Comparison of ab initio quantum chemistry calculations on matrix isolated molecules with Mössbauer effect studies

All-electron ab initio calculations are performed for free FeX-molecules, X=H, C, N, O, CO, (CO)₂. Geometry optimization is obtained at SCF level and correlation effects on electronic configurations are studied at MP4 level. Calculated electron densities and electric field gradients of FeX are compared with measured isomer shifts and quadrupole splittings of matrix-isolated molecules. The influence of a solid noble-gas matrix on Fe^o, Fe⁺ and FeC is estimated from molecular clusters including neon or argon atoms.