Necessity of including the negative energy space in four-component relativistic calculations for accurate solutions

We discuss the four-component relativistic total energies (TEs) and correlation energies (CEs) with and without the no virtual pair approximation (NVPA). The system considered is the He isoelectronic sequence from ₂He to ₁₁₆Uuh¹¹⁴⁺. It is shown that the full configuration interaction TEs and CEs with the NVPA depend on the methods used to generate atomic orbitals (AOs); without the NVPA the values are independent of how the AOs are generated. This result shows the importance of contributions from the Dirac sea where the electrons kinetic energies are negative.