A series of additive corrections are obtained to further refine the energy predictions and justify the approximations used in the geometry optimization. The zero-point vibrational energies (ZPVE) are obtained from the harmonic vibrational frequency computations producing 0 Kelvin enthalpies (H 0K ). Higher order CCSDT and CCSDT(Q) additive corrections are computed with the cc-pVDZ basis set to capture as much electron correlation as possible and demonstrate convergence towards the FCI limit. A frozen core correction (∆ FC ) is obtained using the cc-pCVQZ basis set to account for the difference in the CCSD(T) energy correlating all electrons and the CCSD(T) energy freezing the core. To account for scalar relativistic effects, the spin-free exact two-component one-electron (SF-X2c-1e) method along with the decontracted cc-pCVTZ basis set is used to compute a scalar relativistic correction (∆ rel ). The correction accounts for the difference in energy with and without the (SF-X2c-1e) method turned on correlating all electrons. Finally, a diagonal Born-Oppenheimer correction (∆ DBOC ) is included as a diagnostic to ensure that each stationary point is not influenced by any nearby conical intersection. For the van der Waals complexes, corrections were computed with the appropriate augmented basis functions, which are detailed in Table . All corrections are obtained using the following software packages: CFOUR 2.0, Molpro 2010, and Psi4. Geometries and harmonic frequencies for all species in the H 2 O + RNCO reactions are obtained using the MP2[TZ,QZ] + ∆CCSD(T)/DZ composite method recently highlighted by Sherill and coworkers and implemented in Psi4. The MP2[TZ,QZ] term refers to a density fitted second-order Moller Plesset (MP2) gradient extrapolated to the CBS limit using the cc-pV(X+d)Z (X=T,Q) basis sets and the two-point extrapolation formula of Helgaker.