Intermolecular Dynamics For Weakly Bound Donor-Acceptor Complexes

We have performed ab initio (3-21 G) calculations on the weakly bound donor-acceptor complex between benzene and tetracyanoethylene (TCNE) as a function of various intermolecular motions. The resultant potential energy surface (PES) was then fit to the product of analytical functions. Classical dynamics was performed on this PES by solving Hamilton's equations using a fourth-order Runge Kutta differential equation solver. Time steps were of the order of 1.0 femtosecond and the system was allowed to evolve for 1,000,000 time steps. Fast Fourier analysis of the resulting trajectories indicate that the primary intermolecular vibrational frequencies are 118, 228, and 232 cm-1. These correspond to the motion of the intermolecular separation coordinate, and the two twisting coordinates. We also discuss other possible solutions, zero-point energies, etc.