Assessment of Orbital-Optimized, XXXX -Component Scaled Second-Order Many-Body Perturbation Theory for Thermochemistry and Kinetics.

@entity7959 @entity4885
An efficient implementation of the orbital-optimized second-order M  ller-Plesset perturbation theory (OO- @entity4885 ) within the resolution of the identity (RI) approximation is reported. Both conventional @entity4885 and @entity7959 -component scaled (SCS- @entity4885 ) variants are considered, and an extensive numerical investigation of the accuracy of these approaches is presented. This work is closely related to earlier work of Lochan, R. C.; Head-Gordon, M. J. Chem. Phys. 2007, 126. Orbital optimization is achieved by making the Hylleraas functional together with the energy of the reference determinant stationary with respect to variations of the double excitation amplitudes and the molecular orbital rotation parameters. A simple iterative scheme is proposed that usually leads to convergence within 5-15 iterations. The applicability of the method to larger molecules (up to    1000-2000 basis functions) is demonstrated. The numerical results show that OO-SCS- @entity4885 is a major improvement in electronically complicated situations, such as represented by radicals or by transition states where @entity7959 contamination often greatly deteriorates the quality of the conventional @entity4885 and SCS- @entity4885 methods. The OO-(SCS-) @entity4885 approach reduces the error by a factor of 3-5 relative to the standard (SCS-) @entity4885 . For closed-shell main group elements, no significant improvement in the accuracy relative to the already excellent SCS- @entity4885 method is observed. In addition, the problems of all @entity4885 variants with 3d transition-metal complexes are not solved by orbital optimization. The close relationship of the OO- @entity4885 method to the approximate second-order coupled cluster method (CC2) is pointed out. Both methods have comparable computational requirements. Thus, the OO- @entity4885 method emerges as a very useful tool for computational quantum chemistry.
