filled with electrons from two different atomic orbitals. These atomic orbitals come from separate atoms resulting in molecular orbitals being linear Quantum chemistry, or molecular quantum mechanics, is a branch of physical chemistry which applies quantum mechanics to chemical systems to predict physical and chemical properties of molecules and materials. Calculations, which involve calculating electronic wave functions at the atomic level, make approximations to make simulations computationally feasible while capturing the relevant contribut The Thomas–Fermi model was developed independently by Thomas and Fermi in 1927. This was the first attempt to describe many-electron systems on the basis of electronic density instead of wave functions, although it was not very successful in the treatment of entire molecules. The method did provide the basis for what is now known as density functional theory (DFT). Modern day DFT uses the Kohn–Sham method, where the density functional is split into four terms; the Kohn–Sham kinetic energy, an external potential, exchange and correlation energies. A large part of the focus on developing DFT is on improving the exchange and correlation terms. Though this method is less developed than post Hartree–Fock methods, its significantly lower computational requirements (scaling typically no worse than n3 with respect to n basis functions, for the pure functionals) allow it to tackle larger polyatomic molecules and even macromolecules. This computational affordability and often comparable accuracy to MP2 and CCSD(T)…