SCS-MP2 method

The scaled-opposite-spin MP2 (SOS-MP2) method omits the E l contribution entirely and takes E = 1.3 os [Y. Jung et al., /. Chem. Phys., 121, 9793 (2004) J. Comput. Chem., 28, 1953 (2007)]. The SOS-MP2 method gives results slightly less accurate than the SCS-MP2 method, but is faster and can be applied to larger molecules than SCS-MP2. Of course, the use of empirical parameters makes the SCS-MP2 and SOS-MP2 methods no longer strictly ab initio methods. 

SCS-MP2 and the new perturbative B2-PLYP density functional methods provide accurate reaction barriers and outperform MP2 and B3-LYP methods when applied to the 1,3-dipolar cycloaddition reactions of ethylene and acetylene.39 Phosphepine has been shown to catalyse the asymmetric 3 + 2-cycloaddition of allenes with a variety of enones (e.g. chalcones) to produce highly functionalized cyclopentenes with good enantiomeric excess.40 The AuPPh3SbF6 complex catalysed the intramolecular 3 + 2- cycloaddition of unactivated arenyne- (or enyne)-yne functionalities under ambient conditions.41 A review of the use of Rh(I)-catalysed 3 + 2-cycloadditions of diaryl-and arylalkyl-cyclopropenones and aryl-, heteroaryl-, and dialkyl-substituted alkynes to synthesise cyclopentadienones for use in the synthesis of natural products, polymers, dendrimers, and antigen-presenting scaffolds has been presented.42... 

Kozuch and Martin have proposed combining the double hybrid method with the SCS-MP2 treatment along with a dispersion correction. This so-called DSD-DFT method (dispersion corrected, spin-component-scaled double hybrid) has the exchange-correlation term... 

Just as one can use spin-component scaling to improve the performance of MP2, one can multiply the contribution of the same-spin terms to the CCSD energy by an empirical parameter and multiply the contribution of the opposite-spin terms by another parameter. This gives the SCS-CCSD method [T. Takatani et al., J. Chem. Phys., 128,124111 (2008)]. The same-spin and opposite-spin parameter values 1.13 and 1.27, respectively, were found by fitting a set of known reaction energies. SCS-CCSD performs quite well for intermo-lecular interactions. 

Ab initio HF calculations and geometry optimizations on moderate-size molecules have become routine, and computationally efficient methods (for example, DFT, MP2, and SCS-MP2) for inclusion of electron correlation are available. The degree of reliability of various quantum-mechanical methods and basis sets has been established by numerous calculations. The size of a molecule for which one can do an accurate ab initio or density-functional calculation is limited by the speed and storage capacity of the available electronic computers. As larger and faster computers are developed, it will become feasible to treat larger molecules. 

The MP2.5, MP2.X, SCS(MI)-CCSD, and SCS(MI)-MP2 methods (Section 16.8). An expanded discussion of calculation of NMR shielding constants and spin-spin coupling constants including linear scaling (Section 16.9). 

Table 5 presents benchmark, estimated CCSD(T) binding energies for several noncovalent complexes and the errors for the MP2, SCS-MP2, and SCSN-MP2 methods. Also included are local versions of the SCS-type methods (SCS-LMP2 and SCSN-LMP2). As already mentioned, in every case but the methane dimer, the MP2 method greatly overbinds in the CBS limit. The SCS-MP2 results are greatly improved over MP2 for all cases but the methane dimer, where the error increases from 0.04 to 0.27kcal mol. This remains a... 

The simpler CCSD and MP3 ab initio methods are much less reliable (but still computationally expensive) and should not be used. In the wave function world, MP2 can (with some faults) also be recommend because the cost/performance ratio is rather good. MP2 should not be used for unsaturated or more strongly correlated (metallic) cases for which SCS-MP2 is a reliable alternative. If the systems are not too large (<50 atoms), MP2.5 can be used as well. 

Hesseimann A, Korona T (2011) On the accuracy of DFT-SAPT, MP2, SCS-MP2, MP2C, and DFT plus Disp methods for the interaction energies of endohedral complexes of the Cgo fullerene with a rare gas atom. Phys Chem Chem Phys 13 732-743... 

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