# Custom solvers

In this example, we show how to define custom solvers. Our system will again be silicon, because we are not very imaginative

using DFTK, LinearAlgebra

a = 10.26
lattice = a / 2 * [[0 1 1.];
[1 0 1.];
[1 1 0.]]
atoms = [Si => [ones(3)/8, -ones(3)/8]]

# We take very (very) crude parameters
model = model_LDA(lattice, atoms)
kgrid = [1, 1, 1]
Ecut = 5
basis = PlaneWaveBasis(model, Ecut; kgrid=kgrid);

We define our custom fix-point solver: simply a damped fixed-point

function my_fp_solver(f, x0, max_iter; tol)
mixing_factor = .7
x = x0
fx = f(x)
for n = 1:max_iter
inc = fx - x
if norm(inc) < tol
break
end
x = x + mixing_factor * inc
fx = f(x)
end
(fixpoint=x, converged=norm(fx-x) < tol)
end;

Our eigenvalue solver just forms the dense matrix and diagonalizes it explicitly (this only works for very small systems)

function my_eig_solver(A, X0; maxiter, tol, kwargs...)
n = size(X0, 2)
A = Array(A)
E = eigen(A)
λ = E.values[1:n]
X = E.vectors[:, 1:n]
(λ=λ, X=X, residual_norms=[], iterations=0, converged=true, n_matvec=0)
end;

Finally we also define our custom mixing scheme. It will be a mixture of simple mixing (for the first 2 steps) and than default to Kerker mixing. In the mixing interface δF is $(ρ_\text{out} - ρ_\text{in})$, i.e. the difference in density between two subsequent SCF steps and the mix function returns $δρ$, which is added to $ρ_\text{in}$ to yield $ρ_\text{next}$, the density for the next SCF step.

struct MyMixing
α  # Damping parameter
end
MyMixing() = MyMixing(0.7)

function DFTK.mix(mixing::MyMixing, basis, δF::RealFourierArray, δF_spin=nothing; n_iter, kwargs...)
if n_iter <= 2
# Just do simple mixing on total density and spin density (if it exists)
(mixing.α * δF, isnothing(δF_spin) ? nothing : mixing.α * δF_spin)
else
# Use the KerkerMixing from DFTK
DFTK.mix(KerkerMixing(α=mixing.α), basis, δF, δF_spin; kwargs...)
end
end

That's it! Now we just run the SCF with these solvers

scfres = self_consistent_field(basis;
tol=1e-8,
solver=my_fp_solver,
eigensolver=my_eig_solver,
mixing=MyMixing());
n     Energy            Eₙ-Eₙ₋₁     ρout-ρin   Diag
---   ---------------   ---------   --------   ----
1   -7.231196098783         NaN   3.19e-01    0.0
2   -7.246178074844   -1.50e-02   1.46e-01    0.0
3   -7.248750207429   -2.57e-03   6.61e-02    0.0
4   -7.249063680400   -3.13e-04   3.64e-02    0.0
5   -7.249166356486   -1.03e-04   2.02e-02    0.0
6   -7.249199920160   -3.36e-05   1.13e-02    0.0
7   -7.249211045327   -1.11e-05   6.39e-03    0.0
8   -7.249214819223   -3.77e-06   3.67e-03    0.0
9   -7.249216136003   -1.32e-06   2.14e-03    0.0
10   -7.249216609416   -4.73e-07   1.26e-03    0.0
11   -7.249216784646   -1.75e-07   7.58e-04    0.0
12   -7.249216851247   -6.66e-08   4.61e-04    0.0
13   -7.249216877146   -2.59e-08   2.84e-04    0.0
14   -7.249216887410   -1.03e-08   1.77e-04    0.0
15   -7.249216891541   -4.13e-09   1.11e-04    0.0

Note that the default convergence criterion is on the difference of energy from one step to the other; when this gets below tol, the "driver" self_consistent_field artificially makes the fixpoint solver think it's converged by forcing f(x) = x. You can customize this with the is_converged keyword argument to self_consistent_field.