FieldStrength[mu, nu, a] is the field strength tensor \partial _{\mu } A_{\nu }^a - \partial _{\nu } A_{\mu }^a + g_s A_{\mu }^b A_{\nu }^c f^{abc}.
FieldStrength[mu, nu] is the field strength tensor (\partial _{\mu } A_{\nu}- \partial_{\nu } A_{\mu}).
The name of the field (A) and the coupling constant (g) can be set through the options or by additional arguments. The first two indices are interpreted as type LorentzIndex, except OPEDelta, which is converted to Momentum[OPEDelta].
FieldStrength[\[Mu], \[Nu]]F_{\mu \nu }^{}
FieldStrength[\[Mu], \[Nu], a]F_{\mu \nu }^a
FieldStrength[\[Mu], \[Nu], Explicit -> True]\left.(\partial _{\mu }A_{\nu }\right)-\left.(\partial _{\nu }A_{\mu }\right)
FieldStrength[\[Mu], \[Nu], a, Explicit -> True]g_s f^{a\text{b19}\;\text{c20}} A_{\mu }^{\text{b19}}.A_{\nu }^{\text{c20}}+\left.(\partial _{\mu }A_{\nu }^a\right)-\left.(\partial _{\nu }A_{\mu }^a\right)
FieldStrength[\[Mu], \[Nu], a, CouplingConstant -> -SMP["g_s"], Explicit -> True]-g_s f^{a\text{b21}\;\text{c22}} A_{\mu }^{\text{b21}}.A_{\nu }^{\text{c22}}+\left.(\partial _{\mu }A_{\nu }^a\right)-\left.(\partial _{\nu }A_{\mu }^a\right)