Computational chemistry is a computer technology has become very important in the last century, and it became considered a branch of chemistry that uses results of theoretical chemistry based on the efficiency of the computer programs to predict the properties and structures of molecules represented by calculating atomic charges , bond length, angles bonds, dipoles, vibrational frequencies, etc.<br />Where the theoretical chemistry is known as a mathematical description of chemistry based on the principle of combined mathematical methods with fundamental laws of physics to study the properties of important chemical processes.<br />Computational chemistry may be donated for several major areas such as the design of new drugs and catalysis, that can be done by using uses several methods different in accurate from high to very approximate; The most important numerical techniques are Molecular mechanics, Semi experimental methods, ab initio methods, and density functional theory (DFT). Highly accurate methods are usually successful just for smaller systems.<br />Nowadays it is widely used in drug discovery, agrochemical discovery, and increasingly in flavours and fragrances, computational chemistry is used to predict the physical properties and most importantly the biological activity of compounds before they are synthesised or biologically tested.<br /><br />By: Asst. Lec. Halah.T.Mohammed