COSMO solvation model: Difference between revisions

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[[Image:Pentaacrylat.png|thumb|COSMO surface of a pentaacrylate molecule (red = negative, green = positive equilibrium layer).]]

'''COSMO'''<ref name=":0" /><ref name=":1" /> is the abbreviation for "'''CO'''nductor-like '''S'''creening '''MO'''del", 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.

In COSMO thetreats each solvent is treated as a continuum with a [[permittivity]] ''&epsilon;'', 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. The details of the cavity construction differ in different COSMO implementations. 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 ''&fnof;''(''&epsilon;''):

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:<math>f(\varepsilon)=\frac{\varepsilon-1}{\varepsilon+x},</math>

where the value of ''x'' should be set to 0.5 for neutral molecules and to 0.0 for ions, see original derivation<ref name=":1">{{Cite book|title=From Quantum Chemistry to Fluid Phase Thermodynamics and Drug Design|last=Klamt|first=Andreas|publisher=Elsevier|year=2005|isbn=9780444519948|___location=Boston, MA, USA|pages=}}</ref>.

where the value of ''x'' is set to 0.5 based on theoretical arguments. Some re-implementations of COSMO use ''x''&nbsp;=&nbsp;0.

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&ndash;Fock]]-method calculations or [[density functional theory]] (quantum physics) calculations.<ref name=":0">{{cite journal |last=A.|first=Klamt|last2=G.|first2=Schüürmann|date=1993|title=COSMO: a new approach to dielectric screening in solvents with explicit expressions for the screening energy and its gradient|url=http://pubs.rsc.org/en/content/articlepdf/1993/p2/p29930000799|journal=J. Chem. Soc.|publisher=Perkin Trans.2|volume=2|issue=5|pages=799-805|doi=10.1039/P29930000799|access-date=25 October 2015}}</ref>

==Variants and implementations==

COSMO has been implemented in a number of quantum chemistry or semi-empirical codes such as [[TURBOMOLE]], [[Gaussian (software)|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==