An improved ab initio relativistic zeroth-order regular approximation correct to order 1/c2

The equations of the original ab initio scalar-relativistic zeroth-order regular approximation (ZORA) and the infinite-order regular approximation (IORA) are expanded in orders of 1/c(2). It is shown that previous ZORA/IORA implementations in ab initio quantum chemistry programs were not correct to order 1/c(2), but contained imperfections leading to fictitious self-interactions. These errors can be avoided by adding exchange-type terms (coupling the large and small components) to the relativistic ZORA correction to the Hamiltonian, yielding improved ab initio relativistic zeroth- and infinite-order regular approximations that are correct to order 1/c(2). The new methods have been tested numerically by computing the total energies, orbital energies, and static electric dipole polarizabilities of the rare gas atoms He through Xe. (C) 2000 American Institute of Physics. [S0021-9606(00)31046-7].