SEARCH Articles Figures Tables Approximations order Hamiltonian Zeroth-order regular approximation Hamiltonians zeroth-order regular approximation Regular approximation Zeroth order regular approximation for relativistic effects Zeroth-order Zeroth-order approximation Zeroth-order regular approximation (ZORA Zeroth-order regular approximation Hamiltonian/method Zeroth-order regular approximation energies/results Zeroth-order regular approximation method