diff --git a/.gitignore b/.gitignore
index 2c3bc5d..8583cfa 100644
--- a/.gitignore
+++ b/.gitignore
@@ -6,3 +6,4 @@
 /docs/build/
 
 .DS_Store
+.vscode/
\ No newline at end of file
diff --git a/src/struct.jl b/src/struct.jl
index 041cbbf..fcf7ce0 100644
--- a/src/struct.jl
+++ b/src/struct.jl
@@ -1,33 +1,13 @@
 struct Lattice{T}
-    vectors::AbstractMatrix{T}
-    pbc::AbstractVector{Bool}
+    vectors::Matrix{T}
+    pbc::Vector{Bool}
 end
 
 struct Ion{T}
-    species::String
+    species::Symbol
     charge::Int
-    frac_pos::AbstractVector{T}
+    frac_pos::Vector{T}
 end
 
-function periodic_vectors(lattice::Lattice{T}) where T
-    periodic_vectors = Matrix{T}(undef, size(lattice.vectors)[1], sum(lattice.pbc))
-    for column in range(1, size(lattice.vectors)[2])
-        if lattice.pbc[column]
-            periodic_vectors[:, column] = lattice.vectors[:, column]
-        end
-    end
-    return periodic_vectors
-end
-
-function build_grid(nsize::AbstractVector{Int})
-    function decompose(n::Int)
-        remain = n
-        result = Vector{Int}(undef, length(nsize))
-        for i in range(1, length(nsize))
-            result[i] = remain ÷ prod(nsize[i+1:end])
-            remain = remain % prod(nsize[i+1:end])
-        end
-        return result
-    end
-    return [decompose(n) for n in range(0, prod(nsize) - 1)]
-end
\ No newline at end of file
+periodic_vectors(lattice::Lattice) = lattice.vectors[:, lattice.pbc]
+build_grid(nsize::AbstractVector{Int}) = map(x->collect(x.I[end:-1:1] .- 1), vec(CartesianIndices(Tuple(nsize[end:-1:1]))))
\ No newline at end of file
diff --git a/test/interaction.jl b/test/interaction.jl
index 2435bbb..5a99cfa 100644
--- a/test/interaction.jl
+++ b/test/interaction.jl
@@ -7,8 +7,8 @@ using CrystalStructurePrediction
 end
 
 @testset "real_space_sum" begin
-    ion_a = Ion("", rand(3), 1)
-    ion_b = Ion("", rand(3), 2)
+    ion_a = Ion(:none, 1, rand(3))
+    ion_b = Ion(:none, 2, rand(3))
     lattice = Lattice(rand(3,3), [true, true, true])
     alpha = 2.0
     for depth in range(0,4)
@@ -18,8 +18,8 @@ end
 end
 
 @testset "reciprocal_space_sum" begin
-    ion_a = Ion("", rand(3), 1)
-    ion_b = Ion("", rand(3), 2)
+    ion_a = Ion(:none, 1, rand(3))
+    ion_b = Ion(:none, 2, rand(3))
     lattice = Lattice(rand(3,3), [true, true, true])
     alpha = 2.0
     for depth in range(0,4)
@@ -29,8 +29,8 @@ end
 end
 
 @testset "buckingham_sum" begin
-    ion_a = Ion("Sr", rand(3), 1)
-    ion_b = Ion("O", rand(3), 2)
+    ion_a = Ion(:Sr, 1, rand(3))
+    ion_b = Ion(:O, 2, rand(3))
     lattice = Lattice(rand(3,3), [true, true, true])
     for depth in range(0,4)
         depth_list = [depth for _ in range(1,3)]