In the second half of the nineteenth century, physical chemistry developed as a well￾defined subject, consisting of thermodynamics, kinetics, and transport processes,
and mainly dealing with bulk properties and continuum models. When quantum
mechanics was discovered in 1925, paving the way for modern molecular physics,
this subject was less well received by the chemists. Partly this was due to the morass
of equations and calculations one sinks into, just to get insight into such a simple
concept as the chemical bond. Application of quantum methods in chemistry was
pioneered by people like Henry Eyring, Linus Pauling, Robert Mulliken, Per-Olov
Löwdin, Björn Roos, and many others. Most of these scientists called themselves
“quantum chemists.”