  We present an efficient implementation of the Bethe-Salpeter equation (BSE)
method for obtaining core-level spectra including x-ray absorption (XAS), x-ray
emission (XES), and both resonant and non-resonant inelastic x-ray scattering
spectra (N/RIXS). Calculations are based on density functional theory (DFT)
electronic structures generated either by abinit or Quantumespresso, both
plane-wave basis, pseudopotential codes. This electronic structure is improved
through the inclusion of a GW self energy. The projector augmented wave
technique is used to evaluate transition matrix elements between core-level and
band states. Final two-particle scattering states are obtained with the NIST
core-level BSE solver (NBSE). We have previously reported this implementation,
which we refer to as ocean (Obtaining Core Excitations from Ab initio
electronic structure and NBSE) [Phys. Rev. B 83, 115106 (2011)]. Here, we
present additional efficiencies that enable us to evaluate spectra for systems
ten times larger than previously possible; containing up to a few thousand
electrons. These improvements include the implementation of optimal basis
functions that reduce the cost of the initial DFT calculations, more complete
parallelization of the screening calculation and of the action of the BSE
Hamiltonian, and various memory reductions. Scaling is demonstrated on
supercells of SrTiO_3 and example spectra for the organic light emitting
molecule Tris-(8-hydroxyquinoline)aluminum (Alq_3 ) are presented. The ability
to perform large-scale spectral calculations is particularly advantageous for
investigating dilute or non-periodic systems such as doped materials, amorphous
systems, or complex nano-structures.
