At the heart of condensed matter physics are two key concepts: the emergence of rich phenomena (including spontaneously occurring order - structural, magnetic, or otherwise) in the many-particle limit; and the critical role played by quantum mechanics in describing the many-body states of the system. I've tried to explain this before to lay persons by pointing out that while complicated electronic structure techniques can do an adequate job of describing the electronic and vibrational properties of a single water molecule at zero temperature, we still have a difficult time predicting really emergent properties, such as phase diagram of liquid, solid, and vapor water, or the viscosity or surface tension of liquid water.
Plutonium is an even more striking example, given that we cannot even understand its properties from first principle when we only have a single type of atom to worry about. The thermodynamic phase diagram of plutonium is very complicated, with seven different crystal structures known, depending on temperature and pressure. Moreover, as a resident of the actinide row of the periodic table, Pu has unpaired 5f electrons, though it is not magnetically ordered. At the same time, Pu is very heavy, with 94 total electrons, so that relativistic spin-orbit effects can't be neglected in trying to understand its structure. The most sophisticated electronic structure techniques out there can't handle this combination of circumstances. It's rather humbling that more than 70 years after its discovery/synthesis, we still can't understand this material, despite the many thousands of person-hours spent on it via various nations' nuclear weapons programs.