Collisions of atomic ions in optical clocks with background gas molecules result in systematic frequency shifts. We evaluate the frequency shift in ${\mathrm{Sr}}^{+}$ clocks due to collisions with helium atoms and hydrogen molecules, using ab initio spin-orbit-coupled potential energy surfaces together with a master equation for the collisional shift. Our results lead to improved uncertainty estimates for collisional frequency shifts in ${\mathrm{Sr}}^{+}$ optical clocks.

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• Atomic & molecular collisions
• Atomic, optical & lattice clocks
• Quantum channels