Approximate fourth‐order perturbation theory of the electron correlation energy

Abstract

AbstractsAn approximate fourth-order expression for the electron correlation energy in the MgllerPlesset perturbation scheme is proposed. It takes into account all the contributions to the fourthorder energy neglecting only those of the triple-substituted determinants. It is size consistent and correct to fourth order for an assembly of isolated two-electron systems. Illustrative calculations are reported for a series of small molecules.On propose une expression approchie de quatritme ordre pour l'tnergie de correlation Blectronique dans le schema de perturbation de Mgller-Plesset. Dans ce schema toutes les contributions a I'tnergie d'ordre quatre sont indues, sauf celles des determinants triplernent substituts. Le schema est consistent par rapport a I'ttendue des systtmes et correct jusqu'au quatrikme ordre pour un

Thermochemical Kinetics for Multireference Systems: Addition Reactions of Ozone

“…By retaining only the one-body contributions to 〈Φ ijk abc |H j CCSD |Φ ijk abc 〉, we obtain CR-CC(2,3),B, and by replacing the one-body terms in 〈Φ ijk abc |H j CCSD |Φ ijk abc 〉 by the usual orbital energy differences for triples, that is, (ε a + ε b + ε c -ε i -ε j -ε k ), while neglecting other many-body terms in 〈Φ ijk abc |H j CCSD |Φ ijk abc 〉, we obtain the CR-CC(2,3),A method. As explained in ref 58, variants A and B of the CR-CC(2,3) approach are closely related to the triples parts of the CCSD(2) corrections developed by Hirata et al 47,140 (variant A) and HeadGordon et al [141][142][143][144] (variant B). In particular, the CR-CC(2,3),A method is equivalent to the CCSD(2) T approach of refs 47 and 150 when the canonical Hartree-Fock orbitals are employed.…”

Ab Initio Potential Energy Surface for H + OCS Reactions: Extended Basis Sets and Correlation Treatment

“…the structure with the largest S 2 value of 0.89 corresponds to structure [t] in Figure 1. The UMP2/6-31 l+G(2df,2p) structures were treated with higher level correlation techniques including full fouith-order (SDTQ) PUMP4 (Krishnan and Pople 1978;Krishnan, Frisch, and Pople 1980) and QCISD(T) (Pople, Head-Gordon, and Raghavachari 1987) to generate single-point energies for all poirts on the PES. These PUMP4 and QCISD(T) results will be called "refined energies."…”

An Improved Potential Energy Surface for the H2Cl System and Its Use for Calculations of Rate Coefficients and Kinetic Isotope Effects

“…These calculations employed the McLean-Chandler 109 basis set for Cl, combined with the 311G 110 basis set for H, augmented by a set of five d polarization functions for Cl (exponent ) 0.75) and a set of three p polarization functions for H (exponent ) 0.75). The first step in obtaining the energies was to use unrestricted fourth order many-body perturbation theory, within the unrestricted Møller-Plesset formulation developed by Pople and co-workers 111,112 and referred to as MP4. These MP4/MC-311G(d,p) energies were extrapolated to account for the remaining electron correlation energy using the MP4-SAC (scaling all correlation) extrapolation method 113…”