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

Graphical abstract: The correlation energies (CEs) calculated from s′s″ CI are given in the Figure; the solid line gives CE(NVPA) and the broken line CE(VPA). Highlights: ► We discuss the 4-component relativistic total (TEs) and correlation (CEs) energies. ► The CIs with and without no virtual pair approximation (NVPA) are considered. ► TEs and CEs with NVPA for He-like ions depend on the methods generating the AOs. ► Without NVPA the values are independent of how the AOs are generated. ► The results show the importance of contributions to energetics from the Dirac sea. - Abstract: 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.