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SunnySuite / Sunny.jl

Spin dynamics and generalization to SU(N) coherent states
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Flexible Units #285

Closed kbarros closed 2 weeks ago

kbarros commented 2 weeks ago

Allow more flexibility in the system of Units. Given some specific energy and length scale, e.g. units = Units(:meV, :angstrom), the returned object will provide various physical constants. For example, units provides THz, meV, K, and T in multiples of the base energy unit. For kelvin and tesla, conversions are employed using the Botzmann constant and Bohr magneton, respectively. The function set_external_field! is deprecated in favor of set_field!, which now expects the magnetic field scaled by the Bohr magneton, which has units of energy. The function enable_dipole_dipole! now expects a vacuum permeability constant $μ_0 μ_B^2$ with dimensions of length³-energy, and this can be obtained via units.vacuum_permeability.

Most existing Sunny scripts should continue to work, but deprecation warnings will guide users toward the new interface.

kbarros commented 2 weeks ago

One minor question I have is whether its actually vacuum_permeability in the units. Isn't it vacuum permeability times the square of the Bohr magneton?

Yes, that's true. If we wanted to be more complete, we could hypothetically perform the following renamings of the Units energy constants to include the conversion factors:

THz -> h_THz   # Planck constant
K -> kB_K      # Boltzmann constant
T -> μB_T      # Bohr magneton

But that is sort of unnecessarily verbose. So colloquially, a physicist might say "work in units where $h = k_B = μ_B= 1$". Given that units.T actually means $μ_B T$, I think (within this same context) it's consistent to say that units.vacuum_permeability (namely $μ_0$) actually means $μ_0 μ_B^2$. However, I realize that it's confusing, so there are various places in the docs that I use the full expression $μ_0 μ_B^2$ without suppressing the $μ_B$ factors.

ddahlbom commented 2 weeks ago

Sounds good. I have no objections, just wanted to make sure I'm following.