Geodesic Focusing Conditions in $f(Q)$ Gravity

We study the geodesic deviation equation in symmetric teleparallel geometry (STG), where the relative acceleration is defined with respect to the STG connection. We analyze the modified Raychaudhuri equation along a geodesic congruence in $f(Q)$ gravity under the Weyl-type nonmetricity ansatz. In contrast to the purely geometrical Raychaudhuri equation obtained in general metric-affine settings, the equation derived here contains matter-source contributions through the trace equation of $f(Q)$ gravity. Different from general relativity, focusing in $f(Q)$ gravity is not automatic, and one must impose an appropriate focusing condition. We collect the model-dependent terms in the modified Raychaudhuri equation into an effective energy-momentum trace $T_{\mathrm{eff}}$, so that the focusing condition can be written as the inequality $T\leq T_{\mathrm{eff}}$, where $T$ is the trace of the matter energy-momentum tensor. We also apply this condition to a flat Friedmann--Lema\^{i}tre--Robertson--Walker background. The homogeneous and isotropic STG connection admits three branches, each characterized by a single connection function $\gamma_i$, with $i=1,2,3$. In the symmetric teleparallel equivalent of general relativity, we obtain the corresponding effective traces and derive the resulting constraints on the matter content and the connection functions in each branch.