Quantum-chemical and DFT simulations in-vacuum. Ground and excited state 2D potential energy surfaces for UU and D-UU dimerization were calculated using the Gaus-sian09 package 56 on the Graham cluster operated by Compute Canada. Energies were calculated using the wB97XD functional 57 and 6-31G* basis set. Ground state optimizations involved freezing two carbon-carbon bonds between U and U (using the opt=modredundant keyword) from 1.5 to 4.0 Å. Scans from 1.5 to 2.6 Å were performed with a precision of 0.1 Å, and scans from 2.6 to 4.0 Å were performed with a precision of 0.2 Å. Both unrelaxed and relaxed scans were also performed for the first excited state using TDDFT starting from the ground state geometries. In addition, at the same level of theory, a full ground state minimization was run for each of the basins identified in the PESs for UU and D:UU, followed by TDDFT evaluation of excitation energies and first excited state relaxations. After a proper evaluation of the active spaces, CI optimizations through opt=conical via CASSCF(4,4) for UU and CASSCF (6,6) for DU:U and 6-31G* basis set using Gaussian16 package 59 were successfully performed. These final CASSCF calculations were run at the Isabella cluster in Zagreb, Croatia.