Orthorhombic means that the lattice constants a, b and c are not equal. P means primitive, which is one of the lattice types (see Table 4.4 in users's guide). The unit cell for Na has orthorhombic symmetry and is of lattice type P.

The wave function of the electronic state with the lowest total energy can eventually have a symmetry which is lower than cubic. If this is the case, then you need to reduce the symmetry to obtain this state. The use of an orthorhombic symmetry should be enough in every case. F. Tran If the symmetry is too high (e.g., cubic) On Mon, 17 Aug 2015, Seyyed Amir Abbas Emami wrote:

Thank you dear Tran. It was completely helpful.But what is the role of symmetry in calculating cohesive energy as you said reduce cubic to orthorhombic for open shell atoms.Also for Na which is open sell, you use P instead of orthorhombic.

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