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We present a unified variational treatment of the electric quadrupole (E2)matrix elements, Einstein coefficients, and line strengths for generalopen-shell diatomic molecules in the general purpose diatomic code \Duo.Transformation relations between the Cartesian representation (typically usedin electronic structure calculations) to the tensorial representation (requiredfor spectroscopic applications) of the electric quadrupole moment componentsare derived. The implementation has been validated against accurate theoreticalcalculations and experimental measurements of quadrupole intensities of$^1\text{H}_2$ available in the literature. We also present accurate electronicstructure calculations of the electric quadrupole moment functions for the$X^1\Sigma^+$ electronic states of $\text{CO}$ and $\text{HF}$ at the CCSD(T)and MRCI levels of theory, respectively, as well for the $a^1\Delta_g$ --$b^1\Sigma_g^+$ quadrupole transition moment of $\text{O}_2$ with MRCI level oftheory. Accurate infrared E2 line lists for $^{12}\text{C}^{16}\text{O}$ and$^1\text{H}^{19}\text{F}$ are provided. A demonstration of spectroscopicapplications is presented by simulating E2 spectra for$^{12}\text{C}^{16}\text{O}$, $\text{H}^{19}\text{F}$ and $^{16}\text{O}_2$(Noxon $a^1\Delta_g$ -- $b^1\Sigma_g^+$ band).

Calculation of Electric Quadrupole Linestrengths for Diatomic Molecules: Application to the H2, CO, HF and O2 Molecules