Potential energy surfaces cluster models

KD Ball, RS Beii y, RE Kunz, E-Y Li, A Proykova, DJ Wales. Erom topographies to dynamics of multidimensional potential energy surfaces of atomic clusters. Science 271 963-966, 1996. RS Berry, N Elmaci, JP Rose, B Vekhter. Linking topography of its potential surface with the dynamics of folding of a protein model. Proc Natl Acad Sci USA 94 9520-9524, 1997. Z Guo, D Thii-umalai. J Mol Biol 263 323-343, 1996. 

In siunmary, although the application of detailed chemical kinetic modeling to heterogeneous reactions is possible, the effort needed is considerably more involved than in the gas-phase reactions. The thermochemistry of surfaces, clusters, and adsorbed species can be determined in a manner analogous to those associated with the gas-phase species. Similarly, rate parameters of heterogeneous elementary reactions can be estimated, via the application of the transition state theory, by determining the thermochemistry of saddle points on potential energy surfaces. 

To understand the fundamental photochemical processes in biologically relevant molecular systems, prototype molecules like phenol or indole - the chromophores of the amino acids tyrosine respective trypthophan - embedded in clusters of ammonia or water molecules are an important object of research. Numerous studies have been performed concerning the dynamics of photoinduced processes in phenol-ammonia or phenol-water clusters (see e. g. [1,2]). As a main result a hydrogen transfer reaction has been clearly indicated in phenol(NH3)n clusters [2], whereas for phenol(H20)n complexes no signature for such a reaction has been found. According to a general theoretical model [3] a similar behavior is expected for the indole molecule surrounded by ammonia or water clusters. As the primary step an internal conversion from the initially excited nn state to a dark 7ta state is predicted which may be followed by the H-transfer process on the 7ia potential energy surface. 

IR results show that the tricarbonyl complex is not formed. To check this result modelling was performed. It was not possible to reach a minimum in the potential energy surface for the tricarbonyl Nin complex with a nickel dicoordinated to the cluster the structure evolves toward a dicarbonyl complex. Even if a ligand displacement of the silica surface is assumed by adding a third CO molecule as shown by the reaction ... 

Ab initio and density functional calculations of potential energy surfaces for the ground and excited electronic states of model clusters simulating various point defects, impurities, and their combinations in nanosized silica and germania materials are reported. The accurate geometric and electronic structures of these clusters, calculated photoabsorption and photoluminescence (PL) energies, and predicted absorption and PL spectra are obtained. Our calculations reproduced the experimental excitation energy (1.9-2.0 eV)... 

Equation (3.21) shows that the potential of the mean force is an effective potential energy surface created by the solute-solvent interaction. The PMF may be calculated by an explicit treatment of the entire solute-solvent system by molecular dynamics or Monte Carlo methods, or it may be calculated by an implicit treatment of the solvent, such as by a continuum model, which is the subject of this book. A third possibility (discussed at length in Section 3.3.3) is that some solvent molecules are explicit or discrete and others are implicit and represented as a continuous medium. Such a mixed discrete-continuum model may be considered as a special case of a continuum model in which the solute and explicit solvent molecules form a supermolecule or cluster that is embedded in a continuum. In this contribution we will emphasize continuum models (including cluster-continuum models). 

To conclude this section, it should be noted that the calculations of the potential energy surfaces for heterogeneous catalytic reactions, even by semiempirical methods, still remain a matter for the future. Insufficient accuracy of the semiempirical methods, the approximate nature of cluster modeling, the large volume of a configurational space, a variety of possible reaction paths, etc., considerably restrict the utility of quantum chemistry as applied to this field. There is, however, no doubt that these difficulties will be successfully overcome. The value of conclusive quantum-chemical calculations can hardly be overestimated. They are able to answer questions which the most sophisticated and refined experiments would fail to answer. 

---