Non-existence of the modified first integral by symplectic integration methods

Abstract It is known that the symplectic mapping obtained as a symplectic integration method is formally an exact time evolution of the modified Hamiltonian which is close to the original Hamiltonian. In the case when the original Hamiltonian has an additional first integral, it is shown that the modified first integral, which is defined to be an integral for the modified Hamiltonian, does not necessarily exist in general. This non-existence of the modified first integral is demonstrated by an example of the 2D harmonic oscillator with an integer frequency ratio.