From 989a8a17e6aa6256e1501256f86ad3a330aae800 Mon Sep 17 00:00:00 2001
From: David Luevano Alvarado <55825613+luevano@users.noreply.github.com>
Date: Mon, 9 Mar 2020 21:06:23 -0700
Subject: CM O(n^2) to O(n(n/2+1))

---
 ml_exp/representations.py | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/ml_exp/representations.py b/ml_exp/representations.py
index be567d67a..761e263d4 100644
--- a/ml_exp/representations.py
+++ b/ml_exp/representations.py
@@ -56,14 +56,15 @@ size. Arrays are not of the right shape.')
     cm = np.zeros((size, size), dtype=np.float64)
 
     # Actual calculation of the coulomb matrix.
-    for i, xyz_i in enumerate(coords):
-        for j, xyz_j in enumerate(coords):
-            rv = xyz_i - xyz_j
-            r = np.linalg.norm(rv)/cr
-            if i == j:
-                cm[i, j] = (0.5*nc[i]**2.4)
+    for i in range(n):
+        for j in range(i, n):
+            if i==j:
+                cm[i, i] = 0.5*nc[i]**2.4
             else:
+                rv = coords[i] - coords[j]
+                r = np.linalg.norm(rv)/cr
                 cm[i, j] = nc[i]*nc[j]/r
+                cm[j, i] = cm[i, j]
 
     # Now the value will be returned.
     if as_eig:
-- 
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