Introduction

There are enormous amounts of literature dealing with quantum mechanical tunneling phenomena, andmanymonographs have also been published recently covering various fields of the phenomenon [21-24]. Unfortunately, however, theories of multidimensional tunneling phenomena are not yet satisfactorily developed. Effective numerical algorithms in quantum mechanics such as MCTDH (multichannel timedependent Hartree) method [25] and quantum Monte Carlo method [26] have been developed and successfully implemented to study quantum dynamics in multidimensional systems. However, exact quantum dynamics methods require a formidable task including accurate ab initio calculations of global potential energy surfaces. Besides, it is not straightforward to obtain the physical insights of dynamic processes.