VSEPR (Valance Shell Electron Repulsion Theory) is used to predict molecular shapres.

In a nutshell, VSEPR helps to predict specific geometry. The geometric shapes produced are dictated by regions of electron density. Both unbonded lone electron pairs and bonded electron pairs qualify as regions of electron density. VSEPR tutorial.

Molecular shape is typically determined from the Lewis structure. Once a valid Lewis structure is drawn the regions of electron density are determined by counting up the covalent bonds and lone electron pairs for the central atom. The regions then can be referenced to the Sheffield ChemPuter to determine VSPER geometry.