In such systems, the photoreactive potential energy surfaces feature a CI, which presence in CPDs has been demonstrated in multiple flavors. For both our model systems, we have also performed CASSCF calculations and found that the CI is still encountered regardless of the presence of D. The location of the CIs is also included in Figure . DFT calculations, although inaccurate in describing CI region, can describe the PES shapes and qualitatively point to the regions where the CI is located. The PESs clearly feature the covalent cyclodimer and the stacked dimer as two low-energy basins. Interestingly, in our S 1 relaxed PESs, we found a small barrier from the cyclodimer to the region of the CIs. The presence of D obviously de-symmetrizes the surface and the location of the CI. The main consequence of the hydrogen-bonded D is the possibility of accepting a further hydrogen bond with the U-sandwiched molecules (as shown in Figure ). Although these results are in-vacuum, they still suggest how D could effectively promote U photodimerization by forming hydrogen bonds with two U molecules simultaneously. We speculate that this could dynamically translate into longer-living non-covalent structures in the presence of D, hence increasing the probability of photodimerization. Interestingly, while we limited our analysis to singlet states, we found that for given U:U configurations, an intermolecular proton transfer may take place (see Figure ). However this may not be a stable configuration (see SI Figure ). Exploring all possible photochemical pathways in a complex system, such as D-U The propensity for covalent bond formation is primarily a criterion for judging whether a compound or reaction pathway is plausibly prebiotic. We propose that non-covalent interactions between small building blocks of life may provide new insights into chemical evolution. In particular, intermolecular interactions between nucleobases can significantly alter their (already complex) potential energy landscapes and dynamically opens the possibility to photoactivation-driven reactions. Thus, in the early stages of chemical evolution, nucleobases with complementary supramolecular interactions may have been selected or neglected based on their thermodynamic and kinetic stability or photostability.