Moller-Plesset perturbation theory geometries

Using MP2(full)/6-31+G geometries (second-order Moller-Plesset perturbation theory with core electrons included in the perturbation treatment). 

Second-order Moller-Plesset perturbation theory (MP2) is the computationally least expensive and most popular ab initio electron correlation method [4,15]. Except for transition metal compounds, MP2 equilibrium geometries are of comparable accuracy to DFT. However, MP2 captures long-range correlation effects (like dispersion) which are lacking in present-day density functionals. The computational cost of MP2 calculations is dominated by the integral transformation from the atomic orbital (AO) to the molecular orbital (MO) basis which scales as 0(N5) with the system size. This four-index transformation can be avoided by introduction of the RI integral approximation which requires just the transformation of three-index quantities and reduces the prefactor without significant loss in accuracy [36,37]. This makes RI-MP2 the most efficient alternative for small- to medium-sized molecular systems for which DFT fails. 

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

Figrire 8 An overview of quantum chemical methods for excited states. Bold-italic entries indicate methods that are currently applicable to large molecules. Important abbreviations used Cl (configuration Interaction), TD (time-dependent), CC (coupled-cluster), HF (Hartree-Fock), CAS (complete active space), RAS (restricted active space), MP (Moller-Plesset perturbation theory), S (singles excitation), SD (singles and doubles excitation), MR (multireference). Geometry optimizations of excited states for larger molecules are now possible with CIS, CASSCF, CC2, and TDDFT methods. 

Second-Order Moller-Plesset Perturbation Theory for the Calculation of Molecular Geometries and Harmonic Vibrational Frequencies. 

As mentioned above, Bing et al. assumed that the structures of the binary clusters were closely related to those of the pure ones. This issue has recently been addressed by Wielgus et al+ who considered Si ,Ge clusters with m + n = 5. They performed ab initio calculations with correlation effects added either at the coupled-cluster level or via Moller-Plesset perturbation theory. No assumptions were made on the structure, and due to the small size of the clusters, fairly extensive geometry searches could be carried through. 

The TDDFT excitation energies and oscillator strengths calculated for the lowest allowed excited states of MgP [138], ZnP [138, 140], and NiP [137, 138] complexes in their optimized D4h geometry, are reported in Table 15 and compared to CASPT2 [150] and Multireference Moller-Plesset perturbation (MRMP) theory [151] results. 

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