  We find an explicit closed formula for the $k$'th iterated commutator
$\mathrm{ad}_A^k(H_V(\xi))$ of arbitrary order $k\ge1$ between a Hamiltonian
$H_V(\xi)=M_{\omega_\xi}+S_{\check V}$ and a conjugate operator
$A=\frac{\mathfrak{i}}{2}(v_\xi\cdot\nabla+\nabla\cdot v_\xi)$, where
$M_{\omega_\xi}$ is the operator of multiplication with the real analytic
function $\omega_\xi$ which depends real analytically on the parameter $\xi$,
and the operator $S_{\check V}$ is the operator of convolution with the
(sufficiently nice) function $\check V$, and $v_\xi$ is some vector field
determined by $\omega_\xi$. Under certain assumptions, which are satisfied for
the Yukawa potential, we then prove estimates of the form
$\lVert\mathrm{ad}_A^k(H_V(\xi))(H_0(\xi)+\mathfrak{i})^{-1}\rVert\le
C_\xi^kk!$ where $C_\xi$ is some constant which depends continuously on $\xi$.
The Hamiltonian is the fixed total momentum fiber Hamiltonian of an abstract
two-body dispersive system and the work is inspired by a recent result
[Engelmann-M{\o}ller-Rasmussen, 2015] which, under conditions including
estimates of the mentioned type, opens up for spectral deformation and analytic
perturbation theory of embedded eigenvalues of finite multiplicity.
