Ab initio rotation–vibration energy levels of triatomics to spectroscopic accuracy

Abstract

The factors that need to be taken into account to achieve spectroscopic accuracy for triatomic molecules are considered focusing on H 3 + and water as examples. The magnitude of the adiabatic and non-adiabatic corrections to the Born–Oppenheimer approximation is illustrated for both molecules, and methods of including them ab initio are discussed. Electronic relativistic effects are not important for H 3 +, but are for water for which the magnitude of the various effects is discussed. For H 3 + inclusion of rotational non-adiabatic effects means that levels can be generated to an accuracy approaching 0.01 cm −1; for water the error is still dominated by the error in the correlation energy in the electronic structure calculation. Prospects for improving this aspect of the calculation are discussed.