CO2 is one of the most important materials of the giant planets in the solar system, such as Mars and Venus [1].The study of its high pressure structural changes provides crucial knowledge for planetary science and geophysics [2].Its structural changes under pressure have been intensely studied over the past two decades [3,4].In order to systematically study the structure evolution of CO2 under high pressure, and especially to examine the possibility of attaining hypervalent C and study the chemical nature of its bonding with neighboring atoms, we here present extensive structure searches to explore the high pressure structures of solid CO2 up to 1200 GPa, using our developed calypso method for crystal structure prediction [5,6].Our results show that although undertaking many structural transitions under pressure, CO2 is quite resistive to structures with C beyond 4-fold coordination.For the first time, we are able to identify two 6-fold structures of CO2 solid with Pbcn and Pa-3 symmetries that become stable at pressures close to 1TPa [7].Both structures consist of network of C-O octahedron, showing hypervalence of the central C atoms.The C-O bond length varies from 1.30 to 1.34 A at the 4-fold to 6-fold transition, close to the C-O distance in the transition state of a corresponding SN2 reaction.It has been a long-standing and challenging objective to stabilize C in hypervalent state, particularly when it is bonded with nonmetallic elements.Most of the work so far focused on synthesizing organic molecules with high coordination number of C.Our results provide a good measure of the resistivity of C toward forming hypervalent compounds with nonmetallic elements and of the barrier of reaction involving C-O bonds.