diff --git a/Project.toml b/Project.toml
index 34aadfc..18f25dc 100644
--- a/Project.toml
+++ b/Project.toml
@@ -7,6 +7,7 @@ version = "0.1.3"
 AtomsBase = "a963bdd2-2df7-4f54-a1ee-49d51e6be12a"
 AtomsIO = "1692102d-eeb4-4df9-807b-c9517f998d44"
 Cairo = "159f3aea-2a34-519c-b102-8c37f9878175"
+ChainRulesCore = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"
 ChemistryFeaturization = "6c925690-434a-421d-aea7-51398c5b007a"
 Colors = "5ae59095-9a9b-59fe-a467-6f913c188581"
 CondaPkg = "992eb4ea-22a4-4c89-a5bb-47a3300528ab"
@@ -22,6 +23,8 @@ SimpleWeightedGraphs = "47aef6b3-ad0c-573a-a1e2-d07658019622"
 StaticArrays = "90137ffa-7385-5640-81b9-e52037218182"
 Unitful = "1986cc42-f94f-5a68-af5c-568840ba703d"
 Xtals = "ede5f01d-793e-4c47-9885-c447d1f18d6d"
+Zygote = "e88e6eb3-aa80-5325-afca-941959d7151f"
+ZygoteRules = "700de1a5-db45-46bc-99cf-38207098b444"
 
 [compat]
 AtomsBase = "0.2, 0.3"
@@ -38,7 +41,7 @@ NearestNeighbors = "0.4"
 PythonCall = "0.9.10"
 SimpleWeightedGraphs = "1.2"
 StaticArrays = "1.5"
-Unitful = "1.12"	
+Unitful = "1.12"
 Xtals = "0.3, 0.4"
 julia = "1.6, 1.7"
 
diff --git a/src/AtomGraphs.jl b/src/AtomGraphs.jl
index bd958c4..410defe 100644
--- a/src/AtomGraphs.jl
+++ b/src/AtomGraphs.jl
@@ -10,6 +10,8 @@ using MolecularGraph
 import AtomsBase: atomic_symbol
 
 import ChemistryFeaturization: elements
+using ChainRulesCore
+using ZygoteRules
 
 include("atomgraph.jl")
 export AtomGraph, elements, visualize
@@ -17,4 +19,6 @@ export AtomGraph, elements, visualize
 include("graph_building.jl")
 export inverse_square, exp_decay
 
+include("adjoints.jl")
+
 end
diff --git a/src/adjoints.jl b/src/adjoints.jl
new file mode 100644
index 0000000..7fa6e00
--- /dev/null
+++ b/src/adjoints.jl
@@ -0,0 +1,50 @@
+using Zygote
+using ZygoteRules: @adjoint
+
+@adjoint function Dict(g::Base.Generator)
+  ys, backs = Zygote.unzip([Zygote.pullback(g.f, args) for args in g.iter])
+  Dict(ys...), Δ -> begin
+    dd = Dict(k => b(Δ)[1].second for (b,(k,v)) in zip(backs, pairs(Δ)))
+    ((x for x in dd),)
+  end
+end
+
+@adjoint function _cutoff!(weight_mat, f, ijd,
+                           nb_counts, longest_dists;
+                           max_num_nbr = 12)
+  y, ld = _cutoff!(weight_mat, f, ijd,
+               nb_counts, longest_dists;
+               max_num_nbr = max_num_nbr)
+  function cutoff_pb((Δ,nt))
+    s = size(Δ)
+    Δ = vec(collect(Δ))
+    for (ix, (_,_,d)) in zip(eachindex(Δ), ijd)
+      y_, back_ = Zygote.pullback(f, d)
+      Δ[ix] *= first(back_(Zygote.sensitivity(d)))
+    end
+    (reshape(Δ, s), nothing,
+    collect(zip(fill(nothing, size(Δ,1)),
+                fill(nothing, size(Δ,1)),
+                Δ)),
+    nothing,
+    nothing)
+  end
+
+  (y,ld), cutoff_pb
+end
+
+function Zygote.ChainRulesCore.rrule(::Type{T}, x...) where T <: SArray{D, Ts, ND, L} where {D, Ts, ND, L}
+  y = SArray{D, Ts, ND, L}(x...)
+  function sarray_pb(Δy)
+    Δy = map(t->eltype(x...)(t...), Δy)
+    return NoTangent(), (Δy...,)
+  end
+  return y, sarray_pb
+end
+
+Zygote.@adjoint function Unitful.ustrip(x::Quantity{T, D, U}) where {T, D, U}
+  function back(Δ)
+    (Quantity{T, D, U}(Δ), )
+  end
+  Unitful.ustrip(x), back
+end
\ No newline at end of file
diff --git a/src/graph_building.jl b/src/graph_building.jl
index 0fe0c9b..94b05f7 100644
--- a/src/graph_building.jl
+++ b/src/graph_building.jl
@@ -10,6 +10,8 @@ include("utils.jl")
 inverse_square(x) = x^-2.0
 exp_decay(x) = exp(-x)
 
