Hamiltonian symplectomorphisms and the Berry phase
  On the space ${\cal L}$, of loops in the group of Hamiltonian symplectomorphisms of a symplectic quantizable manifold, we define a closed ${\bf Z}$-valued 1-form $\Omega$. If $\Omega$ vanishes, the prequantization map can be extended to a group representation. On ${\cal L}$ one can define an action integral as an ${\bf R}/{\bf Z}$-valued function, and the cohomology class $[\Omega]$ is the obstruction to the lifting of that action integral to an ${\bf R}$-valued function. The form $\Omega$ also defines a natural grading on $\pi_1({\cal L})$. 
Andr\'es Vi\~na