Ab initio calculations

In the physical sciences, an ab initio calculation is one performed from first principles. The phrase ab initio is Latin for "from the beginning."

For example, an ab initio calculation of the properties of liquid water might start with the properties of the constituent hydrogen and oxygen atoms and the laws of electrodynamics. From these basics, the properties of isolated individual water molecules would be derived, followed by computations of the interactions of larger and larger groups of water molecules, until the bulk properties of water had been determined.