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How can I evaluate specific expressions in FeynCalc? For example, if there is a $\delta^i_j$, how can I set $i$ to 1 and $j$ to 2 and make it evaluate to zero?

I've tried 

TR[GA[µ,v]] /. {µ -> 1, v -> 2}

but that does not work, it just yields $4g^{12}$ instead of 0.

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I think I found a solution, albeit not a very pretty one.

One can manually set the elements of the metric tensor via 

Pair[LorentzIndex[1], LorentzIndex[2]] = 0

however one has to do this for every entry.

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