**4. Concluding Remarks**

It is clear from the above exposition and examples that RRPA, which includes significant many-body correlation including initial state two-particle two-hole terms and final state interchannel coupling, can be suitably applied to a number of aspects of atomic photoionization and give physical insight into what makes the results for these processes what they are. Furthermore, it must be noted that only a few of the many examples that have been studied over the years are presented above. Thus, it is evident that the RRPA methodology has contributed greatly to our understanding of atomic photoionization as well as many other atomic processes [50]. However, it must be recalled that all of this was made possible by the pioneering work on the RPAE method by Miron Amusia and his collaborators.

**Author Contributions:** Both authors contributed equally to the writing. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work of STM was supported by the US Department of Energy, Office of Science, Basic Energy Sciences under Award Number DE-FG02-03ER15428.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** All data is available from the authors upon request.

**Acknowledgments:** This work of STM was supported by the US Department of Energy, Office of Science, Basic Energy Sciences under Award Number DE-FG02-03ER15428. We are thankful to Sourav Banerjee and Soumyajit Saha for their help in the preparation of this article.

**Conflicts of Interest:** The authors declare no conflict of interest.
