Research Area: Computational Methods

Research Area Summary: The Center supports the Office of Naval Research's Grand Challenge "Navy Materials by Design" by developing and maintaining a variety of computational tools. These include first-principles methods based on density-functional theory, specialized models for highly correlated systems, efficient tight-binding and overlapping-atom models, and simulation methods spanning multiple length scales.

Multiscale Methods: We develop a number of computational tools for multiscale simulations, including the the QUIP method and the NRL-MDS suite

The QUIP method combines QUantum mechanics and Interatomic Potentials into a single computational tool. Based on the libAtoms library, it includes a programming interface and some framework programs for doing simulations with interatomic potentials and quantum-mechanical methods using a unified interface. Interatomic potentials and tight-binding models are built in, and density functional theory codes can be used as plug ins. Capabilities include energy minimization, molecular dynamics, and reaction path finding. Methods that enabled simultaneous coupling QM and IP are being developed using this framework.

The NRL-MDS is a suite of tools for DoD researchers involved in computer simulations of material properties, consisting of four components:

MPNRLMOL: Massively-Parallel NRL Molecular Orbital Library
DOD-AE: All-Electron total-energy calculations using APW with local orbitals
LMSCS: Lattice Models for Strongly Correlated Spins and electrons
CLS: atomistic Coupling of Length Scales for mechanical properties

NRL-MDS is being developed as part of the Common HPC Software Support Initiative (CHSSI), under the Computational Chemistry and Materials Science (CCM) Computational Technology Area.

Further Information:

Expressive programming for computational physics in Fortran 95+. G. Csányi, S. Winfield, J. R. Kermode, M. C. Payne, A. Comisso, A. De Vita, and N. Bernstein, IOP Computational Physics Group Newsletter, Spring, p. 1-11 (2007).

Point of contact: Noam.Bernstein@nrl.navy.mil (Privacy Advisory)

FIG. 1. Success of the NRL Tight-Binding Method (click on the image for a larger picture)

NRL Tight-Binding: Tight-binding total energy methods can be used as a link between highly accurate but computationally intensive, first-principles density functional calculations and fast, compact, but less accurate atomistic potential methods. By fitting to LAPW calculations for high symmetry structures, we have constructed tight-binding (TB) Hamiltonians which are used to interpolate between first-principles results, allowing accurate determinations of structural energy orderings, elastic constants, phonon frequencies, surface energies, stacking faults and various defect structures.We have also developed computer programs which can use these TB parameters to do tight-binding molecular dynamics for the study of temperature-dependent properties. Our method is succesful for a variety of materials including both metals and semiconductors as well as magnetic systems. We have developed TB parameters for all the transition elements, most sp metals and the single element semiconductors. We have also extended the method to spin-polarized systems for the magnetic elements, binary materials such as FeAl, NiAl, NbC, Cu₃Au and SiC.

Further Information: NRL Tight-Binding Publications

Point of contact: Michael.Mehl@nrl.navy.mil (Privacy Advisory)

Self-Consistent Atomic Deformation (SCAD) method: A density functional method is being developed, called self-consistent atomic deformation (SCAD) in which the total charge density is represented as a sum of atomic-like (localized) densities. While this approach is generally less accurate than conventional band-structure methods, it offers a relatively simple interpretation of polarization and related properties. The computational labor of the SCAD method increases as the first power of the number of atoms in the system, offering a more efficient method for treating large systems. Each atomic-like density is obtained from the solutions of one-electron Schr\"odinger's equations, one for each atomic site, with basis functions given by the product of tabulated radial functions and spherical harmonics. The potentials at each site are formulated variationally from the total energy, in analogy to the Kohn-Sham formulation of density functional theory. However, the total energy in the SCAD method contains an additional approximation which accounts for kinetic energy due to atomic-like densities overlapping with their neighbors.

The SCAD method has been applied to calculate electronic and vibrational properties of alkali halides. Polarization calculations are used to determine Born effective charges and high frequency dielectric constants which in turn have a pronounced effect on phonon frequencies. The frequencies for high symmetry wave vector phonons in twelve alkali halides are compared with experimental values in figure 1. Calculations for the sequence of compounds NaCl, MgO and AlP indicate that charge densities associated with increasingly covalent bonds are effectively approximated in SCAD by the increasingly deformable negative ions.

Further Reading:

Self-consistent atomic deformation method for application of density functional theory L. L. Boyer, H. T. Stokes, M. M. Ossowski, and M. J. Mehl, Phys. Rev. B 78, 045121 (2008).
Calculation of Polarization Using a Density Functional Method with Localized Charge L. L. Boyer, H. T. Stokes and M. J. Mehl, Phys. Rev. Lett. 84, 709(2000).

Point of contact: Larry.Boyer@nrl.navy.mil (Privacy Advisory)

Modification date : Sep 9, 2008 Send comments or corrections to the webmaster (Privacy Advisory).