Evolution of the ice-mantle and gas-phase chemistry during the cold quasi-static contraction phase. The results from the standard models N(2-5)-T15-CR1 are displayed from top to bottom. Panel a: calculated fractional ice-mantle composition by ice layer. Panel b: cumulative fractional composition of the ices with respect to water, summed over the grain mantle up to an ice thickness given on the abscissa. Panel c: gas-phase fractional abundances (with respect to total hydrogen) of atomic and simple diatomic species as a function of time with the same time axis as panels a and b to facilitate the comparison.