title: A gauge-independent zeroth-order regular approximation to the exact relativistic Hamiltonian--formulation and applications.
abstract: A simple modification of the zeroth-order regular approximation (ZORA) in relativistic theory is suggested to suppress its erroneous gauge dependence to a high level of approximation. The method, coined gauge-independent ZORA (ZORA-GI), can be easily installed in any existing nonrelativistic quantum chemical package by programming simple one-electron matrix elements for the quasirelativistic Hamiltonian. Results of benchmark calculations obtained with ZORA-GI at the Hartree-Fock (HF) and second-order Moller-Plesset perturbation theory (MP2) level for dihalogens X(2) (X=F,Cl,Br,I,At) are in good agreement with the results of four-component relativistic calculations (HF level) and experimental data (MP2 level). ZORA-GI calculations based on MP2 or coupled-cluster theory with single and double perturbations and a perturbative inclusion of triple excitations [CCSD(T)] lead to accurate atomization energies and molecular geometries for the tetroxides of group VIII elements. With ZORA-GI/CCSD(T), an improved estimate for the atomization energy of hassium (Z=108) tetroxide is obtained.
authors.affiliation: Department of Theoretical Chemistry, Göteborg University, Kemigården 3, S-41296 Göteborg, Sweden. filatov@theoc.gu.se
authors.forename: Michael
authors.initials: M
authors.lastname: Filatov
authors.forename: Dieter
authors.initials: D
authors.lastname: Cremer