relativistic effects

https://dx.doi.org/10.1351/goldbook.RT07093
Corrections to exact non-relativistic energy from the fact that inner shell electrons in heavy atoms move with velocities comparable in order of magnitude to the velocity of light. The most important relativistic effect is spin-orbit coupling.
Source:
PAC, 1999, 71, 1919 (Glossary of terms used in theoretical organic chemistry) on page 1960