diff --git a/src/main.rs b/src/main.rs
index 25ab1a6..141fe96 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -119,12 +119,6 @@ fn main() {
     let mut volume = (num_particles as f64)/ density;
     let length  = volume.cbrt();
     let (l_x, l_y, mut l_z) = (length, length, length);
-    if vacuum_slab > 0.0 { // increase space in z
-        let scale = vacuum_slab + 1.0;
-        l_z *= scale;
-        volume *= scale;
-        density /= scale;
-    }
 
     let cutoff_squared = cutoff * cutoff;
     let max_displacement = length / 2.0;
@@ -142,6 +136,13 @@ fn main() {
         if rx.len() == num_particles { break; }
     }
 
+    if vacuum_slab > 0.0 { // increase space in z
+        let scale = vacuum_slab + 1.0;
+        l_z *= scale;
+        volume *= scale;
+        density /= scale;
+    }
+
     let e_shift = if SHIFT { 4.0 * LJ_EPS * ( (LJ_SIG/cutoff).powi(12) - (LJ_SIG/cutoff).powi(6) ) } else { 0.0 };
     let e_corr = if TAILCORR { 8.0/3.0*std::f64::consts::PI*density*LJ_EPS*LJ_SIG.powi(3)*((1.0/3.0*(LJ_SIG/cutoff).powi(9)) - (LJ_SIG/cutoff).powi(3)) } else { 0.0 };
     let p_corr = if TAILCORR { 16.0/3.0*std::f64::consts::PI*density.powi(2)*LJ_EPS*LJ_SIG.powi(3)*((2.0/3.0*(LJ_SIG/cutoff).powi(9)) - (LJ_SIG/cutoff).powi(3)) } else { 0.0 };