ABSTRACT
In the previous chapter, we considered multidimensional dynamics where all the degrees of freedom were treated explicitly. As the number of dimensions in the system gets very large, it becomes intractable to solve the complete problem in such a manner. At that point, it is natural to divide it into two subsystems: one where each degree of freedom is considered explicitly (the “system”), and one where all degrees of freedom are aggregated and not treated individually (the “bath”). A common example is a molecule in solution, where the molecule is the system and the surrounding solvent molecules the bath. Another is an atomic gas with collisions, where any given atom can be the system and the surrounding atoms the bath. No matter the physical situation, for an appropriate choice of system and bath, the coupling between various degrees of freedom in the system is much stronger than between the system and bath. It is then reasonable to calculate only the system properties explicitly using quantum mechanics, while treating the bath in a simpler, often classical, way.
