The series divergence in the perturbation theory should be taken into account in calculation of highly excited energy levels of light molecules by the effective rotation Hamiltonian method. For calculation of the rotation energy levels for the H3+ molecule in the ground vibration state, the well-known Euler method of series transformation is used. It is shown that the use of the elementary approximation function - the diagonal Pade approximant of the first order - allows calculating accurately the high energy levels.