  We consider the simulation of electromagnetic scattering by single and
multiple isotropic homogeneous dielectric particles using boundary integral
equations. Galerkin discretizations of the classical
Poggio-Miller-Chang-Harrington-Wu-Tsai (PMCHWT) boundary integral equation
formulation provide accurate solutions for complex particle geometries, but are
well-known to lead to ill-conditioned linear systems. In this paper we carry
out an experimental investigation into the performance of Calder\'on
preconditioning techniques for single and multiple absorbing obstacles, which
involve a squaring of the PMCHWT operator to produce a well-conditioned
second-kind formulation. For single-particle scattering configurations we find
that Calder\'on preconditioning is actually often outperformed by simple
"mass-matrix" preconditioning, i.e. working with the strong form of the
discretized PMCHWT operator. In the case of scattering by multiple particles we
find that a significant saving in computational cost can be obtained by
performing block-diagonal Calder\'on preconditioning in which only the
self-interaction blocks are preconditioned. Using the boundary element software
library Bempp (www.bempp.com) the numerical performance of the different
methods is compared for a range of wavenumbers, particle geometries and complex
refractive indices relevant to the scattering of light by atmospheric ice
crystals.
