Semiclassical description of nonadiabatic quantum dynamics: Application tothe S-1-S-2 conical intersection in pyrazine

A recently proposed semiclassical approach to the description of nonadiabat ic quantum dynamics [G. Stock and M. Thoss, Phys. Rev. Lett. 78, 578 (1997) , X. Sun and W. H. Miller, J. Chem. Phys. 106, 916 (1997)] is applied to th e S-1-S-2 conical intersection in pyrazine. This semiclassical method is ba sed on a transformation of discrete quantum variables to continuous variabl es, thereby bypassing the problem of a classical treatment of discrete quan tum degrees of freedom such as electronic states. Extending previous work o n small systems, we investigate the applicability of the semiclassical meth od to larger systems with strong vibronic coupling. To this end, we present results for several pyrazine models of increasing dimensionality and compl exity. In particular, we discuss the quality and performance of the semicla ssical approach when the number of nuclear degrees of freedom is increased. Comparison with quantum-mechanical calculations and experimental results s hows that the semiclassical method is able to describe the ultrafast dynami cs in this system. (C) 2000 American Institute of Physics. [S0021-9606(00)0 1323-4].