  Crystal structure prediction methods and first-principles calculations have
been used to explore low-energy structures of carbon monoxide (CO). Contrary to
the standard wisdom, the most stable structure of CO at ambient pressure was
found to be a polymeric structure of Pna21 symmetry rather than a molecular
solid. This phase is formed from six-membered (4 Carbon + 2 Oxygen) rings
connected by C=C double bonds with two double-bonded oxygen atoms attached to
each ring. Interestingly, the polymeric Pna21 phase of CO has a much higher
energy density than trinitrotoluene (TNT). On compression to about 7 GPa, Pna21
is found to transform into another chain-like phase of Cc symmetry which has
similar ring units to Pna21. On compression to 100 GPa it is energetically
favorable for CO to polymerize to form a single-bonded Cmcm phase from another
structure of Cmca symmetry composed of units similar to those found in the
single-bonded I212121 structure. Thermodynamic stability of these structures
was verified using calculations with different density functionals, including
hybrid and van der Waals corrected functionals.
