Solvation

Why account for solvation

Gas-phase calculations describe a molecule in a vacuum. Real chemistry usually happens in solution, however, and a polar solvent can stabilise ions or molecules with large dipoles by anywhere from a few to several tens of kcal/mol. Ignoring solvation can put reaction equilibria, acid–base equilibria, and free-energy barriers far from experiment.

Available models — CPCM · SMD · ALPB

ORCA supports the following implicit-solvent models:

This page focuses on the two you will use most often — CPCM and SMD. The manual activates both via the same keyword, but SMD also includes non-electrostatic terms (cavity formation, dispersion, surface tension) that make it more appropriate for free-energy comparisons.

Basic usage — one extra keyword

CPCM is activated by adding one keyword to the keyword line:

# Benzene optimisation in water (CPCM)
! B3LYP D4 def2-TZVP RIJCOSX def2/J Opt CPCM(water)

* xyzfile 0 1 benzene.xyz

In the formaldehyde example from the manual, the gas-phase n → π* excitation energy of 4.633 eV shifts to 4.857 eV in water with CPCM. The n-orbital is stabilised further in a hydrogen-bonding environment, so the excitation energy increases by 0.224 eV (~5 kcal/mol).

SMD — including the free-energy correction

Plain CPCM gives only the electrostatic free energy. For solvation free energies ΔG_solv or quantitative reaction equilibria, SMD — which includes the non-electrostatic correction — is more appropriate.

# Activate SMD — CPCM keyword + SMD true
! B3LYP D4 def2-TZVP RIJCOSX def2/J Opt Freq CPCM(water)

%cpcm
   SMD       true
   SMDSolvent "water"     # solvent must be repeated here
end

* xyzfile 0 1 mol.xyz

With SMD active, the output gains entries such as:

--------------------
SMD CDS free energy
--------------------
G-CDS (kcal/mol):              1.234567
...

----------------------------------------------------
SMD SOLVATION FREE ENERGY (CONTRIBUTIONS)
----------------------------------------------------
Total electrostatic    :     -8.456 kcal/mol
Total non-electrostatic:      1.235 kcal/mol
                         ===================
SMD ΔG_solv            :     -7.221 kcal/mol

Fine control — the %cpcm block

For precise control over the dielectric constant, refractive index and cavity shape, use the %cpcm block:

%cpcm
   Epsilon      80.4          # static dielectric constant (water)
   RefRac       1.33          # refractive index (for the dynamic response in TD-DFT)
   Rsolv        1.385         # solvent radius (Å)
   SurfaceType  vdw_gaussian  # cavity surface type
end

Two surface types are available for SurfaceType:

With the standard solvent keywords (CPCM(water), CPCM(ethanol), …) all of the above defaults are set automatically; you rarely need to touch them.

Built-in solvent list

The CPCM standard solvents include the entries below; more than 30 are built in, and SMD supports even more. The complete list is in section 7.53 of the manual.

SOLVATOR — automated placement of explicit solvent molecules

Implicit models cannot capture directional interactions such as specific hydrogen bonds. A microsolvation strategy — explicitly modelling the few most important hydrogen bonds while treating the rest with CPCM — is often a worthwhile compromise. ORCA 6 includes the SOLVATOR utility to automate this.

# Auto-place 5 solvent molecules around the solute, then compute
! B3LYP D4 def2-SVP Opt

%solvator
   NSolv     5            # number of solvent molecules
   Solvent   "water"      # built-in name or .xyz file
end

* xyzfile 0 1 solute.xyz

SOLVATOR places solvent molecules around the solute using a pre-defined force field. The resulting geometry is saved to a separate file and used for the subsequent quantum-chemistry calculation. Details are in section 6.10 of the manual.

Excited states + solvation

When you combine TD-DFT with CPCM, both the equilibrium and non-equilibrium solvent responses are handled automatically.

! B3LYP D4 def2-TZVP CPCM(water)

%tddft
   NRoots 10
end

* xyzfile 0 1 mol.xyz

ORCA separates the fast (optical) response, which depends on the refractive index, from the slow (electrostatic) response, which depends on the static dielectric constant. This is the standard recipe for treating absorption (fast) and emission / equilibrium (slow) consistently.