# 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.

In :
using DFTK
using LinearAlgebra


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

In :
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.

In :
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

In :
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.

In :
x1 = 0.2
x2 = 0.8
positions = [[x1, 0, 0], [x2, 0, 0]]
gauss     = ElementGaussian(1.0, 0.5)
atoms     = [gauss, gauss]

Out:
2-element Vector{Main.##340.ElementGaussian}:
Main.##340.ElementGaussian(X)
Main.##340.ElementGaussian(X)

We setup a Gross-Pitaevskii model

In :
C = 1.0
α = 2;
n_electrons = 1  # Increase this for fun
terms = [Kinetic(),
AtomicLocal(),
LocalNonlinearity(ρ -> C * ρ^α)]
model = Model(lattice, atoms, positions; n_electrons, 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 gauss we have to specify a starting density and we choose to start from a zero density.

In :
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            log10(ΔE)   log10(Δρ)   Diag
---   ---------------   ---------   ---------   ----
1   -0.143577066747                   -0.42    8.0
2   -0.156035130922       -1.90       -1.10    1.0
3   -0.156768327093       -3.13       -1.56    2.0
4   -0.157045858392       -3.56       -2.32    2.0
5   -0.157055277264       -5.03       -2.93    2.0
6   -0.157056388742       -5.95       -3.72    1.0
7   -0.157056406007       -7.76       -4.34    2.0
8   -0.157056406917       -9.04       -5.95    2.0

Out:
Energy breakdown (in Ha):
Kinetic             0.0380295
AtomicLocal         -0.3163466
LocalNonlinearity   0.1212607

total               -0.157056406917

Computing the forces can then be done as usual:

In :
compute_forces(scfres)

Out:
2-element Vector{StaticArrays.SVector{3, Float64}}:
[-0.05568221088373597, 0.0, 0.0]
[0.05568170081166464, 0.0, 0.0]

Extract the converged total local potential

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


Extract other quantities before plotting them

In :
ρ = scfres.ρ[:, 1, 1, 1]        # converged density, first spin component
ψ_fourier = scfres.ψ[:, 1]   # first k-point, all G components, first eigenvector
ψ = G_to_r(basis, basis.kpoints, 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")

Out: