Adsorption sites diagonal-fourfold-hollow

In this study four different adsorption sites have been considered, namely the twofold-bridge, the threefold hollow, the diagonal fourfold hollow and the aligned fourfold hollow sites, see Fig. 3. 

To study the adsorption on the twofold-bridge and threefold hollow sites we used a Cu9(5,4) cluster as shown in Fig. 4(a), where the numbers inside brackets indicate the number of metal atoms in the first and second layers respectively. To study the adsorption on the aligned-fourfold-hollow and diagonal-fourfold-hollow sites we used the same cluster but on an inverted position, that is, we used a Cu9(4,5) cluster as shown in Fig. 4(b). 

The difference in stability between the diagonal fourfold-hollow site and the aligned fourfold-hollow site is only 9.2 kJ.moT. This difference is too small to allow us to conclude unambiguously which is the most stable adsorption mode. This served as a motivation to the calculation of the vibrational frequencies for acetylene adsorbed on these two most stable adsorption sites. The results obtained, along with the available experimental results, are presented in Table 4. 

Figure 2 Cluster model (Cui ) used to represent the Cu( 100) surface. Cu( 12,5). C7H4 adsorption on a fourfold hollow site. tcA diagonal adsorption mode. Right aligned adsorption mode...

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