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AD for forces #868

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AD for forces #868

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36 changes: 22 additions & 14 deletions src/terms/local.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -68,7 +68,7 @@ end
Atomic local potential defined by `model.atoms`.
"""
struct AtomicLocal end
function (::AtomicLocal)(basis::PlaneWaveBasis{T}) where {T}
function compute_local_potential(basis::PlaneWaveBasis{T}; positions=basis.model.positions) where {T}
# pot_fourier is <e_G|V|e_G'> expanded in a basis of e_{G-G'}
# Since V is a sum of radial functions located at atomic
# positions, this involves a form factor (`local_potential_fourier`)
Expand Down Expand Up @@ -97,7 +97,7 @@ function (::AtomicLocal)(basis::PlaneWaveBasis{T}) where {T}
pot_fourier = map(enumerate(Gs)) do (iG, G)
q = norm(G_cart[iG])
pot = sum(enumerate(model.atom_groups)) do (igroup, group)
structure_factor = sum(r -> cis2pi(-dot(G, r)), @view model.positions[group])
structure_factor = sum(r -> cis2pi(-dot(G, r)), @view positions[group])
form_factors[(igroup, q)] * structure_factor
end
pot / sqrt(model.unit_cell_volume)
Expand All @@ -106,25 +106,33 @@ function (::AtomicLocal)(basis::PlaneWaveBasis{T}) where {T}
enforce_real!(basis, pot_fourier) # Symmetrize Fourier coeffs to have real iFFT
pot_real = irfft(basis, to_device(basis.architecture, pot_fourier))

TermAtomicLocal(pot_real)
pot_real
end
(::AtomicLocal)(basis::PlaneWaveBasis) = TermAtomicLocal(compute_local_potential(basis))

function compute_δV(::TermAtomicLocal, basis::PlaneWaveBasis{T}, γ, τ;
q=zero(Vec3{T}), positions=basis.model.positions) where {T}
displacement = zero.(positions)
displacement[τ] = setindex(displacement[τ], one(T), γ)
ForwardDiff.derivative(zero(T)) do ε
positions = ε*displacement .+ positions
compute_local_potential(basis; positions)
end
end

@timing "forces: local" function compute_forces(::TermAtomicLocal, basis::PlaneWaveBasis{TT},
@timing "forces: local" function compute_forces(term::TermAtomicLocal, basis::PlaneWaveBasis{TT},
ψ, occupation; ρ, kwargs...) where {TT}
T = promote_type(TT, real(eltype(ψ[1])))
model = basis.model
ρ_fourier = fft(basis, total_density(ρ))

# energy = sum of form_factor(G) * struct_factor(G) * rho(G)
# where struct_factor(G) = e^{-i G·r}
forces = [zero(Vec3{T}) for _ in 1:length(model.positions)]
for group in model.atom_groups
element = model.atoms[first(group)]
form_factors = [Complex{T}(local_potential_fourier(element, norm(G)))
for G in G_vectors_cart(basis)]
for idx in group
r = model.positions[idx]
forces[idx] = _force_local_internal(basis, ρ_fourier, form_factors, r)
# forces = ∫ρδV
n_atoms = length(model.positions)
forces = [zero(Vec3{T}) for _ in 1:n_atoms]
for τ in 1:n_atoms
for γ in 1:3
δV_τγ = compute_δV(term, basis, γ, τ)
forces[τ] = setindex(forces[τ], -real(sum(conj(ρ_fourier) .* fft(basis, δV_τγ))), γ)
end
end
forces
Expand Down