Cumulant expansion method in the density matrix approach to wave packet   dynamics in molecular systems
  A non-Markovian master equation for a molecule interacting with a heat bath is obtained using the cumulant expansion method. The equation is applied to the problem of molecular wave packet relaxtional dynamics. An exact solution is derived that demonstrates classical type of squeezing for the wave packet evolution and a heat-bath dependent frequency shift. 
M. Schreiber and D. Kilin