I want to consider the van-der-waal interaction between two layers of Graphene. How can we include that? I have made a POSCAR file in which i considered the unit cell of both layers and the separation between them (a3) 3 Angstrom and both layers contain Carbon atoms, so i specified 4 atoms in POSCAR file. Then in performing calculations i have used a flag IVDW = 1 but in OUTCAR file there is no information regarding IVDW. Can anyone tell me where i am doing a mistake?