Multiconfigurational short-range density functional theory for nuclear magnetic resonance shielding constants with gauge-including atomic orbitals

In this paper, we present the theory and implementation of NMR shielding constants with gauge-including atomic orbitals (GIAOs) for the hybrid multiconfigurational short-range density functional theory model (MC--srDFT). As a special case, this implementation also includes Hartree--Fock srDFT (HF--srDFT). Choosing a complete-active space (CAS) wave function as the multiconfigurational parameterization of the wave function, we investigate how well CAS--srDFT reproduces experimental trends of nuclear shielding constants compared to DFT and CASSCF. Calculations on the nucleobases adenine and thymine show that CAS--srDFT performs on average best of the tested methods;much better than CASSCF, but only marginally better than HF--srDFT. The performance, compared to regular DFT, is similar when functionals containing exact exchange are used.We generally find that the inclusion of exact exchange is important for an accurate description of the shielding constants. In cases where no exact exchange is included, we observe that the HF-- and CAS--srDFT often outperform regular DFT. For calculations on transition metal nuclei in organometallic compounds with significant static correlation, the CAS--srDFT method again outperforms CASSCF compared to experimental shielding constants, and the change from HF--srDFT is substantial. In conclusion, the static correlation posed by the metal complexes seems to be captured by CAS--srDFT, which is promising since this type of correlation is not well described by regular DFT.