A porous coordination polymer (PCP), {Fe-II(pyrazine)[Pt-II(CN)(4)]}, adsorbs CS2 molecules to induce spin transition from high-spin (HS) to low-spin (LS) state. To elucidate this mechanism, we investigated flexibility of the PCP framework, namely rotation of pyrazine ligands, with DFT method and evaluated the rotational entropy difference (Delta S-rot(HS-LS)) between the HS and LS states with Fourier grid Hamiltonian method. The Delta S-rot(HS-LS) value is considerably large in the absence of CS2. The CS2 adsorption occurs between two pyrazine ligands to suppress the pyrazine rotation in both states, which decreases Delta S-rot(HS-LS) to induce the HS -> LS transition at room temperature. (C) 2011 Elsevier B.V. All rights reserved.