An Ab Initio Model is Used to Determine the Infrared Absorption Cross Sections of CO2 Gas as a Function of Laser Wavelength. the Intra-Molecular Potential Energy and Electric Dipole Moment of the CO2 Molecule as a Function of Molecular Nuclear Configurations Are Obtained by Solving the Kohn-Sham (KS) Equation. the Rotational Constants at Different Vibrational Levels, the Vibrational Energy Eigen Values and Transition Dipole Moments Are Determined by Solving the Pure Vibrational Schro¨dinger Equation. using the Fermi’s Golden Rule and All the Calculated Ab Initio Results, the Absorption Cross Sections of CO2 Gas at Room Temperature and One Atmosphere Pressure Are Obtained. the Calculated Results Have a Good Agreement with Experimental Results. based on the Calculated Ab Initio Results, the Infrared Absorption Cross Sections of CO2 Gas at Higher Pressures Are Calculated. the Absorption Spectra at High Pressures Are Found to Be Much Smoother Due to the overlaps between Neighboring Absorption Line Shapes