Time correlation functions using the Semiclassical Initial Value Representation

PHYS 453

Charulatha Venkataraman, vcharu@berkeley.edu, Department of Chemistry, University of California Berkeley, 17 Gilman Hall, Berkeley, CA 94720 and William H. Miller, millerwh@berkeley.edu, Department of Chemistry, University of California, Berkeley, CA 94720.
Most dynamical quantities of interest in complex systems (reaction rates, spectroscopic line shapes, etc) can be expressed in terms of time correlation functions. However the exact calculation of these correlation functions for a quantum system is extremely difficult. We use semiclassical theories to provide an approximate description of the quantum dynamics of the system.

Application of semiclassical techniques, especially the IVR offers one the possibility to include quantum effects into classical molecular dynamics simulations. The primary difference of an IVR calculation from a conventional CMD is that there is phase information which gives rise to the quantum effects. Our effort is to make the SC-IVR more practical to apply to complex molecular systems.

One approach is to consider the linearized SC-IVR or the classical Wigner model. The dynamics is completely classical, however with a Wigner distribution of initial conditions. Even though the initial position in this distribution is localized by the Boltzmann matrix element, the initial momentum is completely delocalized. This makes it difficult to apply to systems with more than a few degrees of freedom. We have been using the stationary phase filtering method to localize the initial momenta.

The other approach is to implement the full SC-IVR. We have come up with an 'exact' version of the forward backward IVR. In this, the two time propagators are combined into just one forward backward trajectory. This is more practical because many degrees of freedom in large systems approximately back integrate and the oscillations are cancelled out automatically. This version of the SC-IVR is not any more difficult than the Wigner model. We will illustrate these two approaches on some simple model problems.

 

Poster Session
7:30 PM-10:00 PM, Wednesday, 13 September 2006 Moscone Center -- Hall D, Poster

Sci-Mix
8:00 PM-10:00 PM, Monday, 11 September 2006 Moscone Center -- Hall D, Sci-Mix

Division of Physical Chemistry

The 232nd ACS National Meeting, San Francisco, CA, September 10-14, 2006