The Hartree is a unit of energy used in atomic and molecular physics, named after the British physicist Douglas Hartree. It is defined as approximately 4.36 x 10^-18 joules or about 27.2 electronvolts (eV). The Hartree is often used in quantum mechanics to simplify calculations involving the energy levels of electrons in atoms. In computational chemistry, the Hartree is commonly employed in methods like Hartree-Fock to describe the behavior of electrons in a system. This unit helps researchers understand and predict the properties of molecules and materials by providing a standardized measure of energy.