diff --git a/tests/BIndexTest.cs b/tests/BIndexTest.cs
new file mode 100644
index 0000000..7358496
--- /dev/null
+++ b/tests/BIndexTest.cs
@@ -0,0 +1,50 @@
+using System;
+
+namespace VirtualChemistryTest;
+
+public static class BIndexTest
+{
+    private static int BIndex(int index, int multi)
+    {
+        if (multi == 1)
+            return 0;
+        if (multi % 2 == 1)
+        {
+            int center = multi / 2 + 1;
+            return center - index;
+        }
+
+        if (multi % 2 == 0)
+        {
+            double center = multi / 2 + 0.5;
+            return (int)(center > index ? Math.Ceiling(center - index) : Math.Floor(center - index));
+        }
+
+        return 0;
+    }
+    [Test]
+    public static void TestCase1()
+    {
+        Assert.That(BIndex(1, 1), Is.EqualTo(0));
+        Assert.That(BIndex(1, 2), Is.EqualTo(1));
+        Assert.That(BIndex(2,2), Is.EqualTo(-1));
+        Assert.That(BIndex(1, 3), Is.EqualTo(1));
+        Assert.That(BIndex(2, 3), Is.EqualTo(0));
+        Assert.That(BIndex(3, 3), Is.EqualTo(-1));
+        Assert.That(BIndex(1,4), Is.EqualTo(2));
+        Assert.That(BIndex(2, 4), Is.EqualTo(1));
+        Assert.That(BIndex(3, 4), Is.EqualTo(-1));
+        Assert.That(BIndex(4, 4), Is.EqualTo(-2));
+        Assert.That(BIndex(1, 5), Is.EqualTo(2));
+        Assert.That(BIndex(2, 5), Is.EqualTo(1));
+        Assert.That(BIndex(3, 5), Is.EqualTo(0));
+        Assert.That(BIndex(4, 5), Is.EqualTo(-1));
+        Assert.That(BIndex(5, 5), Is.EqualTo(-2));
+        Assert.That(BIndex(1, 6), Is.EqualTo(3));
+        Assert.That(BIndex(2, 6), Is.EqualTo(2));
+        Assert.That(BIndex(3, 6),Is.EqualTo(1));
+        Assert.That(BIndex(4, 6),Is.EqualTo(-1));
+        Assert.That(BIndex(5, 6), Is.EqualTo(-2));
+        Assert.That(BIndex(6,6), Is.EqualTo(-3));
+    }
+}
\ No newline at end of file