Systematic analysis and extension of embedded atom models

Embedded-atom methods (EAM) are among the most popular classical force fields for pure metals and alloys. Despite the relative simplicity of the model, it is capable of reproducing the correct trends for a variety of physical quantities and has been found applicable to a wide range of structural problems. Of particular interest in this study is the transferability of the method between different coordination environments.The EAM potentials are composed of a pair-wise repulsive term and an embedding term which functionally depends on an electronic density, usually formed as a linear combination of atom-centered radial charge distributions. Both the repulsion and the embedding terms can be chosen to reproduce a given equation of state exactly.In this work, we replace each of these EAM components with different functional forms and analyze how well they describe a set of ab initio copper structures ranging from clusters to atomic wires, layer structures and bulk lattices. Since the degrees of freedom related to charge transfer are outside the scope of the conventional EAM description, our structures are chosen to consist only of symmetry equivalent atoms.We find that the simple EAM potentials such as Gupta and Finnis-Sinclair give a satisfactory match against the ab initio data. The more complex component functionals improved the quality of the fit only by trace amounts. We find that letting the embedding function to not only depend on the density but also on the density gradients improves the fit significantly.In a systematic expansion of the embedding energy in terms of the charge density gradients, we find that only a few terms are necessary to yield a model which reproduces the data within its accuracy limits. This model, which we call as systematically modified embedded atom method (SMEAM), overcomes the systematic errors in the EAM dissociation energies at low coordination numbers but still continues to share the difficulties with the compliance properties of the hypothetical copper diamond