Table 1 Mössbauer parameters extracted from the fitting of the spectra at T = 2.2 K and from ab initio calculations

	Parameter	Central site	Peripheral sites^f
Dropcast	Isomer shift (mm s⁻¹)^a	0.409 (2)	0.418 (4)	0.467 (2)	0.467 (2)
	Electric quadrupole shift (mm s⁻¹)^b	−0.149 (2)	0.045 (4)	0.094 (2)	0.094 (2)
	Hyperfine magnetic field (T)	46.39 (1)	52.72 (2)	52.92 (2)	52.92 (2)
Dropcast (theory)		Fe1	Fe2	Fe3	Fe4
Dropcast (theory)	Hyperfine magnetic field, A_iso (T)^c	46.90	52.34	54.49	53.32
Monolayer	Isomer shift (mm s⁻¹)^a	0.27 (5)	0.43 (4)	0.42 (6)	0.42 (6)
	Electric quadrupole shift (mm s⁻¹)^b	−0.07 (4)	−0.06 (2)	−0.03 (2)	−0.03 (2)
	σ of Gaussian broadening (mm s⁻¹)	0.28 (6)	0.25 (5)	0.2 (1)	0.2 (1)
	Hyperfine magnetic field (T)	40.7 (4)	49.9 (3)	45.6 (9)	45.6 (9)
Monolayer (theory)		Fe1	Fe2	Fe3	Fe4
	Hyperfine magnetic field, A_iso (T)^c,d	43.68 (W1)	52.11 (W1)	52.22 (W1)	53.04 (W1)
	Hyperfine magnetic field, A_iso (T)^c,d	45.09 (W3)	52.50 (W3)	51.73 (W3)	49.04 (W3)
	ΔA_iso (T) due to Au substrate^e	−1.77	−0.28	−0.55	−0.60

^aWith respect to α-Fe
^bThe formula of the electric quadrupole shift is \({\it{\epsilon }} = \frac{{e^2qQ}}{4}\left( {\frac{{3{\mathrm{cos}}^2\theta - 1}}{2}} \right)\), where θ is the angle between the electric field gradient principal axis and the direction of the hyperfine magnetic field at the iron atom
^cThe calculated hyperfine fields (Fermi contact terms) take into account the \(\left\langle {s_z^i} \right\rangle\) value and were scaled by a factor of 1.81 (see text and Methods section)
^dGeometries resulting from two different AIMD trajectories as in²⁶, i.e., walker 1 (W1) and walker 3 (W3), were considered
^eReduction of A_iso (T) values due to pure electronic effects induced by the substrate. The ΔA_iso values were calculated as 22Δ\(\left\langle {s_z^i} \right\rangle\) (averaged over all AIMD walkers) from the variation of the spin density, computed at the GPW-DFT level, between-on-surface and extrapolated geometries
^fThe numbering of peripheral sites only applies to computed values; experimental parameters in the last column apply to two indistinguishable sites, errors are given in parentheses

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