THE INTERNAL AXIS SYSTEM OF MOLECULES WITH ONE LARGE-AMPLITUDE INTERNAL MOTION

An old puzzle of theoretical molecular spectroscopy, the derivation of the internal axis system (IAS) of molecules with one large amplitude internal motion and asymmetric top and asymmetric frame has been solve d by a simple and transparent method. The basic idea has been to consi der the large-amplitude-coordinate-dependent IAS as the result of quan tum mechanical evolution in rho of an initial system of axes, where rh o denotes the coordinate describing the large amplitude motion. A spec ial choice of the operator governing rho evolution guarantees that the rho evolved system is the IAS. Floquet theory provides both a simple analytical expression for the evolution-matrix and a means to efficien tly calculate this matrix. Numerical examples have revealed fast conve rgence of the method to the IAS. (C) 1998 American Institute of Physic s. [S0021-9606(98)01534-7].