A force field for mackinawite surface simulations in an aqueous environment

Abstract

attracted considerable experimental [4,5] and theoretical attention [6][7][8][9][10].Like pyrite [11], mackinawite (FeS) is an exceptional scavenger for heavy metals in water [12,13], while the similarity between its structural unit [14] and reactive biological clusters [15] has suggested that FeS structures could have catalysed the formation of the first prebiotic molecules in deep sea hydrothermal vents [16,17]. However, contrary to FeS 2 , the interface between water and FeS has not been widely investigated [18,19], in part because of the lack of force field parameters for the system.In previous work, we have derived a force field, based on pairwise interactions, for molecular mechanics (MM) simulations of mackinawite [20]. In this model, each interaction can be written as a function of the interatomic distances r ij :where A, ρ, and C are the coefficients of Buckingham potentials, and q i the electrostatic charges of the atoms. In addition, to take into account polarisability effects, each sulphur atom is represented by a core and a massless shell (S-S shell ), which interact through a harmonic potential [21]:where k s is the shell force constant, and r ij the core-shell distance. The parameters, reported in Table 1, have highlighted the stability of the (001) surface compared to all other surfaces, in agreement with ab initio calculations [22] and experimental findings [23].Here, we expand the FeS interatomic potential model to include its interaction with water, the latter described byAbstract We introduce a force field for the description of the mackinawite/water interface, which we derive by refining, and consistently merging with the SPC/Fw model of water, a set of existing interatomic potentials for the mineral. The thermal behaviour predicted for bulk mackinawite is in good agreement with experiment. The adsorption of water on the low-index surfaces of mackinawite reproduces results from density functional theory calculations at different water coverages, while the behaviour of water intercalated into the mineral is also remarkably similar to that from ab initio results. The force field is therefore suitable to model bulk mackinawite and its surfaces in an aqueous environment.

Structure and dynamics of water at the mackinawite (001) surface

“…In Ref. 28, we have shown that they accurately reproduce density functional theory results of water adsorption on the low-index surfaces of mackinawite, and that the force field description of the behaviour of water intercalated into the FeS layers under extreme thermodynamic conditions is very similar to that found by ab initio MD. 20 We have employed the NVT ensemble, 29 with a thermostat relaxation time of 0.1 ps.…”

“…MD simulations were carried out with DL_POLY_4, 27 using the force field introduced in Ref. 28, which we derived by refining, and consistently merging with the SPC/Fw model of water, 25 a set of existing interatomic potentials for the mineral. 24 In this force field, the mackinawite pairwise interactions can be written as a function of the interatomic distances r i j ,…”

DFT-D2 simulations of water adsorption and dissociation on the low-index surfaces of mackinawite (FeS)

“…The optimised adsorption geometries of water on the different FeS surfaces are shown in Figure 3, and the calculated adsorption energies, optimized geometry parameters, and vibrational frequencies are listed in Table I. On the FeS{001}, we considered different high-symmetry sites and found that the water molecule interacts very weakly with the hydrogen atoms pointing toward surface sulfur atoms, 74,75 releasing an adsorption energy of 0.15 eV. The contribution from the dispersion correction to this adsorption energy is 0.13 eV, which is about 87% of the total value, highlighting the importance of dispersion forces on stabilising the water molecule on the FeS{001} surfaces.…”

A kinetic model of water adsorption, clustering and dissociation on the Fe3S4{001} surface

“…mackinawite. 69,70 Clusters with five or more members generate geometrical structures such as pentagons and hexagons, whereas square structures are less stable due to the geometry of the water molecule and the tension in the long-range interactions. The [H 2 O] 4 cluster forms a periodic linear structure, similar to the ones found on cubic MgO{001} surfaces.…”