DMRG-CASSCF/CASPT2 calculations were performed with the OpenMolcas 62,63 package interfaced with the CheMPS2 library. We used the aug-cc-pwCV5Z-DK basis set for Fe, 65 cc-pVTZ-DK for H, and aug-cc-pVTZ-DK for the other ligand atoms, as we found that this combination gives the best agreement to the complete basis set limit due to error cancellations. Cholesky decomposition of the two-electron integrals with a threshold of 10 -6 au was used. A second-order Douglas-Kroll-Hess (DKH) Hamiltonian was used to account for scalar relativistic effects. Similar to our previous works, the DMRG-CASSCF calculations made use of Fiedler orbital ordering, residual norm threshold of 10 -5 for the Davidson algorithm, and perturbative noise with a prefactor of 0.05. We chose a value of 1000 for the number of renormalized states m, as it gave almost converged results in other studies. The ionization-potential/electron-affinity (IPEA) shift 77 of 0.25 au and an imaginary shift 78 of 0.1 au were used in the CASPT2 calculations. All core and semicore electrons of Fe (3s and 3p) were kept frozen in the CASPT2 treatment, as they make only a slight contribution to the CASPT2 relative energies in iron-nitrosyl complexes. Point group symmetry was employed as appropriate.