Asymmetric-top molecules containing one methyl-like internal rotor: Methods and codes for fitting and predicting spectra

This paper is a review of various effective Hamiltonians, methods and codes dealing with asymmetric-top molecules containing one internal rotor with C3v (or close to C3v) symmetry. It aims at helping high-resolution spectroscopists to find the most appropriate method(s) and code(s) to assign and fit their internal rotor spectra. The topics included are: description of some of the available effective Hamiltonians, methods (PAM, Principal Axis Method, RAM, Rho-Axis Method and IAM, Internal-Axis Method) and codes to deal with internal rotor spectra (JB95, ERHAM, XIAM, SPFIT/SPCAT/IAMCALC and BELGI). The methods are also classified by the way they are dealing with the set of torsional states associated with the internal rotation large amplitude motion: globally or locally. Examples are given of internal rotors treated by each of those codes with root-mean-square deviations, Jmax, torsional bath levels involved, and parameters obtained to allow the reader to choose the appropriate method for his study. More details are given for the RAM method used in BELGI, with series of examples. Some user-manual-like hints for running the fits are also given.