  This report summarizes the work of the Computational Frontier topical group
on theoretical calculations and simulation for Snowmass 2021. We discuss the
challenges, potential solutions, and needs facing six diverse but related
topical areas that span the subject of theoretical calculations and simulation
in high energy physics (HEP): cosmic calculations, particle accelerator
modeling, detector simulation, event generators, perturbative calculations, and
lattice QCD (quantum chromodynamics). The challenges arise from the next
generations of HEP experiments, which will include more complex instruments,
provide larger data volumes, and perform more precise measurements.
Calculations and simulations will need to keep up with these increased
requirements. The other aspect of the challenge is the evolution of computing
landscape away from general-purpose computing on CPUs and toward
special-purpose accelerators and coprocessors such as GPUs and FPGAs. These
newer devices can provide substantial improvements for certain categories of
algorithms, at the expense of more specialized programming and memory and data
access patterns.
