Electron correlation effects in molecules

There has been dramatic progress in the development of electron correlation techniques for the accurate treatment of the structures and energies of molecules. in this review, we give brief and somewhat qualitative descriptions of the different methods that have been developed in recent years. We also discuss the range of applicability as well as the limitations of the methods with a few selected examples. We focus particular attention on electron correlation methods which start from a Hattree-Fock wave function since such single configuration-based approaches are most easily extended to larger molecules. Multiconfiguration-based correlation techniques are considered briefly. We also present a fairly thorough account of the recent developments and applications using novel quantum Monte Carlo approaches.