+const pymat_structure = pyimport("pymatgen.core.structure")
+
 """
 Build graph from a file storing a crystal structure (will be read in using AtomsIO, which in turn calls ASE). Returns the weight matrix and elements used for constructing an `AtomGraph`.
 
@@ -38,8 +40,7 @@ function build_graph(
 
     if use_voronoi
         @info "Note that building neighbor lists and edge weights via the Voronoi method requires the assumption of periodic boundaries. If you are building a graph for a molecule, you probably do not want this..."
-        s = pyimport("pymatgen.core.structure")
-        struc = s.Structure.from_file(file_path)
+        struc = pymat_structure..Structure.from_file(file_path)
         weight_mat = weights_voronoi(struc)
         return weight_mat, atom_ids, struc
     else
@@ -83,7 +84,7 @@ function build_graph(
         max_num_nbr = max_num_nbr,
         dist_decay_func = dist_decay_func,
     )
-    return weight_mat, String.(atomic_symbol(sys)), sys
+    return weight_mat # , string.(atomic_symbol(sys)), sys
 end
 
 """
@@ -103,21 +104,38 @@ function weights_cutoff(is, js, dists; max_num_nbr = 12, dist_decay_func = inver
 
     # iterate over list of tuples to build edge weights...
     # note that neighbor list double counts so we only have to increment one counter per pair
-    weight_mat = zeros(Float32, num_atoms, num_atoms)
+    weight_mat = zeros(Float64, round(Int,num_atoms), round(Int,num_atoms))
+    weight_mat, longest_dists = _cutoff!(weight_mat,
+                                         dist_decay_func,
+                                         ijd,
+                                         nb_counts,
+                                         longest_dists)
+
+    # average across diagonal, just in case
+    weight_mat = 0.5 .* (weight_mat .+ weight_mat')
+
+    # normalize weights
+    weight_mat = weight_mat ./ maximum(weight_mat)
+    weight_mat
+end
+
+function _cutoff!(weight_mat, f, ijd,
+                  nb_counts, longest_dists; max_num_nbr = 12)
+
     for (i, j, d) in ijd
+        # FiniteDifferences doesn't like non integers as indices
+        # and is used to test
+        i, j = round.(Int, (i,j))
+
         # if we're under the max OR if it's at the same distance as the previous one
         if nb_counts[i] < max_num_nbr || isapprox(longest_dists[i], d)
-            weight_mat[i, j] += dist_decay_func(d)
+            weight_mat[i, j] += f(d)
             longest_dists[i] = d
             nb_counts[i] += 1
         end
     end
 
-    # average across diagonal, just in case
-    weight_mat = 0.5 .* (weight_mat .+ weight_mat')
-
-    # normalize weights
-    weight_mat = weight_mat ./ maximum(weight_mat)
+    weight_mat, longest_dists
 end
 
 """
@@ -183,29 +201,33 @@ function neighbor_list(sys; cutoff_radius::Real = 8.0)
         cutoff_radius = 0.99 * min_celldim
     end
 
