I took two type of POSCAR of O2 molecule which is attached here.

1st Case

O atom in a box

1.0 ! universal scaling parameters

8.0 0.0 0.0 ! lattice vector a(1)

0.0 8.0 0.0 ! lattice vector a(2)

0.0 0.0 8.0 ! lattice vector a(3)

O

2 ! number of atoms

Direct ! positions in cartesian coordinates

0 0 0 ! first atom

0 0 0.22 ! second atom

2nd Case

O

1.0

8.000000000000 0.000000000000 0.000000000000

0.000000000000 8.000000000000 0.000000000000

0.000000000000 0.000000000000 8.000000000000

O

2

Direct

0.470987189148 0.500000000000 0.500000000000

0.529012810852 0.500000000000 0.500000000000

Upper one is giving right ground state energy since one corner O atom is attached to other O atom but in second O2 molecule is middle of the cube.

I want to check bulk energy for more than two atom molecule for example C6H15N molecule. In that case how will make POSCAR for this big molecule .