  In this paper, we present a new method for calculating linear absorption
spectra for large molecular aggregates, called dyadic adaptive HOPS (DadHOPS).
This method combines the adaptive HOPS (adHOPS) framework, which uses locality
to improve computational scaling, with the dyadic HOPS method previously
developed to calculate linear and non-linear spectroscopic signals. To
construct a local representation of dyadic HOPS, we introduce an initial state
decomposition which reconstructs the linear absorption spectra from a sum over
locally excited initial conditions. We demonstrate the sum over initial
conditions can be efficiently Monte Carlo sampled, that the corresponding
calculations achieve size-invariant (i.e. $\mathcal{O}(1)$) scaling for
sufficiently large aggregates, and that it allows for the trivial inclusion of
static disorder in the Hamiltonian. We present calculations on the photosystem
I core complex to explore the behavior of the initial state decomposition in
complex molecular aggregates, and proof-of-concept DadHOPS calculations on an
artificial molecular aggregate inspired by perylene bis-imide.
