Page "Computational chemistry" Paragraph 2

The methods employed cover both static and dynamic situations.

In all cases the computer time and other resources ( such as memory and disk space ) increase rapidly with the size of the system being studied.

That system can be a single molecule, a group of molecules, or a solid.

Computational chemistry methods range from highly accurate to very approximate ; highly accurate methods are typically feasible only for small systems.

Ab initio methods are based entirely on theory from first principles.

Other ( typically less accurate ) methods are called empirical or semi-empirical because they employ experimental results, often from acceptable models of atoms or related molecules, to approximate some elements of the underlying theory.