AnÃlise topolÃgica de redes de ligaÃÃes de hidrogÃnio em um sistema modelo

AUTOR(ES)
DATA DE PUBLICAÇÃO

2006

RESUMO

An analysis of the local and global topological properties of the hydrogen bonds networks between water molecules, generated by Monte Carlo (MC-NPT) simulations near to supercritical conditions and using the TIP5P model potential for water-water interactions, indicates the appearance of small worlds patterns with large clustering coefficient and small path lengths. The H-bonds networks in water at room condition do not present such patterns, and a high degree of clustering is observed with the presence of a giant cluster that percolates through the whole system (Ref. 1). In this work we perform MC-NPT simulations for water in normal conditions (298 K and 1 atm), where we introduce in the electrostatic interaction potential a damping factor that modifies the charges at site of hydrogen and isolated pairs. Therefore we can reduce, in a controlled way, the number of hydrogen onds. Our results for the clustering coefficient (C), the average degree ( k ) and chemical distance (L), as a function of the damping factor (0,7 ≤ λ ≤ 1), indicates that, at λc= 0,75, the model system shows a topological phase transition separating the regions of small and large values of C and k , for λ <λc and λ >λc, respectively. Our statistical analysis of islands in the region λ >λc shows the existence of a giant cluster with about 99% of the components, whereas for λ <λc a large number of monomers, dimers and trimers occurs and we do not have the percolation of the system. The behavior of the clustering coefficient and the chemical distance as a function of the parameter λ shows similarities with the results obtained by Watts and Strogatz (Ref. 2), in which they suggest a region for small worlds patterns. The ratio C/Crand, where Crand is the clustering coefficient for random networks with the same number of nodes (N), is not depend on the parameter λ, and varies linearly with N, with angular coefficient given by ratio C / k = 1/6. The degree distribution for the H-bonds networks in the region λ >λc is very similar to a Poisson distribution, as expected for random networks, but for λ <λc the distribution becomes asymmetrical and different from Poisson distribution. Calculations of the thermodynamics functions, such as density, enthalpy and vaporization enthalpy, are also indicative of the occurrence of a topological phase transition in λc= 0,75

ASSUNTO(S)

fisica statistical mechanics, complex networks, phase transition, monte carlo method mecÃnica estatÃstica, redes complexas, transiÃÃo de fase, mÃtodo de monte carlo

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