The norm of a radius vector is given by
$\rho = \sqrt{ \vphantom{\sum^{2}} \; k_{mm} \, \rho_{m}^{ 2} + k_{ee} \, \rho_{e}^{2} + k_{zz} \, \rho_{z}^{2} + 2 k_{em} \, \rho_{e} \rho_{m} + 2 k_{ez} \, \rho_{e} \rho_{z} + 2 k_{mz} \, \rho_{m} \rho_{z} \; }$
where $k$ stands for various dimensionless numbers in the quark metric. So
(1)
\begin{align} \rho^{2} = k_{mm} \, \rho_{m}^{ 2} + k_{ee} \, \rho_{e}^{2} + k_{zz} \, \rho_{z}^{2} + 2 k_{em} \, \rho_{e} \rho_{m} + 2 k_{ez} \, \rho_{e} \rho_{z} + 2 k_{mz} \, \rho_{m} \rho_{z} \end{align}
Now let $A = \rho^{2}$ and set an average radius by $2 \tilde{\rho} = \rho_{i} + \rho_{f}$ then
$\partial A = A_{f} - A_{i} = \rho^{2}_{f} - \rho^{2}_{i} = \left( \rho_{f} + \rho_{i} \right) \left( \rho_{f} - \rho_{i} \right) = 2 \tilde{\rho} \, \partial \rho$
The average value of $\rho$ can also be written as $2\tilde{\rho} = 2\rho_{i} + \partial \rho = 2\rho_{f} - \partial \rho$. Then if $\partial \rho$ is a lot smaller than both of $\rho_{i}$ and $\rho_{f}$ we obtain $\tilde{\rho} \simeq \rho_{i} \simeq \rho_{f}$ and so can drop the tilde and subscripts to write
$\partial A = 2 \rho \, \partial \rho$
That is, when an interaction causes changes in a particle's shape that are negligible compared to its radii, then we can use the usual result from differential calculus. Similarly Equation (1) implies that
$\begin{align} \rho \, \partial \rho &= k_{mm} \, \rho_{m} \partial \rho_{m} + k_{ee} \, \rho_{e} \partial \rho_{e} + k_{zz} \, \rho_{z} \partial \rho_{z} \\ &+ \ k_{em} \left( \rho_{e} \partial \rho_{m} + \rho_{m} \partial \rho_{e} \right) + k_{ez} \left( \rho_{e} \partial \rho_{z} + \rho_{z} \partial \rho_{e} \right) + k_{mz} \left( \rho_{m} \partial \rho_{z} + \rho_{z} \partial \rho_{m} \right) \end{align}$
Recall that $k_{zz} \equiv 1$ and rearrange terms to get
(2)
\begin{align} \rho \, \partial \rho &= \partial \rho_{m} \left[ k_{mm} \rho_{m} + k_{em} \rho_{e} + k_{mz} \rho_{z} \right] \\ &+ \ \partial \rho_{e} \left[ k_{em} \rho_{m} + k_{ee} \rho_{e} + k_{ez} \rho_{z} \right] \\ &+ \ \partial \rho_{z} \left[ k_{mz} \rho_{m} + k_{ez} \rho_{e} + \rho_{z} \right] \end{align}
The work required to form a particle is $W \equiv \rho k_{\sf{F}}$ so the work done by the interaction is
$\begin{align} \partial W \equiv W_{\it{f}} - W_{\it{i}} = \partial \rho \, k_{\sf{F}} = \frac{ \rho \, \partial \rho \, }{\rho} k_{\sf{F}} \end{align}$
Using Equation (2) this can be written as
$\begin{align} \partial W &= \partial \rho_{m} \frac{ k_{mm} \rho_{m} + k_{em} \rho_{e} + k_{mz} \rho_{z}}{ \rho } k_{\sf{F}} \\ &+ \ \partial \rho_{e} \frac{ k_{em} \rho_{m} + k_{ee} \rho_{e} + k_{ez} \rho_{z} }{ \rho } k_{\sf{F}} \\ &+ \ \partial \rho_{z} \frac{ k_{mz} \rho_{m} + k_{ez} \rho_{e} + \rho_{z} }{ \rho } k_{\sf{F}} \end{align}$
Then in terms of $F_{m}$, $F_{e}$ and $F_{c}$ which note the forces that hold the atom together
$\begin{align} \partial W = F_{m} \partial \rho_{m} + F_{e} \partial \rho_{e} - F_{c} \partial \rho_{z} \end{align}$
Or more explictly, the work done on an atom is related to attributes of the exchange particle by