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We present density functional theory (DFT) simulations and shock-reshock experiments for liquid carbon dioxide (CO2) in the range 100 to 800 GPa. The simulations support the previously suggested dissociation threshold around 50 GPa [W. J. Nellis et al., J. Chem. Phys. 95, 5268 (1991)] for shocked liquid CO2 and describe a very steep Hugoniot past dissociation. We performed the shock-reshock experiments using the Sandia Z machine. The Z machine magnetically accelerated aluminum flyer plates to shock compress cryogenic liquid CO2 to 550 GPa and attained reshock states up to 840 GPa. The plate impact experiments combined with well-characterized impedance matching standards and laser velocimetry results in high-accuracy measurements of the principal Hugoniot and reshock states of liquid CO2. The experimental results validated the DFT simulations at extreme conditions and the combination of experiment and DFT provide reliable data for evaluating existing and constructing future wide-range equations of state models for molecular compounds such as CO2. [ABSTRACT FROM AUTHOR]