# [MPB-discuss] Group velocity of guided mode

```I need to calculate the group velocity of the guided mode in 1-D line defect
photonic crystal waveguide. When I extract the group velocities using grep
I'm getting velocities for different k-points and different band gaps```
```
tevelocity:, 1, #(0.0 0.0 0.0), #(3.08591790567814e-8 -3.17361897850985e-10
0.0), #(-3.18411484190156e-8 8.71342196808855e-11 0.0),
#(1.84721203025619e-8 8.50943462313198e-11 0.0), #(-1.74134645752838e-8
-8.68616295160761e-11 0.0), #(-3.7772400907894e-7 8.03424804528978e-11 0.0),
#(-2.11572047798556e-7 -7.02123429895677e-11 0.0), #(-1.11656829528599e-5
3.29740544390964e-10 0.0), #(5.88498385990865e-5 2.54748780062977e-10 0.0)

tevelocity:, 2, #(0.307842626631501 -1.05710206152388e-15 0.0),
#(-0.306375565942897 4.39169137546245e-12 0.0), #(0.306191451559405
-5.2147365791715e-11 0.0), #(-0.302374230904649 5.21430531539878e-12 0.0),
#(0.301861636273583 -7.89021001345105e-12 0.0), #(-0.295952089064426
-1.08606617225004e-10 0.0), #(0.2943892343767 -1.2497561620612e-11 0.0),
#(-0.27378153579097 -2.61950834357133e-12 0.0), #(0.246972085727235
6.78484070946508e-12 0.0)

tevelocity:, 3, #(0.307874225357897 2.98191519834776e-15 0.0),
#(-0.30722391019847 8.68970169510119e-11 0.0), #(0.306812900930046
-1.84388853008345e-10 0.0), #(-0.305355084193399 -5.3571389939689e-12 0.0),
#(0.304265426376774 4.91908787004334e-13 0.0), #(-0.301341960337996
-3.28548624382275e-11 0.0), #(0.298125554089018 -6.44688040255094e-12 0.0),
#(-0.283177075609072 -4.84518748634126e-12 0.0), #(0.222551861651346
5.59117224725802e-12 0.0)

tevelocity:, 4, #(0.307661346795037 -6.32456627857244e-15 0.0),
#(-0.307443720320429 4.36350600459636e-12 0.0), #(0.306504436817747
-8.22640550826007e-11 0.0), #(-0.30575617804152 -3.29745306772771e-12 0.0),
#(0.303752119135192 -3.29477877939267e-13 0.0), #(-0.302041943401158
-2.71566240944536e-11 0.0), #(0.296866132176718 -1.51944298931579e-12 0.0),
#(-0.287532581139554 2.45493582564361e-12 0.0), #(0.177059998067604
1.43028979341483e-11 0.0)

and so on

How do I calculate the group velocity of the guided mode?

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• [MPB-discuss] Group velocity of guided mode ASHINA