Using quantum mechanics to predict the chemical bonding patterns, optimal geometries, and physical and chemical properties of molecules is a large and active field of research known as molecular quantum mechanics or more commonly as quantum chemistry. The density functional theory referred to in the previous lecture, for which the chemistry Nobel prize was given in 1998, has had a tremendous impact in quantum chemistry, with some of the papers in this subject having acquired some 10,000 citations each since their publication. In fact, the 1998 chemistry Nobel prize was shared between Walter Kohn, one of the inventors of density functional theory and John Pople, the developer of a commonly used quantum chemistry software package.
Quantum chemistry calculations allow the geometries of molecules
to be computed as well as a wide range of properties. Quantum
chemistry can also be used in a novel way, in which the
electrons are treated using quantum mechanics but the nuclei
are treated as classical particles. We use quantum mechanics to
calculate the internuclear forces but then use these forces
in Newton's Second Law to study the motion of the nuclei
during chemical reactions. This gives us a microscopic window
into the specific motions, the complex dance, executed by the
nuclei during a simple or complex chemical process.
The methods of quantum chemistry have become very sophisticated,
and there are various software packages that can be downloaded
for carrying out the calculations of quantum chemistry. It should
be noted that these packages use a series of approximations
to solve the Schrödinger equation because for all but the simplest
of molecules, exact solutions are not available. We will
discuss some of these methods, but first we need to introduce
some of the underlying theory.