Coordinate system
The lattice vectors describing the unit cell are named \(\mathbf{a}\), \(\mathbf{b}\), and \(\mathbf{c}\). \(\mathbf{a}\) and \(\mathbf{b}\) must be in the \((x, y)\) plane, and \(\mathbf{c}\) must have a positive \(z\) component. The \(+z\) direction is considered to be pointing outwards from the surface (i.e., towards vacuum). For (1 × 1) cells, it is common (and highly recommended) to choose the \(\mathbf{a}\) and \(\mathbf{b}\) vectors such that \(|\mathbf{a}| \leq |\mathbf{b}|\).