Largest Lyapunov exponent in molecular systems: Linear molecules and application to nitrogen clusters

The computation of the largest Lyapunov lambda exponent is carried out for a Hamiltonian system made of linear molecules. The internal rotational degrees of freedom are described with unit vectors parallel to molecular axes. As an application, we investigate with classical molecular dynamics the behavior of lambda with internal energy in small nitrogen clusters with 3 and 13 molecules. lambda sharply rises when the molecular rotational degrees of freedom are released. However, no particular changes are observed at the solidlike-rigidlike phase transition. Thus the amount of chaos in the cluster seems mainly governed by the orientational degrees of freedom. [S1063-651X(98)01411-1].