The Earth's core is thought to composed of Fe–Ni alloys with large amounts of light elements. The composition of the present Earth's outer core reflects various processes, such as core formation, inner core growth, and core-mantle chemical interactions. Although oxygen, silicon, carbon, nitrogen, sulfur, and hydrogen have been proposed as candidates for light components (Stevenson, 1981), little is known yet about amounts and species. This is partly because experimental determination of the physical properties of liquid states is still not practical at the outer core pressure and temperature due to technical limitations. However, the ab initio density functional computation method is quite powerful for investigating liquid properties under such extreme conditions. The thermodynamic properties of liquid iron alloys may provide fundamental data for thermochemical modeling of the Earth's core. Presented are comprehensive discussions covering density jump at the inner-outer core boundary, phase relations of iron alloys, geochemical constraints on the bulk composition of the Earth, heterogeneities of P-wave velocity in the outer core, and partitioning of elements during core formation processes, along with density and P-wave velocity of pure Fe and Fe-light elements alloy liquids by means of an ab initio molecular dynamics simulation at whole outer core P, T conditions.