# Polarizability using automatic differentiation

Simple example for computing properties using (forward-mode) automatic differentiation. For a more classical approach and more details about computing polarizabilities, see Polarizability by linear response.

using DFTK
using LinearAlgebra
using ForwardDiff

# Construct PlaneWaveBasis given a particular electric field strength
# Again we take the example of a Helium atom.
function make_basis(ε::T; a=10., Ecut=30) where {T}
lattice=T(a) * I(3)  # lattice is a cube of a Bohrs
# Helium at the center of the box
positions = [[1/2, 1/2, 1/2]]

model = model_DFT(lattice, atoms, positions, [:lda_x, :lda_c_vwn];
extra_terms=[ExternalFromReal(r -> -ε * (r[1] - a/2))],
symmetries=false)
PlaneWaveBasis(model; Ecut, kgrid=[1, 1, 1])  # No k-point sampling on isolated system
end

# dipole moment of a given density (assuming the current geometry)
function dipole(basis, ρ)
@assert isdiag(basis.model.lattice)
a  = basis.model.lattice[1, 1]
rr = [a * (r[1] - 1/2) for r in r_vectors(basis)]
sum(rr .* ρ) * basis.dvol
end

# Function to compute the dipole for a given field strength
function compute_dipole(ε; tol=1e-8, kwargs...)
scfres = self_consistent_field(make_basis(ε; kwargs...); tol)
dipole(scfres.basis, scfres.ρ)
end;

With this in place we can compute the polarizability from finite differences (just like in the previous example):

polarizability_fd = let
ε = 0.01
(compute_dipole(ε) - compute_dipole(0.0)) / ε
end
1.773558078729286

We do the same thing using automatic differentiation. Under the hood this uses custom rules to implicitly differentiate through the self-consistent field fixed-point problem.

polarizability = ForwardDiff.derivative(compute_dipole, 0.0)
println()
println("Polarizability via ForwardDiff:       $polarizability") println("Polarizability via finite difference:$polarizability_fd")
n     Energy            log10(ΔE)   log10(Δρ)   Diag   Δtime
---   ---------------   ---------   ---------   ----   ------
1   -2.770755867506                   -0.52    9.0    287ms
2   -2.772059359418       -2.88       -1.33    1.0    106ms
3   -2.772082900347       -4.63       -2.42    1.0    107ms
4   -2.772083329953       -6.37       -3.10    1.0    141ms
5   -2.772083417673       -7.06       -4.41    2.0    126ms
6   -2.772083417771      -10.01       -4.67    1.0    111ms
7   -2.772083417807      -10.44       -5.62    1.0    119ms
8   -2.772083417811      -11.49       -6.64    2.0    148ms
9   -2.772083417811      -14.15       -7.02    1.0    124ms
10   -2.772083417811   +  -14.35       -7.98    1.0    134ms
11   -2.772083417811      -14.12       -8.19    2.0    156ms

Polarizability via ForwardDiff:       1.772534979666139
Polarizability via finite difference: 1.773558078729286