Plane-wave density functional theory has been applied in a novel way to help interpret the molecular crystal structure disorder observed in the orthorhombic zigzag phase of plumbocene, Pb(C5H5)(2). A crystal lattice comprising uniformly staggered C5H5 rings was found to be lower in energy by 2,8 kJ mol(-1) per unit cell, compared to a uniformly eclipsed packing arrangement, This energy difference has been attributed to the difference in the strength of intermolecular interactions between the Pb(C5H5)2 chains for the two different lattices. The calculations performed allowed the determination of the crystallographic occupancy factors by a quantum mechanical technique for the first time.