Implicit Solvent Models
When modeling processes in solution, it is essential to account for solvent effects in calculations. Solvent interactions significantly influence not only energies but also molecular geometries.
In xtb, two implicit solvent models are available: GBSA and ALPB. These models can be easily applied to any calculation. They can be invoked using:
xtb struc.xyz --alpb <solvent>
xtb struc.xyz --gbsa <solvent>
Parameterized solvent and method combinations are:
| Solvent | GFN1(ALPB) | GFN1(GBSA) | GFN2(ALPB) | GFN2(GBSA) | GFN-FF |
|---|---|---|---|---|---|
| Acetone | x | x | x | x | x |
| Acetonitrile | x | x | x | x | x |
| Aniline | x | x | x | ||
| Benzaldehyde | x | x | x | ||
| Benzene | x | x | x | x | x |
| CH₂Cl₂ | x | x | x | x | x |
| CHCl₃ | x | x | x | x | x |
| CS₂ | x | x | x | x | x |
| Dioxane | x | x | x | ||
| DMF | x | x | x | x | |
| DMSO | x | x | x | x | x |
| Ether | x | x | x | x | x |
| Ethylacetate | x | x | x | ||
| Furane | x | x | x | ||
| Hexadecane | x | x | x | ||
| Hexane | x | x | x | x | |
| Methanol | x | x | x | x | x |
| Nitromethane | x | x | x | ||
| Octanol | x | x | x | ||
| Octanol (wet) | x | x | x | ||
| Phenol | x | x | x | ||
| Toluene | x | x | x | x | x |
| THF | x | x | x | x | x |
| Water (H₂O) | x | x | x | x | x |
An up-to-date list of available solvents can be found in the documentation.
Warning: If you want to compute a solvent that is not yet parameterized in xtb, use a solvent with a similar dielectric constant that is parameterized. They can be found from several sources, e.g. here. Never do a gas phase computation in such cases.
In principle, both solvent models perform similarly but do not reach the accuracy of more advanced and computationally expensive models like COSMO-RS or SMD.
Note: GBSA and ALPB treat the solvent as an electrostatic continuum that is self-consistently relaxed during the SCC, directly influencing the wavefunction. Thus, the Wafefunction and SCC energies are not identical compared to a gas-phase calculation. Non-electrostatic contributions are added after SCC convergence.
More details on the theoretical background and accuracy of ALPB and GBSA can be found in this publication.
Tip: Using a solvent model can improve SCC or geometry optimization convergence when gas-phase calculations fail to converge. The resulting wavefunction or structure can serve as a good starting point for a gas-phase calculation.