James B. Adams

Interatomic Potentials from First-Principles Calculations: The Force-Matching Method

We present a new scheme to extract numerically optimal interatomic potentials from large amounts of data produced by first-principles calculations. The method is based on fitting the potential to ab initio atomic forces of many atomic configurations, including surfaces, clusters, liquids and crystals at finite temperature. The extensive data set overcomes the difficulties encountered by traditional fitting approaches when using rich and complex analytic forms, allowing to construct potentials with a degree of accuracy comparable to that obtained by ab initio methods. A glue potential for aluminium obtained with this method is presented and discussed.

EAM STUDY OF SURFACE SELF-DIFFUSION OF SINGLE ADATOMS OF FCC METALS NI, CU, AL, AG, AU, PD AND PT

Abstract Self-diffusion of single adatoms on the (100), (110), (111), (311), and (331) surfaces of fcc metals is investigated with the embedded atom method (EAM). The general trend of activation energies for these surfaces is consistent with experimental observations. The calculated activation energies for Ni are in excellent agreement with experimental data, but those for Al and Pt differ from experimental values by up to a factor of 3. The estimated pre-exponential factors are in the range of 10 −4 −10 −2 cm 2 s , in good agreement with experiment.

The application of the analytic embedded atom method to bcc metals and alloys

Johnson and Oh have recently developed Embedded Atom Method potentials for bcc metals (Na, Li, K, V, Nb, Ta, Mo, W, Fe). The predictive power of these potentials was first tested by calculating vacancy formation and migration energies. Due to the results of these calculations, some of the functions were slightly modified to improve their fit to vacancy properties. The modified potentials were then used to calculate phonon dispersion curves, surface relaxations, surface energies, and thermal expansion. In addition, Johnsons alloy model, which works well for fcc metals, was applied to the bcc metals to predict dilute heats of solution.

Diffusion mechanisms on Ni surfaces

Abstract The embedded atom method (EAM) was previously used to study diffusion of single Ni adatoms on various Ni surfaces; the calculated diffusion rates were in excellent agreement with experiment (C.L. Liu, J.M. Cohen, J.B. Adams and A.F. Voter, Surf. Sci. 253 (1991) 334 [1]). This paper presents calculations of the formation and migration energies of vacancies in Ni surfaces, including (100), (110), (111), (311), and (331). In all cases, the activation energies for vacancy diffusion are higher than those for single adatoms. Formation energies of ledges and kinks on (100), (110), and (111) Ni surfaces were calculated. The interactions of single adatoms with ledges and kinks and diffusion of single adatoms on the stepped Ni surfaces were determined. Finally, MD simulations of surface diffusion of a single adatom on a Ni(111) surface were carried out to confirm the earlier molecular statics calculations and to test a simple Debye model.

Vacancy diffusion along twist grain boundaries in copper

Vacancy diffusion along two different high-angle twist grain boundaries ({Sigma}5 and {Sigma}13) was studied using the Embedded Atom Method (EAM). Vacancy formation energies in all the possible sites were calculated and found to be directly related to the degree of coincidence with the neighboring crystal planes. Optimal migration paths and migration energies were determined and found to be very low. The activation energies for self-diffusion at the boundaries were found to be less than half of the bulk value.

Structure and diffusion of clusters on Ni surfaces

Abstract The optimal structure of Ni clusters on Ni(100) and Ni(111) and Pt clusters on Ni(100) are studied using the embedded atom method (EAM). Unlike Ni and Pt clusters on Pt(100) [1,2], the clusters always preferred relatively closed-packed islands over linear chains. The structure of Ni clusters on Ni(111) were generally similar to those of Ir on Ir(111) [3]. Significant variations in dissociation energy indicate that certain “magic number” cluster sizes are favored over others. The diffusion mechanisms of the small clusters were investigated to determine the optimal migration paths and corresponding migration energies. The energy barriers for small clusters of Ni on Ni(100) are comparable to monomers, in qualitative agreement with recent FIM experiments for Ptx/Pt(100) [4]. For Nix/Ni(111) migration energies generally increased with increasing cluster size, in qualitative agreement with experimental results for Ir clusters on Ir(111) [3].

Fourth moment approximation to tight binding: application to bcc transition metals

Abstract Using Carlssons approach of the low-order moments approximation to tight binding, we develop improved potentials for Mo and W, also a potential for V. This model was fit to twelve bulk properties, namely cohesive energy, lattice constant, elastic constants, vacancy properties, bcc — fcc and bcc — A15 structural energy differences and four zone edge phonons. Also, for Mo and W, the functions were required to yield a reconstruction of the (100) surface, although the details of the surface were not fit. The potentials were tested by calculating surface properties. Most importantly, both the W(100) (√2 × √2 )R45° and Mo(100) c(7√2 × √2 )R45° surface reconstructions are observed, which is a good indication of the physical correctness of the angular terms, although for Mo(100) the lowest energy occurred on c(5√2 × √2 )R45° structure.

Diffusion behavior of single adatoms near and at steps during growth of metallic thin films on Ni surfaces

Abstract Diffusion of single adatoms approaching both descending and ascending steps on Ni(111), (110) and (100) has been investigated with the embedded atom method (EAM) and molecular statics (MS). (1) It was found that there exists a “forbidden” region near both descending and ascending steps on Ni(111). Adatoms have to overcome a slightly higher energy barrier to get into the “forbidden” region, which extends 2–3 nearest-neighboring spacings from the steps. This is consistent with FIM experiments for incorporation of Ir adatoms into ascending steps of Ir clusters. (2) Detailed calculations for incorporation of adatoms over descending steps of type B on Ni(111) have revealed that exchange diffusion, in which an adatom replaces the position of an atom in the step and exchange their roles, is energetically favored over ordinary jumps and is the dominant diffusion mechanism at the descending steps. This is consistent with recent FIM experiments of W adatom diffusion at Ir cluster edges of type B on Ir(111). (3) Exchange diffusion was also found to be favored over direct jumps at the descending steps on Ni(110) and Ni(100).

CO on Pd(110): determination of the optimal adsorption site

We present ab initio pseudo-potential plane-wave total-energy calculations for the geometric and electronic structure of the CO-covered Pd(110) surface. Our calculations were performed within the local-density approximation (LDA) of density functional theory (DFT). There has been some controversy as to whether CO prefers to adsorb at a bridge or on-top site when exposed to Pd(110). Total energy calculations for a CO monolayer adsorbed at the on-top and bridge adsorption sites revealed the bridge site adsorption to be favored by 0.59 eV per CO molecule. The preferential adsorption of CO to the bridge site was further corroborated by our band-structure calculations, with only the bridge site results being in good agreement with recent inverse photoemission experiments.

Structure of seven W surfaces

Abstract We present a theoretical study of the structure of seven W surfaces, using a modified fourth moment approximation to tight-binding theory. The seven W surfaces include three low-index surfaces ((100), (110) and (111)) and four high-index ((210), (211), (310) and (321)) surfaces. Perpendicular to the surface, most surfaces followed an oscillatory pattern of contraction and expansion, but the (111) and (321) surfaces relaxed with successive contractions of the first and second interplanar spacing, followed by an expansion of the third interplanar spacing. Also, the (100) surface and high-index (210), (211), (310) and (321) surfaces formed reconstructions with the top layers shifting parallel to the surface. The details of the atomic rearrangement can be understood with a simple bonding coordination argument.