Observations (Goldsmith et al., 1986) indicate that the gas in dark clouds with embedded low-mass stars experiences a cycle, driven by stellar winds, between low and high density phases. The cycle time is sufficiently short that the chemistry never attains steady state. Icy mantles accumulating on grains during the collapse and dense core phases are thought to be removed in each cycle by low velocity shocks that terminate the low density phase. Models of this type give detailed point-by-point descriptions of both gas and solid phases in molecular clouds (Charnley et al., 1988; Nejad et al., 1990).