Mpller-Plesset perturbation theory second-order

Specifies the calculation of electron correlation energy using the Mpller-Plesset second order perturbation theory (MP2). This option can only be applied to Single Point calculations. 

MP2 = Mpller-Plesset second-order perturbation theory. 

We have above discussed the CASSCF method and how we can choose the active space. We noted that this choice was closely connected to the method we use to compute the effects of dynamic correlation, in this case the CASPT2 method. The development of this approach was inspired by the success of the Mpller-Plesset second order perturbation theory (MP2), which has been used for a long time to treat electron correlation for ground states, where the reference function is a single determinant. It was assumed that such an approach would be even more effective with the more accurate CASSCF reference function. A first attempt was made soon... 

To see the overall picture of the benchmark test, the mean absolute deviations A are given for several methods and basis sets in Table II. These methods include the previously mentioned ACPF and CCSD methods, but also the MCPF (modified coupled pair functional) method [11], the MP2 (Mpller-Plesset second-order perturbation theory) method and the CCSD(T) method [12], where a perturbational estimate of the triple excitations has been added. The basis sets include the DZP basis discussed above and the nearly equivalent VDZ basis set, a DZ basis set... 

The ab initio HF calculations reported below have been performed with the GAUSSIAN 76 [26] program package. The atomic basis sets applied are a minimal (STO-3G [26]) one, a split valence (6-31G [26]) one, a split-valence one plus a set of five d-functions on carbon (6-31G [26]), and one with an additional set of p-functions on hydrogen (6-31G [26]). The correlation energy has been computed using Mpller-Plesset many body perturbation theory of second order (MP2) [27], the linear approximation of Coupled Cluster Doubles theory (L-CCD)... 

MC approaches [30] involve the optimization of molecular orbitals within a restricted subspace of electronic occupations provided such active space is appropriately chosen, they allow for an accurate description of static electron correlation effects. Dynamical correlation effects can also be introduced either at the perturbation theory level [complete active space with second-order perturbation theory (CASPT2), and multireference Mpller-Plesset (MR-MP2) methods] [31] or via configuration interaction (MR-CI). 

Carbenes A Testing Ground for Electronic Structure Methods Complete Active Space Self-consistent Field (CASSCF) Second-order Perturbation Theory (CASPT2) Configuration Interaction Mpller-Plesset Perturbation Theory Natural Orbitals Transition State Theory Unimolecu-lar Reaction Dynamics. 

Most of the theory in the present chapter is concerned with perturbational corrections to states that are dominated by a single electronic configuration, usually represented by a Hartree-Fock wave function. However, in Section 14.7, we consider multiconfigurational generalizations of Mpller-Plesset perturbation theory, in particular the second-order perturbation theory developed within the framework of CASSCF theory. 

Perturbative approximation methods are usually based on the Mpller-Plesset (MP) perturbation theory for correcting the HF wavefunction. Energetic corrections may be calculated to second (MP2), third (MP3), or higher order. As usual, the open- versus closed-shell character of the wavefunction can be specified by an appropriate prefix, such as ROMP2 or UMP2 for restricted open-shell or unrestricted MP2, respectively. 

H. J. Werner, F. R. Manby, and P. J. Knowles, Fast linear scaling second order Mpller Plesset perturbation theory (MP2) using local and density fitting approximations. J. Chem. Phys. 118, 8149 8160 (2003). 

There is also a hierarchy of electron correlation procedures. The Hartree-Fock (HF) approximation neglects correlation of electrons with antiparallel spins. Increasing levels of accuracy of electron correlation treatment are achieved by Mpller-Plesset perturbation theory truncated at the second (MP2), third (MP3), or fourth (MP4) order. Further inclusion of electron correlation is achieved by methods such as quadratic configuration interaction with single, double, and (perturbatively calculated) triple excitations [QCISD(T)], and by the analogous coupled cluster theory [CCSD(T)] [8],... 

Prior to stretching C-S bond, we optimized the geometry of the anionic Me-S-Me molecule and parent neutral molecule at the unrestricted second-order Mpller-Plesset (UMP2) perturbation level of theory (in order to take into account the effect of electron correlation) with aug-cc-pVDZ basis sets [9]. We also... 

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