How to determine rotational states of a molecule? How can you determine the possible rotational states of a molecule? Atkins Physical Chemistry book says that for linear molecules there are two possible rotational states, but what is the general method for finding the amount of rotational states for other relatively more complex molecules, say for example $\ce{H2O}$ and $\ce{CH4}$?
Molecules have only 3 (or 2 if linear) rotational modes.  A molecule with n atoms has 3n-6 vibrational states, 3n-5 if it is linear.  The "6" or "5" that is subtracted represents the 3 translational modes that all molecules can move in (x, y, and z directions) plus the 3 rotational modes (rotation about the x, y and z axes).  The 3 rotational modes reduce to 2 if the molecule is linear since you can't observe rotation about its molecular axis (the axis that contains the $C_{\infty}$ symmetry element).