Theoretical Chemistry is a branch of chemical physics which studies on a theoretical fundamental level energetics, dynamics and reactivity of chemical and biological systems, drugs and medicaments. In short, it harnesses Quantum Mechanics and the Computer's capability to understanding the regime of Chemistry.
Theoretical Chemistry develops and uses methods common to Atomic and Molecular Physics, Thermodynamics, and Physical Chemistry. As such, depending on the size and complication of the chemical system, as well as on the problem in question and the accuracy needed, Theoretical Chemistry utilizes methods spanning a wide range from empirical and semi empirical methods up to ab initio quantum chemistry ones.
Main sub-branches include Computational Chemistry, Quantum Chemistry, Molecular Dynamics, Molecular Modeling and Informatics.