# Regarding POSCAR file for bulk Silicon

I am newbie to DFT and I am trying to understand the different input parameters in the VASP POSCAR file of bulk silicon system, provided in the examples here. I am trying to find the lattice constant of Silicon. My question is the following:

Do I need to know the lattice structure of silicon beforehand to perform this calculation? Can't I give a different unit cell structure (lines 3, 4 and 5 in the link) and still arrive at the same lattice constant?

The given example deals with with fitting energy and volume (or lattice parameter). The minima for this relation will give equilibrium lattice parameter where energy of the system is lowest. Example have taken primitive lattice vector of FCC as only one atom is needed for calculation, hence calculation is faster. For your second query, you can also choose FCC conventional cell with 4 atom, and theoretically you should have end at same result. Do I need to know lattice parameter before hand? My answer is yes, we should have rough idea about equilibrium. It will save your lot of time and if you are doing volume relaxation ISIF=3 for any complex system, starting point far from equilibrium may lead to some other minima.