EXACT QUANTUM MOLECULAR HAMILTONIANS .2. ON THE CHOICE OF THE MOVING FRAME OF REFERENCE - THE PRINCIPAL AXIS SYSTEM

It is demonstrated that, for an N-atom molecular system, a body-fixed (BF) frame of reference such that the rotational kinetic energy considered a quadratic form of the components of the total angular momentum measured in the BF frame: [GRAPHICS] where q collectively denotes the internal coordinates and mu is a 3 x 3 symmetric matrix, can always be taken as instantaneously block-diagonal. Next, it is shown that it is not much complication to use the principal axis system (PAS) as the BF frame of reference. An explicit solution of this problem is proposed. Using the PAS to treat the dynamics of a molecular system can be very helpful, in particular when the system does not fragment into more than two pieces. Applications are presented for the three-body problem, using various parameterizations of the internal deformation motion.