  We present a method of extracting the exchange parameters of the classical
Heisenberg model from first-principles calculations of spin-spiral total
energies based on density functional theory. The exchange parameters of the
transition-metal monoxides MnO and NiO are calculated and used to estimate
magnetic properties such as transition temperatures and magnon energies.
Furthermore we show how to relate the magnon energies directly to differences
in spin-spiral total energies for systems containing an arbitrary number of
magnetic sublattices. This provides a comparison between magnon energies using
a finite number of exchange parameters and the infinite limit.
