The Atiyah-Hitchin bracket for the cubic nonlinear Schrodinger equation.   ii. Periodic potentials
  This is the second in a series of papers on Poisson formalism for the cubic nonlinear Schr\"{o}dinger equation with repulsive nonlinearity. In this paper we consider periodic potentials. The inverse spectral problem for the periodic auxiliary Dirac operator leads to a hyperelliptic Riemann surface $\G$. Using the spectral problem we introduce on this Riemann surface a meromorphic function $\P$. We call it the Weyl function, since it is closely related to the classical Weyl function discussed in the first paper. We show that the pair $(\G,\P)$ carries a natural Poisson structure. We call it the deformed Atiyah--Hitchin bracket. The Poisson bracket on the phase space is the image of the deformed Atiyah--Hitchin bracket under the inverse spectral transform. 
K.L. Vaninsky