Molecular geometries at sixth order Moller-Plesset perturbation theory. At what order does MP theory give exact geometries?

Sixth order Moller-Plesset perturbation theory (MP6) in connection with correlation consistent basis sets cc-pVDZ, cc-pVTZ, and cc-pVQZ was used to calculate accurate equilibrium geometries for 14 molecules and to establish the complete basis set (CBS) limit for MP6 with an extrapolation method that is based on CBS limit geometries obtained at second order MP (MP2) and at fourth order MP (MP4) perturbation theory. MP6 equilibrium geometries are more accurate than MP2 or MP4 geometries provided a sufficiently large basis set is used. However, improvements in the geometry are small relative to MP4 geometries where in the latter case a cancellation of correlation and basis set errors may even lead in some cases to better results than for MP6. Analysis of correlation effects reveals that MP6 will be superior to other MP methods if a bond situation is described not involving more than six electrons (single, double, or triple bonds). As soon as there is the influence of additional electron pairs as for example in the case of multiple bonds involving heteroatoms with electron lone pairs, bond lengths are slightly exaggerated due to missing disconnected eight and ten electron correlation effects. This reflects the importance of infinite order effects as provided by couple cluster methods such as CCSD or CCSD(T), which are often superior to MPn methods with n less than or equal to 6.