Re-add nlopt test

Author: mfherbst

test/nlopt.jl

@@ -0,0 +1,25 @@
+@testitem "Test NLopt solver via Optimization.jl interface" begin
+    using AtomsBase
+    using AtomsBuilder
+    using AtomsCalculators
+    using EmpiricalPotentials
+    using GeometryOptimization
+    using OptimizationNLopt
+    using Unitful
+    using UnitfulAtomic
+    AC = AtomsCalculators
+    calculator = StillingerWeber()
+    solver = NLopt.LD_TNEWTON()
+
+    # Get a minimised reference
+    reference = minimize_energy!(bulk(:Si; cubic=true), calculator;
+                                 tol_forces=1e-10, maxiters=200)
+
+    # Equilibrate a perturbed Si crystal using a solver from Optimization.jl
+    silicon_init = rattle!(bulk(:Si; cubic=true), 0.2u"Å")
+    energy_init  = AC.potential_energy(silicon_init,  calculator)
+    results = minimize_energy!(silicon_init, calculator, solver;
+                               tol_energy=1e-10, tol_forces=1e-4u"eV/Å", maxeval=100)
+    @test results.energy ≤ energy_init
+    @test austrip(abs(results.energy - reference.energy)) < 1e-10
+end