摘要
<正> Ⅰ. INTRODUCTION The symmetry of non-rigid molecules can be described by Longuet-Higgins’ permutation-inversion groups (PI groups). A PI group is composed of (1) the identity and permutations on the coordinates of spins and orbitals of identical atoms in a molecule, (the structure of the molecule after being operated by permutations must be one that is allowable to the motion of the molecule under study) and (2) P~*, the
