Another field of activity concerned molecular complexes. For one thing, some of these complexes might be present in space. One example are cation-He complexes as investigated in project B2. Another example is the CO-H2 complex which comprises the two most abundant molecules in space. For example, the weakly-bound (H2)2 dimer has been detected in astronomical sources like Jupiter. Spectra of the CO-H2 complex had been recorded in our laboratory already some time ago by P. Jankowski et al., (J. Chem. Phys. 138, 084307, 2013). Further details on the potential energy surface (PES) were also inferred from more recent studies of the CO-HD complex by one of the postdocs in this project, A. Potapov, (L. Surin, et al., J. Mol. Spectrosc. 307, 18, 2015). Based on these investigations a collaboration between projects A6 and B4 searched for the CO-H2 complex in a cold, dense core in TMC-1C (with a temperature of 10 K) using the IRAM 30m telescope. All the brightest CO-H2 transitions in the 3 mm (80–110 GHz) band were observed with a spectral resolution of 0.5–0.7 km/s, reaching a rms noise level of 2 mK. Although no lines belonging to the CO-H2 complex were detected, a stringent upper limit for its abundance was determined to be [CO-H2]/[CO] < 5·10−6 compared to estimates lying in the range 10−8 to 10−3. The spectra of more complexes (CO-N2 L. A. Surin, A. Potapov, A. A. Dolgov, et al., J. Chem. Phys. 142, 114308, 2015, CO-NH3 L. A. Surin, A. Potapov, H. S. P. Müller, et al., J. Mol. Spectrosc. 307, 54, 2015, and H2-NH3 L. A. Surin, I. V. Tarabukin, et al., ApJ 838, 27, 2017) have been recorded with the very sensitive jet spectroscopy instrumentation of this project. These studies are related to the collision work in progress because the existing collaboration with theoreticians like Ad van der Avoird (Radboud University, Nijmegen, NL) reveals very accurate PESs which are necessary to predict the spectra of these weakly bound complexes, as well as rate coefficients for rotational inelastic collisions of which so far we only studied NH3 self collisions.