An exact Eckart-embedded kinetic energy operator in Radau coordinates for triatomic molecules

We present exact kinetic energy operators in Radau coordinates for two general families of molecule-fixed frames. The members of one molecule-fixed frame family are distinguished by different values of a parameter f. Special values of f yield the Radau vector- and bisector-embedded kinetic energy operators. The members of the other family of molecule-fixed frames are all Eckart frames. The members of this family are distinguished by different values of a parameter ηe. The Coriolis terms of the exact Eckart kinetic energy operator do not depend on ηe. The exact Eckart embedded Radau kinetic energy operator should be useful for calculating and analysing ro-vibrational spectra.