  In many problems in particle cosmology, interaction rates are dominated by
${2}\leftrightarrow{2}$ scatterings, or get a substantial contribution from
them, given that ${1}\leftrightarrow{2}$ and ${1}\leftrightarrow{3}$ reactions
are phase-space suppressed. We describe an algorithm to represent, regularize,
and evaluate a class of thermal ${2}\leftrightarrow{2}$ and
${1}\leftrightarrow{3}$ interaction rates for general momenta, masses, chemical
potentials, and helicity projections. A key ingredient is an automated
inclusion of virtual corrections to ${1}\leftrightarrow{2}$ scatterings, which
eliminate logarithmic and double-logarithmic IR divergences from the real
${2}\leftrightarrow{2}$ and ${1}\leftrightarrow{3}$ processes. We also review
thermal and chemical potential induced contributions that require resummation
if plasma particles are ultrarelativistic.
