We use the DFTK and Optim packages in this example to find the minimal-energy
bond length of the `H_2`

molecule. We setup `H_2`

in an
LDA model just like in the Tutorial for silicon.

In [1]:

```
using DFTK
using Optim
using LinearAlgebra
using Printf
kgrid = [1, 1, 1] # k-Point grid
Ecut = 5 # kinetic energy cutoff in Hartree
tol = 1e-8 # tolerance for the optimization routine
a = 10 # lattice constant in Bohr
lattice = a * Diagonal(ones(3))
H = ElementPsp(:H, psp=load_psp("hgh/lda/h-q1"));
```

We define a blochwave and a density to be used as global variables so that we can transfer the solution from one iteration to another and therefore reduce the optimization time.

In [2]:

```
ψ = nothing
ρ = nothing
```

First, we create a function that computes the solution associated to the
position `x \in \mathbb{R}^6`

of the atoms in reduced coordinates
(cf. Reduced and cartesian coordinates for more
details on the coordinates system).
They are stored as a vector: `x[1:3]`

represents the position of the
first atom and `x[4:6]`

the position of the second.
We also update `ψ`

and `ρ`

for the next iteration.

In [3]:

```
function compute_scfres(x)
atoms = [H => [x[1:3], x[4:6]]]
model = model_LDA(lattice, atoms)
basis = PlaneWaveBasis(model, Ecut; kgrid=kgrid)
global ψ, ρ
if ρ === nothing
ρ = guess_density(basis)
end
scfres = self_consistent_field(basis; ψ=ψ, ρ=ρ,
tol=tol / 10, callback=info->nothing)
ψ = scfres.ψ
ρ = scfres.ρ
scfres
end;
```

Then, we create the function we want to optimize: `fg!`

is used to update the
value of the objective function `F`

, namely the energy, and its gradient `G`

,
here computed with the forces (which are, by definition, the negative gradient
of the energy).

In [4]:

```
function fg!(F, G, x)
scfres = compute_scfres(x)
if G != nothing
grad = forces(scfres)
G .= -[grad[1][1]; grad[1][2]]
end
scfres.energies.total
end;
```

Now, we can optimize on the 6 parameters `x = [x1, y1, z1, x2, y2, z2]`

in
reduced coordinates, using `LBFGS()`

, the default minimization algorithm
in Optim. We start from `x0`

, which is a first guess for the coordinates. By
default, `optimize`

traces the output of the optimization algorithm during the
iterations. Once we have the minimizer `xmin`

, we compute the bond length in
cartesian coordinates.

In [5]:

```
x0 = vcat(lattice \ [0., 0., 0.], lattice \ [1.4, 0., 0.])
xres = optimize(Optim.only_fg!(fg!), x0, LBFGS(),
Optim.Options(show_trace=true, f_tol=tol))
xmin = Optim.minimizer(xres)
dmin = norm(lattice*xmin[1:3] - lattice*xmin[4:6])
@printf "\nOptimal bond length for Ecut=%.2f: %.3f Bohr\n" Ecut dmin
```

We used here very rough parameters to generate the example and
setting `Ecut`

to 10 Ha yields a bond length of 1.523 Bohr,
which agrees with ABINIT.

!!! note "Degrees of freedom"
We used here a very general setting where we optimized on the 6 variables
representing the position of the 2 atoms and it can be easily extended
to molecules with more atoms (such as `H_2O`

). In the particular case
of `H_2`

, we could use only the internal degree of freedom which, in
this case, is just the bond length.