"Electron density is the measure of the probability of finding an electron in a particular location in an atom or molecule. It is observable and can be measured experimentally. Density Functional Theory (DFT) is a quantum chemical computational method based on electron density. DFT can be used to determine structural, magnetic and electronic properties of molecules. DFT constitutes the work horse for modern computational chemistry and physics and has been very successful in analyzing and rationalizing numerous physicochemical processes. However, there are also challenging situations where DFT fails to predict and/or analyze several experimentally observed phenomena, and application is difficult for very large systems such as biological systems due to the limitations on computational resources"-- Provided by publisher.