Geometry optimizations in the zero order regular approximation for relativistic effects

Analytical expressions are derived for the evaluation of energy gradients i n the zeroth order regular approximation (ZORA) to the Dirac equation. The electrostatic shift approximation is used to avoid gauge dependence problem s. Comparison is made to the quasirelativistic Pauli method, the limitation s of which are highlighted. The structures and first metal-carbonyl bond di ssociation energies for the transition metal complexes W(CO)(6), Os(CO)(5), and Pt(CO)(4) are calculated, and basis set effects are investigated. (C) 1999 American Institute of Physics. [S0021-9606(99)30317-2].