Molecular Geometries at Sixth Order Møller−Plesset Perturbation Theory. At What Order Does MP Theory Give Exact Geometries?

Abstract

Sixth order Møller-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 e 6.

Parameterization of OPLS–AA force field for the conformational analysis of macrocyclic polyketides

“…To test if this overestimation leads to errors in conformational energies, low energy conformers of ethane, propane, isobutane, methanol, ethanol, 1‐propanol, 2‐propanol, EME, methoxymethanol, HOP, methyl formate, methyl acetate, and ethyl formate were reoptimized at the MP2/aug‐cc‐pVTZ level. It has been shown previously that this level of theory predicts bond lengths with an accuracy of 0.006 Å 45, 46. The optimization with the triple‐zeta basis set did not change conformational energies significantly.…”

High‐level ab initio methods for calculation of potential energy surfaces of van der Waals complexes

“…A large amount of attention has been spent on the prediction of properties in the limit of a CBS based on extrapolation procedures 21–26, 54, 58–80, including studies with particular emphasis on the prediction of equilibrium geometries 26, 62, 69, 76, 77, 80. Specifically, comparisons have been made between CBS extrapolations of geometrical parameters, that is, bond lengths, as observed from PESs calculated with and without counterpoise correction.…”

Vibrational Properties of the Isotopomers of the Water Dimer Derived from Experiment and Computations

“…For each basis set, the equilibrium geometry of the complex was determined so that via extrapolation of calculated geometry parameters a reliable CCSD(T)/CBS (complete basis set) limit geometry could be obtained. [44][45][46] In the same way, CCSD(T)/CBS values for D e and the 12 vibrational frequencies of the WD were determined. For both energy and geometry, we used a 3-point and a 2-point extrapolation procedure always including the CCSD(T)/ aug-cc-pV5Z results.…”