The Madelung constant is de¯ned as
A =
X
i6=j
§1
pij
Now, consider the linear chain shown below[1]:
+ − + − + − + −
The § in the de¯nition of the Madelung constant represents the § ions. Starting with ion i, as
indicated in the diagram, we add up all of the ions to the right. Using rij = rpij , where r is the
nearest neighbor distance:
A =
X
i6=j
§1
pij
=
X
i6=j
§r
rij
A = r
X
i6=j
§1
rij
= r
µ
+
1
r ¡
1
2r
+
1
3r ¡
1
4r
+ ¢ ¢ ¢
¶
Since there are just as many ions to the left of the reference ion as there are to the right we must
multiply by a factor of 2. Thus,
A = 2r
µ
+
1
r ¡
1
2r
+
1
3r ¡
1
4r
+ ¢ ¢ ¢
¶
= 2
µ
1 ¡
1
2
+
1
3 ¡
1
4
+ ¢ ¢ ¢
¶
Now, using
ln(1 + x) = x ¡
x2
2
+
x3
3 ¡
x4
4
+ ¢ ¢ ¢
with x = 1 we get for the Madelung constant
A = 2 ln(2) = 1:38629
Now