Bonding of NH3, CO, and NO to NiO and Ni-doped MgO: a problem for density functional theory

Abstract

Recent experimental results (Hoeft et al 2001 Phys. Rev. Lett. 87 086101) have questionedthe capability of current theoretical methods for describing the bonding ofNH3, CO, and NO with the NiO(100) surface. We show that these systems doindeed represent a challenge to theory. For different reasons, density functionaltheory (DFT) fails in describing the bonding of these molecules to the NiO surface.The gradient-corrected functionals which work better for the properties ofNH3/NiO and CO/NiO (energies, geometries, vibrations) provide wrong answers for NO/NiO andvice versa. This is not due to the well-known difficulty as regards DFT describing theinsulating character of NiO. In fact, exactly the same problem is found for isolatedNi2+ impurities in MgO. A correct description of the bonding of both closed-shell(NH3 and CO) and open-shell (NO) molecules toNixMg1−xO is obtained only after inclusion of dynamical correlation and dispersion forces viawavefunction-based methods. However, even with correlated calculations someuncertainties exist regarding the predicted value of the energy of adsorption of NO onNiO. While CASPT2 calculations reach reasonable agreement with experiment,the results of approximate coupled-cluster calculations (the multi-configurationcoupled-electron-pair approach) substantially underestimate the adsorption energy.