(sec:solvationmodels.typical)=
# Solvation

ORCA features several implicit solvation models, including the fully 
integrated "conductor-like polarizable continuum (C-PCM)" and "Minnesota SMD"
solvation models, which are available in all its components. 
With these models, various types of calculations can be performed 
using a polarizable continuum with a realistic van der Waals cavity
as summarized below:


-   Energies of molecules in solution with a finite dielectric constant
    $\varepsilon$ using HF or any DFT method.

-   Optimization of molecular structures in solution using HF or any DFT
    method with analytic gradients.

-   Calculation of vibrational frequencies using the analytic Hessian
    for HF or any DFT method, provided that the same calculation is 
	available in vacuum.

-   Calculation of solvent effects on response properties like
    polarizabilities through coupled-perturbed SCF theory. For magnetic
    response properties, such as the g-tensor, the C-PCM response
    vanishes.

-   Calculations of solvent shifts on transition energies using the
    time-dependent DFT or CIS method. The refractive index of the solvent 
	needs to be provided in addition to the dielectric constant.

-   First order perturbation estimate of solvent effects on state and
    transition energies in multireference perturbation and
    configuration-interaction calculations.

Other implicit solvation strategies are available in ORCA. In particular, an interface to
 the open source implementation of the COSMO-RS model (openCOSMO-RS), as well as different solvation
models that can be used in XTB (ALPB, ddCOSMO, and CPCM-X). A detailed overview of the
 available implicit solvation methods and their usage is provided in Sections {ref}`sec:oniom`,
and {ref}`sec:solvationmodels.detailed`.

As a simple example, let us compute the solvent effect on the $n\to \pi^{\ast }$ 
transition energy in formaldehyde with the C-PCM model. This effect can be 
obtained by subtracting the solution-phase and gas-phase transition energies. 
The gas-phase transition energy (4.633 eV) can be computed by using the 
following input:

```{literalinclude} ../../orca_working_input/C05S11_199.inp
:language: orca
```

By adding the `CPCM(water)` flag to the input used for the gas-phase calculation, 
the transition energy can now be computed using the C-PCM model with water 
as the solvent:

```{literalinclude} ../../orca_working_input/C05S12_200.inp
:language: orca
```

This C-PCM calculation yields a transition energy of 4.857 eV:

```
-----------------------------
CIS-EXCITED STATES (SINGLETS)
-----------------------------

the weights of the individual excitations are printed if larger than 1.0e-02

STATE  1:  E=   0.178499 au      4.857 eV    39176.0 cm**-1 <S**2> =   0.000000
7a ->   8a  :     0.929287 (c= -0.96399514)
7a ->  13a  :     0.039268 (c=  0.19816055)
7a ->  18a  :     0.016344 (c=  0.12784298)
```

Hence, water environment increases the transition energy by 0.224 eV. This increase 
can be attributed to the stabilization of lone pair orbitals by the presence of 
water molecules.