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 (Thet a(e)) of C2H2 is established. including estimates of the small effects of r elativity and correlation beyond CCSD(T). the value Theta(e) = (21.88 +/- 1 0.07) X 10(-40) C m(2) is obtained. The rovibrationally averaged value is T heta = (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 prope rly, and the improved value Theta = (20.9 +/- 0.8)X 10(-40) C m(2) is in go od agreement with the theoretical one. Another recent experimentally deduce d value Theta = (20.5 +/- 0.6)X 10(-40) C m(2) is slightly lower than the t heoretical value. This discrepancy may originate in the rigid-rotor approxi mation assumed when deducing Theta from experimental data. (C) 1999 Elsevie r Science B.V. All rights reserved.