Exact geometries from quantum chemical calculations

For seventeen molecules, complete basis set (CBS) geometries are obtained for Moller-Plesset perturbation methods at second (MP2), fourth (MP4), and sixth order (MP6) as well as for the Coupled Cluster methods CCD, CCSD, and CCSD(T). The correlation consistent basis sets cc-pVDZ, cc-pVTZ, and cc-pVQZ were systematically applied and calculated geometries extrapolated to the limit of an infinitely large basis set. MP6 equilibrium geometries are more accurate than MP2 or MP4 geometries at the CBS Limit and provide AH bond lengths with an accuracy of 0.001 Angstrom. However, AB bonds are always predicted too long because of the lack of sufficient coupling effects between p-electron correlation at MP6. CCSD(T) provides reasonable AB bond lengths although these are in general too short by 0.003 Angstrom. Due to error cancellation very accurate geometries are obtained at the CCSD(T)/cc-pVTZ and CCSD(T)/cc-pVQZ level of theory. With the help of the accurate equilibrium geometries obtained in this work, several experimentally based geometries could be corrected. The effects of HF-optimized basis sets, diffuse functions or the frozen core approximation on geometry optimizations are discussed. It is emphasized that the use of the cc-pVDZ or any other VDZ + P basis set should be avoided in correlation corrected ab initio calculations. (C) 2001 Elsevier Science B.V. All rights reserved.