The nuclear quadrupole moment of Ge-73 from molecular microwave data

Large basis set calculations of the electric field gradient at the Ge nucle us have been carried out for GeO and GeS with the purpose to determine the 'molecular' value of the quadrupole moment of the Ge-73 nucleus from the kn own quadrupole coupling constants. The electron correlation contribution to the electric field gradient has been determined at the level of the couple d cluster approximation with iterative determination of the effect of singl e and double substitutions followed by the perturbative estimate of the eff ect of triple excitations (CCSD(T)). Also the relativistic effects have bee n taken into account within the framework of the Douglas-Kroll approximatio n. The vibrationally corrected Douglas-Kroll CCSD(T) results for electric f ield gradients at Ge in GeO and GeS led to the 'molecular' value of the qua drupole moment of the Ge-73 nucleus equal to -0.196 b with the estimated er ror bars in the range of at most a few per cent. This result improves upon the currently used, highly inaccurate 'atomic' value (-0.173 +/- 0.026 b) o f the nuclear quadrupole moment of Ge and is suggested as a new recommended value to be used in nuclear physics.