The Kirchhoff sum index 
is a graph index defined for a graph on 
nodes by
 |
where 
is the resistance distance matrix and 
is the graph
distance matrix.
Unless otherwise stated, hydrogen atoms are usually ignored in the computation of such indices as organic chemists usually do when they write a benzene ring as a hexagon (Devillers and Balaban 1999, p. 25).
Precomputed values for many graphs are implemented in the Wolfram Language as GraphData[g, "KirchhoffSumIndex"].
The following table summarizes values of the Kirchhoff sum index for various special classes of graphs.
| graph class | OEIS |  ,  , ... |
| Andrásfai graph | A000000 | 1, 7, 183/14, 2085/109, 177683/7059, 3526539/113017, ... |
| antiprism graph | A000000 | X, X, 23/4, 437/42, 181/11, 853/36, 291887/9048, ... |
| Apollonian network | A000000 | 3, 672/85, 18269/475, 1359691/5295, 13731955316/7104375, ... |
cocktail party graph  | A000000 | X, 4, 23/4, 23/3, 77/8, 58/5, 163/12, 109/7, 281/16, ... |
complete bipartite
graph  | A000000 | 1, 4, 7, 10, 13, 16, 19, 22, 25, 28, ... |
complete
graph  | A001477 | 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, ... |
complete
tripartite graph  | A000000 | 2, 23/4, 19/2, 53/4, 17, 83/4, ... |
| crossed prism graph | A000000 | 227/18, 117/4, 925/18, 7091/90, 5008/45, 9374/63, ... |
| crown graph | A000000 | X, X, 21/2, 227/18, 551/36, 363/20, 379/18, 3023/126, ... |
| cube-connected cycle graph | A000000 | X, X, 18037/150, 52720110713/81144000, ... |
cycle graph  | A000000 | X, X, 2, 4, 7, 21/2, 15, 20, 26, 65/2, 40, 48, 57, 133/2, ... |
| folded cube graph | A000000 | 1, 3, 10, 65/2, 869/9, 5327/18, 7996/9, 33277/12, ... |
| gear graph | A000000 | X, X, 229/20, 170/9, 6475/228, 7189/180, 45563/852, 69, ... |
grid graph  | A000000 | 0, 4, 170/9, 44299/840, 39633487/346500, 133000367/623700, ... |
grid graph  | A000000 | 0, 227/18, 1900321/18900, 3012514861/7197120, ... |
| halved cube graph | A000000 | 0, 1, 3, 23/3, 115/6, 13123/270, 5698/45, 21445/63, ... |
hypercube
graph  | A000000 | 1, 4, 227/18, 334/9, 96661/900, 140663/450, 10194871/11025, ... |
Möbius ladder  | A000000 | 7, 183/14, 1999/99, 1145/39, 14600/369, 60311/1164, 2929/45, ... |
| Mycielski graph | A000000 | 0, 1, 7, 5935/278, 10873314919/187685230, ... |
odd graph  | A000000 | 0, 2, 21, 169, 5425/4, 69991/6, ... |
| pan graph | A000000 | 14/3, 89/12, 169/15, 1117/72, 125/6, 6379/240, 9013/270, ... |
| path graph | A000000 | 0, 1, 3, 6, 10, 15, 21, 28, 36, 45, 55, 66, 78, ... |
permutation
star graph  | A000000 | 0, 1, 21/2, 2173/20, 31688/21, 5223853/168, ... |
prism graph  | A000000 | X, X, 36/5, 227/18, 384/19, 3461/120, 16855/426, 32317/630, ... |
rook graph  | A000000 | X, 4, 13, 57/2, 52, 85, 129, 371/2, ... |
star graph  | A000290 | 0, 1, 3, 6, 10, 15, 21, 28, 36, 45, ... |
| sun graph | A000000 | X, X, 95/12, 79/6, 7175/372, 20479/780, 84069/2468, ... |
sunlet graph  | A000000 | X, X, 38/3, 137/6, 37, 1067/20, 737/10, 577/6, 3679/30, ... |
| tetrahedral graph | A000000 | X, X, X, X, X, 367/12, 799/12, 3475/27, 16471/72, 13741/36, ... |
| triangular graph | A000000 | X, 0, 2, 23/4, 93/8, 20, 125/4, 183/4, 511/8, 86, ... |
| web graph | A000000 | X, X, 116/5, 2863/72, 3536/57, 19601/225, 50081/426, 27107/180, ... |
wheel graph  | A000000 | X, X, X, 3, 14/3, 75/11, 189/20, 364/29, 564/35, 765/38, 123/5, ... |
Closed forms are summarized in the following table.
