Relativistic Quantum Chemistry
en.wikipedia.org
en.wikipedia.org
I believe that it’s literally one of the most overlooked parts of modern physics. We literally don’t know how to correctly and accurately calculate heavy atoms and molecules and include both special relativity AND electron correlation (and preferably also QED). Somehow most of the physicists left that problem behind and focused on nuclear and high energy physics.
I’m my opinion solving it, contrarily to focusing on abstract mathematical theories that are verifiable in energies beyond what humanity will be capable of in next 100 years or so, is truly one of the most challenging, yet verifiable problems of XXI century physics.
Edit: I see it was resubmitted.
[1]https://blogs.sciencemag.org/pipeline/archives/2019/10/11/co...
Mercury’s place in periodic table is such that these effects are taken to the extreme due to its electron configuration.
Here is nice popular article with references that explain more: https://www.chemistryworld.com/news/relativity-behind-mercur...