## Non-rigid rotor

https://sci.tanta.edu.eg/files/Microwave%20spectroscopy%20BSc-Lect-2.pdf
irregular not-equidistant levels

## Tunneling - https://en.wikipedia.org/wiki/Pyramidal_inversion

[Ammonia - more is different](http://www.tcm.phy.cam.ac.uk/~bds10/aqp/anderson72more_is_different.pdf)