sentence1: the simulation cell is a slab with periodic boundary conditions applied in the plane orthogonal to the growth direction , the substrate is four layers thick and its bottom layer is rigid , while the second and third layers are thermostat .
sentence2: the simulation cell consists of 5,070 molecules in a three-dimensional rectangular box of size 35 41 , where lengths are measured in units of the lennard-jones parameter σ , and each molecule is seven units long and one unit wide .