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.