Least-squares finite element methods for quantum electrodynamics

A significant amount of the computational time in large Monte Carlo simulations of lattice field theory is spent inverting the discrete Dirac operator. Unfortunately, traditional co-variant finite difference discretizations of the Dirac operator present serious challenges for standard iterative methods. For interesting physical parameters, the discretized operator is large and ill- conditioned and has random coefficients. More recently, adaptive algebraic multigrid (AMG) methods have been shown to be effective preconditioners for Wilson's discretization [J. Brannick et al., Lect. Notes Comput. Sci. Eng., 55 (2006), pp. 499-506], [J. Brannick et al., Phys. Rev. Lett., 100 (2008), pp. 041601-041604] of the Dirac equation. This paper presents an alternate discretization of the two-dimensional Dirac operator of quantum electrodynamics (QED) based on least-squares finite elements. The discretization is systematically developed and, physical properties of the resulting matrix system are discussed. Finally, numerical experiments are presented that demonstrate the effectiveness of adaptive smoothed aggregation (αSA) multigrid as a preconditioner for the discrete field equations.