Prediction of a novel monoclinic carbon allotrope
  A novel allotrope of carbon with $P2/m$ symmetry was identified during an \emph{ab-initio} minima-hopping structural search which we call $M10$-carbon. This structure is predicted to be more stable than graphite at pressures above 14.4 GPa and consists purely of $sp^3$ bonds. It has a high bulk modulus and is almost as hard as diamond. A comparison of the simulated X-ray diffraction pattern shows a good agreement with experimental results from cold compressed graphite. 
Maximilian Amsler, Jos\'e A. Flores-Livas, Silvana Botti, Miguel A.L.   Marques, Stefan Goedecker
cond-mat.mtrl-sci physics.comp-ph