  We investigate the chemical evolution of a collapsing core that starts from a
hydrostatic core and finally form a low-mass protostar. New multiphase
gas-grain models that include bulk diffusion and photon penetration are
simulated by the macroscopic Monte Carlo method in order to derive the chemical
evolution. There are two types of species in the ice bulk in the new multiphase
models. Interstitial species can diffuse and sublime at their own sublimation
temperatures while normal species are locked in the ice bulk. Photodissociation
rates of icy species are reduced by the exponential decay of UV flux within the
ice mantle. Two-phase models and basic multiphase models without bulk diffusion
and photon penetration are also simulated for comparison. Our physical model
for the collapsing core is base on a one-dimensional radiation hydrodynamics
model. Abundant icy radicals are produced at around 10 K in the new multiphase
models. Interstitial radicals can diffuse inside ice mantles to form complex
organic molecules (COMS) upon warming-up. Thus, COMs produced by radical
recombination at higher temperatures in the new multiphase models are more than
one order of magnitude higher than those in the two-phase and basic multiphase
models. Moreover, COMs produced at around 10 K in the new multiphase models are
about one order of magnitude higher than those in the two-phase model. Our
model shows a reasonable agreement with observations toward low-mass
protostars. Moreover, molecular oxygen abundances predicted by our new
multiphase models agree reasonably well with that found in cometary materials.
