# 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
positions = [[x1, 0, 0], [x2, 0, 0]]
gauss = ElementGaussian(1.0, 0.5)
atoms = [gauss, gauss]
```

2-element Vector{Main.ex-custom_potential.ElementGaussian}: Main.ex-custom_potential.ElementGaussian(X) Main.ex-custom_potential.ElementGaussian(X)

We setup a Gross-Pitaevskii model

```
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.

```
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 0.0380285 AtomicLocal -0.3163444 LocalNonlinearity 0.1212595 total -0.157056406834

Computing the forces can then be done as usual:

`compute_forces(scfres)`

2-element Vector{StaticArraysCore.SVector{3, Float64}}: [-0.05565918729291715, 0.0, 0.0] [0.055660288082649556, 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")
```