Quadratic F_ij_ force field in l-CH_4_^-^

Three of them are relevant for astrophysics and astrochemistry. These computations are performed using highly correlated ab initio methods and the aug-cc-pVXZ (X=T,Q) basis sets.

In addition to the linear isomers, we predict the existence of several cyclic and branched forms for these molecules. For all the molecular species of interest here, sets of spectroscopic parameters are determined with perturbation theory, which compare quite well with experiment. For l-C_4_H^-^(X^1^{Sigma}^+^), the quartic force field is computed at the coupled cluster level of theory.