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\textit{Ab initio} calculations of line-mixing effects in $CO2$ infrared bands are presented and compared with experiments. The predictions were carried using requantized Classical Dynamics Molecular Simulations (rCDMS) based on an approach previously developed and successfully tested for $CO2$ isolated line shapes, \textit{Phys.~Rev.~Lett. A} nderline{\textbf{87}} (2013), 013403.}. Using classical dynamics equations, the force and torque applied to each molecule by the surrounding molecules (described by an \textit{ab initio} intermolecular potential) are computed at each time step. This enables, using a requantization procedure, to predict dipole and isotropic polarizability auto-correlation functions whose Fourier-Laplace transforms yield the spectra. The quality of the rCDMS calculations is demonstrated by comparisons with measured spectra in the spectral regions of the $3\nu 3$ and $2\nu 1+2\nu 2+\nu 3$ Infrared bands nderline{\textbf{112}} (2011), 925-936.}.