Modification of the Hamiltonian in the principal axes of inertia for nonrigid molecules

Advantages of hamiltonians in the principal axes of inertia which are used for studying nonrigid molecules are set forth. Representation of orbital angular momentum through non-holonomic components of rotations of dynamical axes is stated. Peculiarities of three modifications of rovibronic hamiltonians are considered, with the dynamical axes an associated with the nuclear subsystem; nuclei and electrons are treated uniformly when the principal axes are brought into use; a hamiltonian of linear molecules is represented as the hamiltonian in the principal axes. Special attention is paid to the use of non-holonomic variables.