The orientation of water molecules on the Fe(100) surface and their influence on the interfacial properties need to be set up. Based on previous studies, three distinct water orientations that considered in the study were given as H-up, H-down, and H-mix, which refer to the direction of one of the O-H bonds of water relative to the surface. Table summarizes the key details of each water configuration, including adsorption energy, potential of zero charge vs. SHE, interlayer distances, and change in total charges. Both the Hdown and the H-mix configurations are more stable than the H-up configurations, while the Hdown configuration is slightly more stable than the H-mix. For example, the average water adsorption energy on bare Fe(100) surface is -0.69 eV and -0.67 eV for H-down and H-mix, respectively, whereas the same energy for H-up is given by -0.50 eV. The potential of zero charge (PZC) refers to the specific potential at which the surface has zero net charge with respect to its surroundings. The calculated PZC values for the H-down, H-mix, and H-up configurations are -1.19 V, -1.81 V, and -2.16 V vs. SHE, respectively, highlighting the sensitivity of interfacial properties to the arrangement of water molecules. The order of PZC vs SHE is H-down > H-mix > H-up. It is noted that the experimental value of PZC is -0.72 V vs SHE. It is noted that there is a large discrepancy between the calculated PZC value of the Fe(100) surface with different water configurations. The smallest deviates by -0.47 V for the H-down configuration. Such discrepancies are likely due to the localized adsorption of the water molecules which can influence the PZC by redistributing the charge between water and the surface. In all three water orientations, the presence of the water layer resulted in the accumulation of the electron density at the surface as shown by Bader charge analysis, which effectively lowered the surface potential. Therefore, the PZC value would shift to the more negative values. This can be confirmed by PZC calculations using a continuous dielectric medium model, which yielded a PZC of -0.74 V vs. SHE, much closer to the experimental value (without explicit adsorption of water molecules). Since the aim of this work is to explore the interfacial interactions between water molecules and the iron surface, we proceed with the H-down water-surface system.