Abstract
Chemistry is the field of discussing the behavior of electrons in a molecule. Sixty years ago, the mathematical foundation for predicting the behavior of electrons was given by the quantum mechanics. Now a days, the rapid development of high speed digital computer makes it possible to perform a large scale quantum chemical calculation. Two powerful tools of the quantum chemistry reviewed here are the non-empirical molecular orbital calculation program series: GAUSSIAN and the semi-empirical one: MOPAC. Especially, the treatment of the reaction process in the MOPAC is shown in detail.
