FF: fix compute_force! for constrained systems

Fixes: #138
Signed-off-by: Thomas Kemmer <[email protected]>

src/forcefields/common/nonbonded_component.jl

@@ -643,18 +643,18 @@ function compute_forces!(esi::ElectrostaticInteraction{T}) where {T<:Real}
 end
 
 function compute_forces!(nbc::NonBondedComponent{T}) where {T<:Real}
-    constrained_ids = getproperty.(getindex.(Ref(atoms(nbc.ff.system)), nbc.ff.constrained_atoms), :idx)
-
-    filter_pairs = (
-        isempty(constrained_ids)
-            ? identity
-            : s -> filter(p -> (p.a1.idx ∉ constrained_ids) || (p.a2.idx ∉ constrained_ids), s)
-    )
-
-    map(compute_forces!, filter_pairs(nbc.lj_interactions))
-    map(compute_forces!, filter_pairs(nbc.hydrogen_bonds))
-    map(compute_forces!, filter_pairs(nbc.electrostatic_interactions))
+    if isempty(nbc.ff.constrained_atoms)
+        map(compute_forces!, nbc.lj_interactions)
+        map(compute_forces!, nbc.hydrogen_bonds)
+        map(compute_forces!, nbc.electrostatic_interactions)
+    else
+        constrained_ids = getproperty.(getindex.(Ref(atoms(nbc.ff.system)), nbc.ff.constrained_atoms), :idx)
+        filter_pairs = s -> (p for p in s if (p.a1.idx ∉ constrained_ids) || (p.a2.idx ∉ constrained_ids))
 
+        map(compute_forces!, filter_pairs(nbc.lj_interactions))
+        map(compute_forces!, filter_pairs(nbc.hydrogen_bonds))
+        map(compute_forces!, filter_pairs(nbc.electrostatic_interactions))
+    end
     nothing
 end