COSMO solvation model
COSMO[1][2] is the abbreviation for "COnductor-like Screening MOdel", a calculation method for determining the electrostatic interaction of a molecule with a solvent. The method is commonly used in computational chemistry to model solvation effects.
COSMO treats each solvent as a continuum with a permittivity ε and therefore belongs to the "continuum solvation" group of models. As in all these models COSMO approximates the solvent by a dielectric continuum, surrounding the solute molecules outside of a molecular cavity. In most cases it is constructed as an assembly of atom-centered spheres with radii approximately 20% larger than the Van der Waals radius. For the actual calculation the cavity surface is approximated by segments, e.g., hexagons, pentagons, or triangles.
Unlike other continuum solvation models, COSMO derives the polarization charges of the continuum, caused by the polarity of the solute, from a scaled-conductor approximation. If the solvent were an ideal conductor the electric potential on the cavity surface must disappear. If the distribution of the electric charge in the molecule is known, e.g. from quantum chemistry, then it is possible to calculate the charge q* on the surface segments. For solvents with finite dielectric constant this charge q is lower by approximately a factor ƒ(ε):
The factor ƒ(ε) is approximately
where the value of x should be set to 0.5 for neutral molecules and to 0.0 for ions, see original derivation[2].
From the thus determined solvent charges q and the known charge distribution of the molecule, the energy of the interaction between the solvent and the solute molecule can be calculated.
The COSMO method can be used for all methods in theoretical chemistry where the charge distribution of a molecule can be determined, for example semiempirical calculations, Hartree–Fock-method calculations or density functional theory (quantum physics) calculations.[1]
Variants and implementations
COSMO has been implemented in a number of quantum chemistry or semi-empirical codes such as TURBOMOLE, Gaussian, GAMESS and MOPAC. A COSMO version of the polarizable continuum model PCM has also been developed. Depending on the implementation, the details of the cavity construction and the used radii, the segments representing the molecule surface and the x value for the dielectric scaling function may vary.
Comparison with other methods
While models based on the multipole expansion of the charge distribution of a molecule are limited to small, quasi-spherical or ellipsoidal molecules, the COSMO method has the advantage that it can be applied to large and irregularly formed molecular structures.
The COSMO method is more accurate for solvents with a higher permittivity because a solvent with infinite permittivity behaves like an ideal conductor. With water (ε ≈ 80) a very good accuracy is achieved. Nevertheless, with the choice of x = 0.5 even at low permittivities it is almost as accurate as a complete solution of the electrostatic equations, though at much lower numerical costs. Apart from the numerical effciciency, another big advantage of COSMO compared to other dielectric continuum methods is its huge reduction of the artifacts caused by the small part of the electron density reaching outside of the cavity, the so-called outlying charge errors.
See also
References
- ^ a b A., Klamt; G., Schüürmann (1993). "COSMO: a new approach to dielectric screening in solvents with explicit expressions for the screening energy and its gradient". J. Chem. Soc. 2 (5). Perkin Trans.2: 799–805. doi:10.1039/P29930000799. Retrieved 25 October 2015.
- ^ a b Klamt, Andreas (2005). From Quantum Chemistry to Fluid Phase Thermodynamics and Drug Design. Boston, MA, USA: Elsevier. ISBN 9780444519948.
Further reading
- A. Klamt, G. Schüürmann (1993). "COSMO: A new approach to dielectric screening in solvents with explicit expressions for the screening energy and its gradient". Journal of the Chemical Society, Perkin Transactions 2: 799–805. doi:10.1039/P29930000799.