  With the Poincar\'e group $\mathbf{R}^{3,\,1}\rtimes O(3,\,1)$ as the model
of departure, we focus, for $n=p+q$, $p+q\ge 4$, on the affine indefinite
orthogonal group $\mathbf{R}^{p,q}\rtimes SO(p,\,q)$. Denote by
$\mathfrak{h}_{p,\,q}$ the Lie algebra of the affine indefinite orthogonal
group. We compute the Leibniz cohomology of $\mathfrak{h}_{p,\,q}$ with adjoint
coefficients, written $HL^*(\mathfrak{h}_{p,\,q};\,\mathfrak{h}_{p,\,q})$. We
calculate several indefinite orthogonal invariants, and $\mathfrak{h}_{p,\,q}
$-invariants and provide the Leibniz cohomology in terms of these invariants.
