All DFT calculations were performed using the JDFTx software package 34 using the Perdew-Burke-Ernzerhof (PBE) GGA functional , 60 Ryd planewave cutoff, and the SG15 ONCV norm-conserving pseudopotentials. Implicit solvation effects were included using the SaLSA model. The Coulomb interaction between periodic cells was numerically truncated at both the DFT and GW levels. All simulation boxes measured 10 Å×10 Å×10 Å and the molecules were relaxed using the PBE functional. The full-frequency GW calculations were run in Berke-leyGW using the static subspace approximation and the static remainder approach to correct for the slow convergence of the self-energy with number of empty states. The polarizability used for GW calculations was com-puted with a cutoff of 25 Ryd, and 81 real frequencies spanning 20 eV and 15 imaginary frequencies. Real and imaginary frequencies are required for evaluation of the self energy using contour deformation. Increasing the number of imaginary frequencies to 25 lowered the CO GW 0 HOMO-LUMO gap by 8 meV. The total number of unoccupied bands in the calculation was approximately 14000 and reduced to approximately 500-800 bands using a stochastic pseudobands approach for bands at least 3 Ryd above the LUMO using a 2% accumulation window. The quasiparticle eigenvalues were converged to within 10 meV by performing two additional iterations in G following the initial G 0 W 0 calculation. We found that increasing the planewave and screened cutoffs to 100 and 30 Ryd, respectively, increased the predicted CO GW 0 HOMO-LUMO gap by 24 and 18 meV, respectively. The change in CO and H 2 O GW 0 HOMO-LUMO gap calculated using experimental structure parameters from those calculated using their PBE-relaxed structures were -16 and 37 meV, respectively.