Transitions between metastable states in silica clusters

Relaxation phenomena in glasses can be related to jump processes between di fferent minima of the potential energy in the configuration space. These tr ansitions play a key role in the low temperature regime, giving rise to tun nelling systems responsible for the anomalous specific heat and thermal con ductivity in disordered solids with respect to crystals. A recently develop ed numerical algorithm is used to study the potential energy landscape of s ilica clusters, taking as a starting point the location of first-order sadd le points. This makes it possible to find a great number of adjacent minima . An analysis is made of the degree of cooperativity of these transitions a nd the connection between physical properties and the topology of the confi guration space. Two-level systems (pairs of minima constituting a tunnellin g system) are identified, and the quantum mechanical ground state splitting is calculated by means of the Wentzel-Kramers-Brillouin approximation.