Since scalar-biquadratic is such a common use case, this PR uniformly supports again it with the biquad argument to set_exchange!, regardless of mode.
I was originally hoping we could entirely remove the biquad argument, and instead direct people to the more general set_pair_coupling! function. That idea hit a snag with mode = :dipole_large_S. In particular, Sunny doesn't have a way to represent symbolic spin operators ($S \to \infty$) in the tensor product space along a bond. It would be complicated to implement, and there is not a strong motivation, especially since we are de-emphasizing :dipole_large_S.
Since scalar-biquadratic is such a common use case, this PR uniformly supports again it with the
biquadargument toset_exchange!, regardless of mode.I was originally hoping we could entirely remove the
biquadargument, and instead direct people to the more generalset_pair_coupling!function. That idea hit a snag withmode = :dipole_large_S. In particular, Sunny doesn't have a way to represent symbolic spin operators ($S \to \infty$) in the tensor product space along a bond. It would be complicated to implement, and there is not a strong motivation, especially since we are de-emphasizing:dipole_large_S.Fixes #276.