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 custom potential
struct ElementCustomPotential <: DFTK.Element
pot_real::Function # Real potential
pot_fourier::Function # Fourier potential
end
We need to extend two methods to access the real and Fourier forms of the potential during the computations performed by DFTK
function DFTK.local_potential_fourier(el::ElementCustomPotential, q::Real)
return el.pot_fourier(q)
end
function DFTK.local_potential_real(el::ElementCustomPotential, r::Real)
return el.pot_real(r)
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;
We define the width of the Gaussian potential generated by one nucleus
L = 0.5;
We set the potential in its real and Fourier forms
pot_real(x) = exp(-(x/L)^2)
pot_fourier(q::T) where {T <: Real} = exp(- (q*L)^2 / 4);
And finally we build the elements and set their positions in the atoms
array. Note that in this example pot_real
is not required as all applications
of local potentials are done in the Fourier space.
nucleus = ElementCustomPotential(pot_real, pot_fourier)
atoms = [nucleus => [x1*[1,0,0], x2*[1,0,0]]];
Setup the 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.
Ecut = 500
basis = PlaneWaveBasis(model, Ecut, 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 Diag --- --------------- --------- -------- ---- 1 +0.293977096351 NaN 4.29e-01 8.0 2 +0.442071143377 1.48e-01 5.35e-01 1.0 3 +0.260880214052 -1.81e-01 1.72e-01 2.0 4 +0.243058294552 -1.78e-02 2.70e-02 2.0 5 +0.242660926714 -3.97e-04 5.98e-03 2.0 6 +0.242635893634 -2.50e-05 1.85e-03 1.0 7 +0.242633412289 -2.48e-06 3.10e-04 2.0 8 +0.242633389813 -2.25e-08 1.45e-04 2.0 9 +0.242633376491 -1.33e-08 1.54e-05 2.0 10 +0.242633376330 -1.61e-10 6.30e-07 1.0
Energy breakdown: Kinetic 0.0304520 AtomicLocal 0.0972467 PowerNonlinearity 0.1149346 total 0.242633376330
Computing the forces can then be done as usual:
hcat(compute_forces(scfres)...)
2×1 Matrix{StaticArrays.SVector{3, Float64}}: [-0.038689357456715534, 0.0, 0.0] [0.03868888948444092, 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")