The minimal stage, energy preserving Runge-Kutta method for polynomial   Hamiltonian systems is the Averaged Vector Field method
  No Runge-Kutta method can be energy preserving for all Hamiltonian systems. But for problems in which the Hamiltonian is a polynomial, the Averaged Vector Field (AVF) method can be interpreted as a Runge-Kutta method whose weights $b_i$ and abscissae $c_i$ represent a quadrature rule of degree at least that of the Hamiltonian. We prove that when the number of stages is minimal, the Runge-Kutta scheme must in fact be identical to the AVF scheme. 
Elena Celledoni, Brynjulf Owren, Yajuan Sun
math.NA