+    get_pairdist((i,j)) = sqrt(sum((sc_pos[:, i] .- sc_pos[:, j]).^2))
+    index_map(i) = (i - 1) % n_atoms + 1 # I suddenly understand why some people dislike 1-based indexing
+    
     # todo: try BallTree, also perhaps other leafsize values
     # also, the whole supercell thing could probably be avoided (and this function sped up substantially) by doing this using something like:
     # ptree = BruteTree(hcat(ustrip.(position(s))...), PeriodicEuclidean([1,1,1]))
     # but I don't have time to carefully test that right now and I know the supercell thing should work
-    tree = BruteTree(sc_pos)
-    is_raw = 13*n_atoms+1:14*n_atoms
-    js_raw = inrange(tree, sc_pos[:, is_raw], cutoff_radius)
-
-    index_map(i) = (i - 1) % n_atoms + 1 # I suddenly understand why some people dislike 1-based indexing
-
-    # this looks horrifying but it does do the right thing...
-    #ijraw_pairs = [p for p in Iterators.flatten([Iterators.product([p for p in zip(is_raw, js_raw)][n]...) for n in 1:4]) if p[1]!=p[2]]
-    split1 = map(zip(is_raw, js_raw)) do x
-        return [
-            p for p in [(x[1], [j for j in js if j != x[1]]...) for js in x[2]] if
-            length(p) == 2
-        ]
+    is, js, ijraw_pairs = Zygote.ignore() do
+        tree = BruteTree(sc_pos)
+        is_raw = 13*n_atoms+1:14*n_atoms
+        js_raw = inrange(tree, sc_pos[:, is_raw], cutoff_radius)
+
+
+        # this looks horrifying but it does do the right thing...
+        #ijraw_pairs = [p for p in Iterators.flatten([Iterators.product([p for p in zip(is_raw, js_raw)][n]...) for n in 1:4]) if p[1]!=p[2]]
+        split1 = map(zip(is_raw, js_raw)) do x
+            return [
+                p for p in [(x[1], [j for j in js if j != x[1]]...) for js in x[2]] if
+                length(p) == 2
+            ]
+        end
+        ijraw_pairs = [(split1...)...]
+        is = index_map.([t[1] for t in ijraw_pairs])
+        js = index_map.([t[2] for t in ijraw_pairs])
+        is, js, ijraw_pairs
     end
-    ijraw_pairs = [(split1...)...]
-    get_pairdist((i,j)) = sqrt(sum((sc_pos[:, i] .- sc_pos[:, j]).^2))
     dists = get_pairdist.(ijraw_pairs)
-    is = index_map.([t[1] for t in ijraw_pairs])
-    js = index_map.([t[2] for t in ijraw_pairs])
     return is, js, dists
 end
 
diff --git a/src/utils.jl b/src/utils.jl
index af132ad..1e113c4 100644
--- a/src/utils.jl
+++ b/src/utils.jl
@@ -12,23 +12,23 @@ using AtomsBase
 """
 function build_supercell(sys::AbstractSystem, repfactors)
     @assert length(repfactors) == n_dimensions(sys) "Your list of replication factors doesn't match the dimensionality of the system!"
-
+    # @show repfactors
     old_box = bounding_box(sys)
     new_box = repfactors .* old_box
-    symbols = repeat(atomic_symbol(sys), prod(repfactors))
+    symbols = Zygote.ignore() do
+        repeat(atomic_symbol(sys), prod(repfactors))
+    end
 
-    integer_offsets = Iterators.product(range.(Ref(0), repfactors .- 1, step=1)...)
+    integer_offsets = Iterators.product(range.(0, repfactors .- 1, step=1)...)
     position_offsets = [sum(offset .* old_box) for offset in integer_offsets]
-
     old_positions = position(sys)
-    new_positions = repeat(MArray.(position(sys)), prod(repfactors))
-
-    for (i, offset) in enumerate(position_offsets)
-        indices = (1:length(sys)) .+ (i-1) * length(sys)
-        for (j, pos) in enumerate(old_positions)
-            new_positions[indices[j]] = pos .+ offset
-        end
-    end
 
-    return new_box, symbols, SVector.(new_positions)
-end
\ No newline at end of file
+    a = map(enumerate(position_offsets)) do (i,offset)
+         map(enumerate(old_positions)) do (j, pos)
+           pos .+ offset
+         end
+       end
+    new_positions = reduce(vcat, vec(a))
+    
+    return new_box, symbols, new_positions
+end