For all practical purposes the basic physical equations governing the behaviour of a system at the molecular level can only be solved approximately. The key issue in any reliable and accurate computational study in molecular physics and quantum chemistry is the adoption of a suitable model which contains the essential physics and chemistry, is computationally tractable, and preferably amenable to systematic refinement. The provision of advice on the choice of an appropriate model for a specific problem has so far received scant attention. This issue is becoming acute as `standard' software packages are becoming widely available and are being increasingly heavily used in both the academic and industrial sectors by researchers who have received no special training in the theoretical physics and chemistry that underpins them. This volume provides researchers whose background may not be in the computational molecular sciences with the necessary background to make intelligent use of the methods available by performing reliable calculations of appropriate accuracy and making a considered interpretation of the data so obtained.