The vibrational relaxation rate of a "quantum" molecule embedded in a "quan
tum" host is approached from the perspective of a short-time expansion of t
he quantum force-force time correlation function. A general path integral M
onte Carlo scheme is developed to compute the coefficients of the expansion
, which involve the quantum thermal average of operators that depend on bot
h position and momentum. A simple ansatz is used to connect the short-time
and long-time behavior of the force-force correlation function. The method
is tested on a simple model problem. Improvements of the approach and furth
er applications are discussed.