  We have explored the unknown structure of the phosphorus in the phase IV
(P-IV phase) based on the first-principles calculations using the metadynamics
simulation method. Starting from the simple cubic structure, we found a new
modulated structure of a monoclinic lattice. The modulation is crucial to the
stability of the structure. Refining further the structure by changing the
modulation period, we have found the structure which shows the X-ray powder
pattern in the best agreement with the experimental one.We can not exclude the
possibility that the unknown structure of the phase IV of phosphorus is an
incommensurately modulated one.
