Pii: S1386-1425(01)00663-1

The factors that need to be taken into account to achieve spectroscopic accuracy for triatomic molecules are considered focusing on H3 + 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 H3 , but are for water for which the magnitude of the various effects is discussed. For H3 + inclusion of rotational non-adiabatic effects means that levels can be generated to an accuracy approaching 0.01 cm; 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. © 2002 Elsevier Science B.V. All rights reserved.