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
    atoms     = [ElementPsp(:He; psp=load_psp("hgh/lda/He-q2"))]
    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

1.7735581387894268

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   -2.770757225891                   -0.52    9.0    164ms
  2   -2.772059962939       -2.89       -1.33    1.0    105ms
  3   -2.772082925821       -4.64       -2.42    1.0    107ms
  4   -2.772083332830       -6.39       -3.11    1.0    108ms
  5   -2.772083417766       -7.07       -4.72    2.0    283ms
  6   -2.772083417790      -10.60       -4.74    1.0    181ms
  7   -2.772083417807      -10.79       -5.47    1.0    147ms
  8   -2.772083417811      -11.40       -6.16    2.0    180ms
  9   -2.772083417811      -13.45       -6.60    1.0    604ms
 10   -2.772083417811      -14.05       -7.48    1.0    133ms
 11   -2.772083417811      -14.40       -8.35    2.0    142ms

Polarizability via ForwardDiff:       1.7725349619077673
Polarizability via finite difference: 1.7735581387894268