Quantum mechanics COSMO

A. Julg, From Atoms and Molecules to the Cosmos A Quasi-Ergodic Interpretation of Quantum Mechanics, Springer, Berlin, 1998. 

Computational quantum mechanics continues to be a rapidly developing field, and its range of application, and especially the size of the molecules that can be studied, progresses with improvements in computer hardware. At present, ideal gas properties can be computed quite well, even for moderately sized molecules. Complete two-body force fields can also be developed from quantum mechanics, although generally only for small molecules, and this requires the study of pairs of molecules in a large number of separations and orientations. Once developed, such a force field can be used to compute the second virial coefficient, which can be used as a test of its accuracy, and in simulation to compute phase behavior, perhaps with corrections for multibody effects. However, this requires major computational effort and expert advice. At present, a much easier, more approximate method of obtaining condensed phase thermodynamic properties from quantum mechanics is by the use of polarizable continuum models based on COSMO calculations. 

A recent alternative to group-contribution activity-coefficient estimation methods is based on interactions between surface charge distributions (determined by quantum-mechanical calculations) of molecules in solution. The solvation model used for the charge-distribution calculation is known as COSMO the most widely used method based on this technique is called COSMO-RS [47]. 

For many chemical problems, it is crucial to consider solvent effects. This was demonstrated in our recent studies on the hydration free energy of U02 and the model reduction of uranyl by water [232,233]. The ParaGauss code [21,22] allows to carry out DKH DF calculations combined with a treatment of solvent effects via the self-consistent polarizable continuum method (PCM) COSMO [227]. If one aims at a realistic description of solvated species, it is not sufficient to represent an aqueous environment simply as a dielectric continuum because of the covalent nature of the bonding between an actinide and aqua ligands [232]. Ideally, one uses a combination model, in which one or more solvation shells (typically the first shell) are treated quantum-mechanically, while long-range electrostatic and other solvent effects are accounted for with a continuum model. Both contributions to the solvation free energy of U02 were... 

Quantum-Mechanical Continuum Solvation Models. Several ab initio continuum solvation models were discussed in Section 15.22. One can calculate AG, , by such SCRF methods as the dipole-in-a-sphere, the multipole expansion, or the PCM methods using semiempirical methods such as AMI or PM3 instead of an ab initio electronic-structure method. Thus the program MOPAC-93 implements the PCM calculation of solvent effects with semiempirical methods and the program AMPAC 6.0 implements the COSMO method. 

The COnductor-like Screening MOdel (COSMO) is a method that computes the electrostatic interaction of the analyzed molecule with a certain solvent by considering the dielectric continuum surrounding the solute molecule outside of molecular cavities (Klamt and Schuiirmann 1993). The COSMO method can be used by all methods that compute the net atomic charges in analyzed molecules, for example, the semiempirical quantum mechanics method PM6. 

The PSP approach [39-43] is a novel predictive thermodynamic framework, which combines elements from the solubility parameter approach [112,114,116,126] detailed earlier, the solvatochromic/ LSER approach [127-133], and the COSMO-RS theory of solutions [134-136]. It retains the simplicity of the solubility parameter approach, it uses molecular descriptors that can be mapped one to one to the Abraham/LSER descriptors, and these descriptors are derived from the moments of the a-profiles of the quantum mechanics-based COSMO-RS model. Because of this combination, the PSP approach has a broader range of applications compared to each of the earlier three... 

Briefly, the PSP approach heavily resides on the quantum mechanics-based COSMO-RS theory of solutions [17-22], The COSMO model belongs to the class of continuum solvation models (CSM) of quantum mechanics. For the solvation picture, it considers the molecule embedded in a conductor of infinite permittivity that screens perfectly the molecular charges on the surface of its molecular cavity. This molecular cavity is characterized by a volume, Fcogni, and a molecular surface area, The crucial information is contained in the so-called COSMO tile of each compound obtained from quantum chemical calculations at various levels of theory. COSMO tiles give the detailed surface charge distribution or the o-protile of each molecule. The o-protile may be analyzed into its moments of various orders, known as COSMOments, out of which a large number of properties may be calculated, among them the molecular descriptors of Abraham s QSPR/LSER model [23,24]. 

On the other hand, Panayiotou has proposed a novel and potential groundbreaking method for estimating the surface tension components. This is a method based on a quantum mechanical approach and, in particular, the COSMO-RS method pioneered by Klamt and co-workers (see Klamt (2005) for a detailed review of the method). 

COSMO = conductor-like screening model PCM = polarizable continuum method QM/MM = quantum mecha-nics/molecular mechanics. 

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