Electronic structure of molecules using one‐component wave functions and relativistic effective core potentials

A one‐component approach to molecular electronic structure is discussed that includes the dominant relativistic effects on valence electrons and yet allows the use of the traditional quantum‐chemistry techniques. The approach starts with one‐component Cowan–Griffin relativistic orbitals that successfully incorporate the effects of the mass‐velocity and Darwin terms present in more complicated wave functions such as the Dirac–Hartree–Fock. The approach then constructs “relativistic” effective core potentials (RECPS) from these orbitals, and uses these to bring the relativistic effects into the molecular electronic calculations. The use of effective one‐electron spin‐orbit operators in conjunction with these one‐component wave functions to include the effects of spin‐orbit coupling is discussed. Applications to molecular systems involving heavy atoms and comparisons with available spectroscopic data on molecular geometries and excitation energies are presented. Finally, a new approach to the construction of RECPS encompassing the Hamiltonian and shapeconsistent approach is presented together with a novel analysis of the long‐range behavior of the RECPS.