Essentials of relativistic quantum chemistry.

Relativistic quantum chemistry has evolved into a fertile and large field and is now becoming an integrated part of mainstream chemistry. Yet, given the much-involved physics and mathematics (as compared with nonrelativistic quantum chemistry), it is still necessary to clean up the essentials underlying the relativistic electronic structure theories and methodologies (such that uninitiated readers can pick up quickly the right ideas and tools for further development or application) and meanwhile pinpoint future directions of the field. To this end, the three aspects of electronic structure calculations, i.e., relativity, correlation, and QED, will be highlighted.