State-to-state quantum three-dimensional rotational total cross sections from two-dimensional close-coupled equations

A method for calculating state-to-state rotational three-dimensional (3D) t otal cross sections from two-dimensional (2D) close-coupled equations is de scribed. The idea is to replace the S-matrices in the 3D cross sections by those obtained from the 2D calculations. The correctness of the approximati on is proved from the quantum 3D hard shape model. Classical trajectory cal culations are performed for very high collision energy (0.2 eV) and compare d with the proposed method showing satisfactory agreement. The accuracy of the method is checked on a few examples by comparing the results with the s tandard close-coupling calculations and classical trajectories. The method is a powerful tool for studying systems in which a large amount of energy i s transferred between translation of atoms and rotations of molecules.