ABSTRACT
For most of the 80 years that quantum mechanics has been employed to characterize chemical systems, its value has been conceptual and qualitative. This is, of course, familiar territory to chemistry, and invaluable to our task of developing insight. In 1960, a division was evident; there were those who were content with qualitative theory and hastened to apply its concepts to large chemical systems. But there were those who were not satisfied with this level of description and sought ever more accurate accounts of small chemical systems. Recently, the persistence of this latter group has generated methods of great value. We will explore two branches of their endeavor. The more thoroughly developed and widely adopted approach focuses on the systematic treatment of correlation by introducing refinements on the Hartree-Fock approximation either entirely rigorously (coupled cluster theory) or by empirical estimates of small effects not captured directly in other ways (extrapolation methods). The second branch, which may now be poised to make its impact on chemistry, deals more directly with the motion of electrons in a field of nuclear charges. The first approach aspires to ‘‘chemical accuracy’’ while the second, called quantum Monte Carlo, aspires to exact solution of the Schroedinger equation.
