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
n Energy Eₙ-Eₙ₋₁ ρout-ρin α Diag --- --------------- --------- -------- ---- ---- 1 -0.143558520868 NaN 3.81e-01 0.80 8.0 2 -0.156032524694 -1.25e-02 7.92e-02 0.80 1.0 3 -0.156768415148 -7.36e-04 2.74e-02 0.80 2.0 4 -0.157045816374 -2.77e-04 4.82e-03 0.80 2.0 5 -0.157055207782 -9.39e-06 1.22e-03 0.80 2.0 6 -0.157056386565 -1.18e-06 1.97e-04 0.80 1.0 7 -0.157056406524 -2.00e-08 3.10e-05 0.80 2.0 8 -0.157056406917 -3.92e-10 1.57e-06 0.80 2.0
Energy breakdown: Kinetic 0.0380295 AtomicLocal -0.3163466 PowerNonlinearity 0.1212607 total -0.157056406917
Computing the forces can then be done as usual:
hcat(compute_forces(scfres)...)
2×1 Matrix{StaticArrays.SVector{3, Float64}}: [-0.05568295449851397, 0.0, 0.0] [0.05568183401379799, 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 kpoint, 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")