Polarizability using automatic differentiation

Simple example for computing properties using (forward-mode) automatic differentation. 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
    He = ElementPsp(:He, psp=load_psp("hgh/lda/He-q2"))
    atoms = [He => [[1/2; 1/2; 1/2]]]  # Helium at the center of the box

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

# 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

# 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=tol)
    dipole(scfres.basis, scfres.ρ)

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

Forward-mode implicit differentiation

Right now DFTK has no out-of-the-box support for implicit differentiation through the SCF. However one can easily work around this as follows. We keep both a non-dual basis and a basis including duals for easy bookkeeping (but redundant computation ...).

function self_consistent_field_dual(basis::PlaneWaveBasis, basis_dual::PlaneWaveBasis{T};
                                    kwargs...) where T <: ForwardDiff.Dual
    scfres = self_consistent_field(basis; kwargs...)
    ψ, occupation = DFTK.select_occupied_orbitals(basis, scfres.ψ, scfres.occupation)

    # promote everything eagerly to Dual numbers
    occupation_dual = [T.(occupation[1])]
    ψ_dual = [Complex.(T.(real(ψ[1])), T.(imag(ψ[1])))]
    ρ_dual = compute_density(basis_dual, ψ_dual, occupation_dual)

    _, δH = energy_hamiltonian(basis_dual, ψ_dual, occupation_dual; ρ=ρ_dual)
    δHψ = δH * ψ_dual
    δHψ = [ForwardDiff.partials.(δHψ[1], 1)]
    δψ = DFTK.solve_ΩplusK(basis, ψ, -δHψ, occupation)
    δρ = DFTK.compute_δρ(basis, ψ, δψ, occupation)
    ρ = ForwardDiff.value.(ρ_dual)
    ψ, ρ, δψ, δρ

This function is now used in the following to provide a dual version for the compute_dipole function:

function compute_dipole(ε::ForwardDiff.Dual; tol=1e-8, kwargs...)
    T = ForwardDiff.tagtype(ε)
    basis = make_basis(ForwardDiff.value(ε); kwargs...)
    basis_dual = make_basis(ε; kwargs...)
    ψ, ρ, δψ, δρ = self_consistent_field_dual(basis, basis_dual; tol)
    ρ_dual = ForwardDiff.Dual{T}.(ρ, δρ)
    dipole(basis_dual, ρ_dual)

This setup allows to compute the polarizability via automatic differentiation:

polarizability = ForwardDiff.derivative(compute_dipole, 0.0)
println("Polarizability via ForwardDiff:       $polarizability")
println("Polarizability via finite difference: $polarizability_fd")
n     Energy            Eₙ-Eₙ₋₁     ρout-ρin   α      Diag
---   ---------------   ---------   --------   ----   ----
  1   -2.770798363028         NaN   2.99e-01   0.80    8.0
  2   -2.772055194430   -1.26e-03   4.81e-02   0.80    1.0
  3   -2.772083087027   -2.79e-05   3.45e-03   0.80    2.0
  4   -2.772083417154   -3.30e-07   4.95e-05   0.80    2.0
  5   -2.772083417790   -6.36e-10   1.29e-05   0.80    2.0

Polarizability via ForwardDiff:       1.7725573430001202
Polarizability via finite difference: 1.77373982324435