Abstract
We present a comparison of relativistic and nonrelativistic CASSCF and CASPT2 calculations of low-lying quintet 5D, 5F, and 5P terms of the Group 8 transition-metal elements, Fe, Ru, and Os. Our analysis of relativistic effects brings a useful insight on the understanding of the electron configurations and corresponding spectroscopic ground states of Fe (5D4): [Ar]3d64s2, Ru (5F5): [Kr]4d75s1, and Os (5D4): [Xe]4f145d66s2. We show that the ground state of Os is 5D4, the same as that of Fe, due to relativistic effects. Using relatively simple computational tools of quantum chemistry we demonstrated that upon switching-off the relativistic contribution the ground state of Os would be 5F5, the same as for Ru. The lowest term of Ru remains 5F at both nonrelativistic and relativistic levels, but the next term is 5D, in accord with experiment, only upon considering relativistic effects. The pattern of lowest quintet terms of Fe remains the same at the relativistic and nonrelativistic levels but excitation energies change quite significantly due to relativistic effects.
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