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 (Theta(e)) of C2H2 is established. including estimates of the small effects of relativity and correlation beyond CCSD(T). the value Theta(e) = (21.88 +/- 10.07) X 10(-40) C m(2) is obtained. The rovibrationally averaged value is Theta = (21.48 +/- 0.12) X 10(-10) C m(2). One recent experimentally deduced value of Theta is improved to include a higher-order correction term properly, and the improved value Theta = (20.9 +/- 0.8)X 10(-40) C m(2) is in good agreement with the theoretical one. Another recent experimentally deduced value Theta = (20.5 +/- 0.6)X 10(-40) C m(2) is slightly lower than the theoretical value. This discrepancy may originate in the rigid-rotor approximation assumed when deducing Theta from experimental data. (C) 1999 Elsevier Science B.V. All rights reserved.