Matrix mechanics theory of matter (MMTM) is a better molecular theory than the ball-and-stick molccular theory. In thcal> initio MMTM molecular theory the only parameters required by the procedure are Planck's constant, the chargc and mass of the electron, and the number and kind of nucici. This theory can for many molcculcs prcdict with high accuracy their equilibrium geometries and their force constants. Unfortunately it predicts other properties, e. g., dissociation energy, clcctronic spectra, etc., with a lower accuracy. The accuracy can be increased by the use of larger vector spaces, a technique that can be very difficult, very expensive, and/or impossible with the currently available scalar computers. The problem becomes easier with supercomputers that employ vector and/or parallel processors and larger memories, but there will always be some upper limit to the size of a molcculc on which accuratc ab initio calculations can be made. A theory which is less strongly computcr-dependent is the semiempirical molecular theory. Its procedure employs a smaller vector space and its parameters are determined by comparison of predictions with a small number of observations.
An example of a semiempirical molccular theory is the n-electron theory of conjugated, unsaturated hydrocarbons. Here the size of the vcctor space is reduced by ignoring corc and 5-bondcd clcctrons and employing a single ÿ-orbital for cach carbon site. The size of the vcctor spacc can be further reduced by the use of cither of two more approximate theories: the n-Hiickel molecular orbital theory, which resembles the Bohr theory of the atom, and die n-valence bond theory, which resembles the ball-and-stick theory.