During structure relaxation, the unit cell parameters remain fixed while the atoms are allowed to move to minimize the energy of the structure. We elected to fix the unit cell lattice parameters during structural relaxation based on the comparison of ( ) the lattice parameters and ( ) the simulated Xe isotherm ( K) in (a) the experimental Ni(PyC) 2 structure and (b) the DFT-optimized Ni(PyC) 2 structure without unit cell constraints. When we allow, in the DFT structure optimization of Ni(PyC) 2 , (i) the lattice constants to vary (but fixed angles) or (ii) the lattice constants and unit cell angles to vary, the resulting lattice parameters are inconsistent with those of the experimental structure; moreover, the simulated Xe isotherms for these structures over-predict the experimentally measured adsorption at saturation. See Fig. and Sec. S . . A fraction of the functionalized structures failed to converge on the first round of structural relaxation. For those, to encourage convergence and produce a reasonable structure (i.e., no overlapping atoms, drifting atoms, or contorted aromatic rings) as a starting configuration for DFT relaxation, we apply one of the following protocols: (i) start with loose convergence criteria, then run multiple relaxations with progressively stricter convergence criteria in each consecutive iteration and (ii) perform an energy minimization using a classical force field, using LAMMPS (see Sec. S . ) [ ] (with a fixed unit cell constraint), to obtain a more reasonable starting configuration for the DFT structure optimization. Nevertheless, a number of structures, typically containing large or easily ionized functional groups, failed to converge to reasonable structures and resulted in incorrectly dissociated bonds or distorted aromatic rings. Tab. S lists the functional groups which we initially investigated, but failed to converge under the DFT geometry optimization routines.