COSMO Solvation Model

COSMO (short for " Conductor -like screening model" ) is a method for calculating the electrostatic interaction of a molecule with a solvent.

Basics

In COSMO, the solvent is treated using a dielectric constant and other parameters (volume of the molecules, Atomradii, ...) as a continuum. Therefore, the method belongs to the class of continuum models (English continuum solvation models ). It is further assumed that the solvent reaches up to the " interface " of the solute molecule. This interface is recognized as an envelope of spheres around the individual atoms (van der Waals radius of the atoms plus a fixed distance for the solvent molecules ). For the actual calculation, this area is by plane elements, eg Triangles approximated.

When the solvent would be an ideal conductor, the electric potential should vanish on this surface, from the charges could be given a known distribution in the molecule easily determine the charge q * at the surface.

For real solvent can be approximated by assuming that the charge q by a factor f is less than the charge q *:

This factor is approximately

The summand 0.5 in the denominator is a size empirically found.

From the thus-determined charge of the solvent, and the charge distribution of the molecule known to the energy of the interaction between the solute molecule and the solvent can then be determined.

Application

The COSMO method can be used for all calculation methods of theoretical chemistry, in which the charge distribution of a molecule can be determined, for example, semi -empirical calculations, Hartree- Fock or density functional theory calculations.

Comparison with other methods

While models based on multipole expansion of the charge distribution of a molecule, are limited to small or approximately spherical or ellipsoidal -shaped molecules, the COSMO method has the advantage that it is also applicable for large and irregularly shaped molecular structures.

The COSMO method is more accurate, the higher the dielectric constant of the solvent is because the fluid behaves like an ideal conductor in the limit of infinitely high dielectric constant; in water ( ε ≈ 80) a fairly good accuracy is already achieved. A complete solution of the electrostatic equations would be more accurate for solvents with low dielectric constant, but is associated with higher costs.

In contrast to the molecular dynamics calculations in which the movement of solvent molecules and their location is calculated and density is averaged over some time, the COSMO model, as all continuum models, the advantage of a much lower computational complexity. However, it is generally not in a position to phenomena associated with the granularity of the solvent to describe correctly.