Dimerization is typically the first step towards oligomerization and eventual polymerization of a chemical compound under pressure. A well-known example is the dimerization of cyclopentadiene, which proceeds via a [4+2] cycloaddition pathway, but the examples presented in this section go beyond Diels-Alder reactions. Experimentally, a plethora of reactions, ranging from dimerizations to polymerizations, have been observed, e.g., in anthracene, isoprene, formic acid, C 60 , and butadiene. In the latter case, Car-Parrinello Molecular Dynamics (CPMD) simulations have suggested an ionic pathway for the pressure-induced transformation from butadiene to polybutadiene. The finding that pressure leads to an increase in polymerization rate in butadiene is in line with experimental data. Carbon dioxide (CO 2 ) has been the target of many computational studies employing high-pressure simulation techniques. Besides the well-founded theoretical and practical interest in the high-pressure behavior of CO 2 , the compound has been discussed as a source of carbon in Earth's interior and some relevant chemical reactions have been studied computationally. In an early work, liquid CO 2 was scrutinized with AIMD simulations and the variable-cell method at pressures of up to 50 GPa and temperatures as high as 4000 K. The formation of a metastable dimer with a symmetric four-membered ring was observed at 20 GPa, which remained stable when the pressure was decreased. A similar observation was made with the X-HCFF method, however, CO 2 dimerization was observed at higher pressures than in the aforementioned AIMD simulations (90-100 GPa), which was traced back to the omission of thermal effects in the static X-HCFF calculations. Since thermal oscillations lead to random collisions that may help overcome remaining energy barriers, it can be speculated that static methods based on geometry optimizations at 0 K generally yield higher activation pressures than dynamic methods, e.g., AIMD simulations.