On the molecular electric quadrupole moment of C2H2

From accurate ab initio calculations the non-relativistic CCSD(T) basis set limit result of the equilibrium molecular electric quadrupole moment (Θe) of C2H2 is established. Including estimates of the small effects of relativity and correlation beyond CCSD(T), the value Θe=(21.88±0.07)×10−40 C m2 is obtained. The rovibrationally averaged value is Θ=(21.48±0.12)×10−40 C m2. One recent experimentally deduced value of Θ is improved to include a higher-order correction term properly, and the improved value Θ=(20.9±0.8)×10−40 C m2 is in good agreement with the theoretical one. Another recent experimentally deduced value Θ=(20.5±0.6)×10−40 C m2 is slightly lower than the theoretical value. This discrepancy may originate in the rigid-rotor approximation assumed when deducing Θ from experimental data.