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

► 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.