section 12.2.2), continuum solvent models such as C-PCM allow one
to include solvent effect in the calculations. TDDFT/C-PCM allows
excited-state modeling in solution. Q-Chem also features TDDFT coupled with
C-PCM which extends TDDFT to calculations of properties of
electronically-excited molecules in solution. In particular, TDDFT/C-PCM allows one to perform geometry optimization and vibrational
analysis.^{581}

When TDDFT/C-PCM is applied to calculate vertical excitation
energies, the solvent around vertically excited solute is out of equilibrium.
While the solvent electron density equilibrates fast to the density of the
solute (electronic response), the relaxation of nuclear degrees of freedom
(*e.g.*, orientational polarization) takes place on a slower timescale. To
describe this situation, an optical dielectric constant is employed. To
distinguish between equilibrium and non-equilibrium calculations, two
dielectric constants are used in these calculations: a static constant
(${\epsilon}_{0}$), equal to the equilibrium bulk value, and a fast constant
(${\epsilon}_{fast}$) related to the response of the medium to high frequency
perturbations. For vertical excitation energy calculations (corresponding to
the unrelaxed solvent nuclear degrees of freedom), it is recommended to use the
optical dielectric constant for ${\epsilon}_{fast}$), whereas for the geometry
optimization and vibrational frequency calculations, the static dielectric
constant should be used.^{581}

The example below illustrates TDDFT/C-PCM calculations of vertical excitation energies. More information concerning the C-PCM and the various PCM job control options can be found in Section 12.2.

$molecule 0 1 C 0.0 0.0 0.0 O 0.0 0.0 1.21 $end $rem EXCHANGE B3lyp CIS_N_ROOTS 10 CIS_SINGLETS true CIS_TRIPLETS true RPA TRUE BASIS 6-31+G* XC_GRID 1 SOLVENT_METHOD pcm $end $pcm Theory CPCM Method SWIG Solver Inversion Radii Bondi $end $solvent Dielectric 78.39 OpticalDielectric 1.777849 $end