  We apply the atom-atom potentials to molecular crystals of iron (II)
complexes with bulky organic ligands. The crystals under study are formed by
low-spin or high-spin molecules of Fe(phen)$_{2}$(NCS)$_{2}$ (phen =
1,10-phenanthroline), Fe(btz)$_{2}$(NCS)$_{2}$ (btz = 5,5$^{\prime
}$,6,6$^{\prime}$-tetrahydro-4\textit{H},4$^{\prime}$\textit{H}-2,2$^{\prime
}$-bi-1,3-thiazine), and Fe(bpz)$_{2}$(bipy) (bpz =
dihydrobis(1-pyrazolil)borate, and bipy = 2,2$^{\prime}$-bipyridine). All
molecular geometries are taken from the X-ray experimental data and assumed to
be frozen. The unit cell dimensions and angles, positions of the centers of
masses of molecules, and the orientations of molecules corresponding to the
minimum energy at 1 atm and 1 GPa are calculated. The optimized crystal
structures are in a good agreement with the experimental data. Sources of the
residual discrepancies between the calculated and experimental structures are
discussed. The intermolecular contributions to the enthalpy of the spin
transitions are found to be comparable with its total experimental values. It
demonstrates that the method of atom-atom potentials is very useful for
modeling organometalic crystals undergoing the spin transitions.
