Recent experiments on the kagome metals ${\mathrm{AV}}_3{\mathrm{Sb}}_5$ (A = K, Rb, Cs) have reported signatures of rotational symmetry breaking and electronic nematicity within the charge-density-wave (CDW) phase, raising fundamental questions about the underlying mechanisms driving this phenomenon. Through first-principles density functional theory calculations for ${\mathrm{RbV}}_3{\mathrm{Sb}}_5$, we reveal a novel bond-order mechanism that naturally gives rise to an intrinsic twofold anisotropic response. Specifically, we demonstrate that a lateral $\pi$ phase shift between adjacent kagome layers induces an anisotropic rearrangement of the V–V dimers, leading to the formation of a bond-order wave. This bond-order wave breaks the $C_6$ symmetry in the kagome plane, lowering the crystal symmetry from $P6/mmm$ to $Fmmm$. This lattice-driven distortion, arising from atomic interactions and geometric constraints, results in pronounced $C_2$-symmetric features in the reconstructed Fermi surface, phonon spectrum, and in-plane resistivity, consistent with experimentally observed twofold anisotropic responses. Our results provide a microscopic structural basis for intrinsic twofold anisotropy in the CDW phase of ${\mathrm{RbV}}_3{\mathrm{Sb}}_5$. Our findings introduce a novel chemical mechanism, in which bond-order anisotropy—originating from lattice distortions—plays a pivotal role in the emergence of nematicity in kagome ${\mathrm{AV}}_3{\mathrm{Sb}}_5$ materials. This challenges the current understanding that rotational symmetry breaking is solely driven by electronic interactions, offering a new perspective on how bond-order effects, shaped by the lattice, can influence electronic properties in complex materials.

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• Charge density waves
• Electronic structure
• First-principles calculations