  By employing $ab$ $initio$ molecular dynamics simulations at constant
pressure, we investigated behavior of amorphous carbon dioxide between 0-100
GPa and 200-500 K and found several new amorphous forms. We focused on
evolution of the high-pressure polymeric amorphous form known as a-carbonia on
its way down to zero pressure, where it eventually converts into a molecular
amorphous solid. During decompression, two nonmolecular amorphous forms with
different proportion of three and four-coordinated carbons and two mixed
molecular-nonmolecular forms were observed. Transformation from a-carbonia to
the molecular state thus appears to proceed discontinuously via several
intermediate stages suggesting that solid CO$_2$ might exhibit interesting
polyamorphism. We also studied relations of the amorphous forms to their
crystalline counterparts. The tetrahedral-like a-carbonia is most probably
related to phase-V according to their structural properties, while presence of
the mixed forms may reflect hypothetical existence of metastable
three-coordinated polymeric phase that is composed of linear chains. Our
molecular amorphous form seems to be related to phase-I according to molecular
coordination and their relative bond orientations.
