Solvent effects: COSMO

The conductor-like screening model (COSMO) has been implemented in ADF and BAND to include solvent effects for reactions and material properties in solution rather than in the gas phase. In this method the environment effects are treated in an implicit fashion. More advanced solvation options are also available in ADF.

Solvent effects at the liquid-solid interface can be included with the unique combination of BAND and COSMO, as shown here modeling a dye-sensitized solar cell: a solvated N3 ruthenium complex on a TiO₂ surface.

The solvent is handled as a dielectric medium that induces charge polarization on a suitably defined surface around the molecule 'the cavity' immersed in the solution. Any detailed properties of the solvent are reduced to an assumed rigid sphere size of the solvent molecules, which aspect is only used to determine how far the solvent can penetrate into the region occupied by the molecule. This thereby plays a role to determine the effective molecular surface, the solvent accessible surface (SAS).

Technically the SAS is defined as the envelope of a series of spheres. These are atom-centered spheres with Van der Waals-type radii (a bit larger, usually, than standard Van der Waals radii), augmented by auxiliary spheres, if necessary, to describe the effective cavity that contains the molecule and that is not accessible for the solvent.

The surface-describing spheres are partitioned, typically in 60 surface triangles, and each triangle is assigned a point charge, the strength of which is adjustable. Because the spheres defining the SAS may intersect, only those triangles are used that are actually on the surface of the molecule and not, for instance, inside one of the other spheres. In the SCF procedure, the surface-point charges are determined from the charge density and corresponding potential of the molecule itself, and the electrostatic equations assuming, for the moment, a perfect conductor solvent, i.e., a vanishing potential on the cavity surface. This particular assumption makes the equations a lot easier and faster to solve. The actual dielectric value of the medium defines a correction factor for computed properties such as the energy.