The accurate determination of molecular equilibrium structures

Equilibrium structures have been determined for 19 molecules using least-sq uares fits involving rotational constants from experiment and vibrational c orrections from high-level electronic-structure calculations. Equilibrium s tructures obtained by this procedure have a uniformly high quality. Indeed, the accuracy of the results reported here likely surpasses that reported i n most experimental determinations. In addition, the accuracy of equilibriu m structures obtained by energy minimization has been calibrated for the fo llowing standard models of ab initio theory: Hartree-Fock, MP2, CCSD, and C CSD(T). In accordance with previous observations, CCSD(T) is significantly more accurate than the other models; the mean and maximum absolute errors f or bond distances of the 19 molecules are 0.09 and 0.59 pm, respectively, i n CCSD(T)/cc-pCVQZ calculations. The maximum error is obtained for R-OO in H2O2 and is so large compared with the mean absolute error that an experime ntal reinvestigation of this molecule is warranted. (C) 2001 American Insti tute of Physics.