Relativistic self-consistent-field calculations with the squared Dirac operator

A four-component relativistic self-consistent-field scheme based on the squared Dirac operator is presented. Just as in the one-electron case, discussed previously [Chem. Phys. Lett. 78, 341 (1981)], the variation principle holds and no variational collapse appears if the expansion method is applied. Numerical results with scalar Gaussian-lobe basis sets are given for He, Li, Be, H₂, and LiH.