Custom potential

We solve the 1D Gross-Pitaevskii equation with a custom potential. This is similar to Gross-Pitaevskii equation in one dimension and we show how to define local potentials attached to atoms, which allows for instance to compute forces.

using DFTK
using LinearAlgebra

First, we define a new element which represents a nucleus generating a Gaussian potential.

struct ElementGaussian <: DFTK.Element
    α  # Prefactor
    L  # Width of the Gaussian nucleus
end

We extend the two methods providing access to the real and Fourier representation of the potential to DFTK.

function DFTK.local_potential_real(el::ElementGaussian, r::Real)
    -el.α / (√(2π) * el.L) * exp(- (r / el.L)^2 / 2)
end
function DFTK.local_potential_fourier(el::ElementGaussian, q::Real)
    # = ∫ V(r) exp(-ix⋅q) dx
    -el.α * exp(- (q * el.L)^2 / 2)
end

We set up the lattice. For a 1D case we supply two zero lattice vectors

a = 10
lattice = a .* [[1 0 0.]; [0 0 0]; [0 0 0]];

In this example, we want to generate two Gaussian potentials generated by two "nuclei" localized at positions $x_1$ and $x_2$, that are expressed in $[0,1)$ in fractional coordinates. $|x_1 - x_2|$ should be different from $0.5$ to break symmetry and get nonzero forces.

x1 = 0.2
x2 = 0.8
nucleus = ElementGaussian(1.0, 0.5)
atoms = [nucleus => [[x1, 0, 0], [x2, 0, 0]]];

We setup a Gross-Pitaevskii model

C = 1.0
α = 2;
n_electrons = 1  # Increase this for fun
terms = [Kinetic(),
         AtomicLocal(),
         PowerNonlinearity(C, α),
]
model = Model(lattice; atoms=atoms, n_electrons=n_electrons, terms=terms,
              spin_polarization=:spinless);  # use "spinless electrons"

We discretize using a moderate Ecut and run a SCF algorithm to compute forces afterwards. As there is no ionic charge associated to nucleus we have to specify a starting density and we choose to start from a zero density.

basis = PlaneWaveBasis(model; Ecut=500, kgrid=(1, 1, 1))
ρ = zeros(eltype(basis), basis.fft_size..., 1)
scfres = self_consistent_field(basis, tol=1e-8, ρ=ρ)
scfres.energies

Energy breakdown (in Ha):
    Kinetic             0.0380294 
    AtomicLocal         -0.3163465
    PowerNonlinearity   0.1212606 

    total               -0.157056406918

Computing the forces can then be done as usual:

hcat(compute_forces(scfres)...)

2×1 Matrix{StaticArrays.SVector{3, Float64}}:
 [-0.05568134049390375, 0.0, 0.0]
 [0.05568050525361551, 0.0, 0.0]

Extract the converged total local potential

tot_local_pot = DFTK.total_local_potential(scfres.ham)[:, 1, 1]; # use only dimension 1

Extract other quantities before plotting them

ρ = scfres.ρ[:, 1, 1, 1]  # converged density, first spin component
ψ_fourier = scfres.ψ[1][:, 1];    # first k-point, all G components, first eigenvector
ψ = G_to_r(basis, basis.kpoints[1], ψ_fourier)[:, 1, 1]
ψ /= (ψ[div(end, 2)] / abs(ψ[div(end, 2)]));

using Plots
x = a * vec(first.(DFTK.r_vectors(basis)))
p = plot(x, real.(ψ), label="real(ψ)")
plot!(p, x, imag.(ψ), label="imag(ψ)")
plot!(p, x, ρ, label="ρ")
plot!(p, x, tot_local_pot, label="tot local pot")