Introduction

Abstract

When trying to identify the molecules whose pure rotational or rovibrational transitions give rise to lines observed in astronomical sources, astrochemists and astrophysicists often face the problem that very limited spectroscopic information is contrasted by a vast variety of possible carriers. In order to reduce the number of possibilities, an increasing knowledge about the conditions in the sources, their molecular composition and in particular highly precise predictions from ab initio calculations are desirable. This is especially the case since many of the molecules identified so far are highly reactive species such as radicals, carbenes or molecular ions, so that the measurement of spectroscopic parameters in the laboratory is very challenging.