Duo is a diatomic code for solving a fully coupled rovibronic Schroedinger equation by Yurchenko, Lodi, Tennyson and Stolyarov, Comp. Phys. Comm, 202, 262 (2016) https://doi.org/10.1016/j.cpc.2015.12.021
The code can be obtain from https://github.com/Trovemaster/Duo
See the Duo mamual at https://duo.readthedocs.io and tutorial at https://github.com/Trovemaster/Duo/wiki