Geometry optimization

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.

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.

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.

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).

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.

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
Iter     Function value   Gradient norm
     0    -1.061170e+00     6.234648e-01
 * time: 4.792213439941406e-5
     1    -1.065558e+00     4.382196e-02
 * time: 1.719054937362671
     2    -1.065592e+00     8.811360e-04
 * time: 3.2049639225006104
     3    -1.065592e+00     9.221025e-07
 * time: 4.078300952911377
     4    -1.065592e+00     3.820797e-08
 * time: 5.3124840259552
     5    -1.065592e+00     2.423934e-08
 * time: 6.156683921813965

Optimal bond length for Ecut=5.00: 1.557 Bohr

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.

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.