caa a22 4500 465818900 CHVBK 20180323112130.0 cr unu---uuuuu 170327e19900301xx s 000 0 eng 10.1007/BF01119191 doi (NATIONALLICENCE)springer-10.1007/BF01119191 Coupled cluster approaches with an approximate account of triexcitations and the optimized inner projection technique [Elektronische Daten] I. General orthogonally spin-adapted formalism [Piotr Piecuch, Josef Paldus] Summary: A general orthogonally spin-adapted formalism for coupled cluster (CC) approaches, with an approximate account of triexcited configurations, and for optimized inner projection (OIP) technique is described. Modifying the linear part of the CC equations for pair clusters (CCD) we obtain the orthogonally spin-adapted, non-iterative version of the CCDT-1 method of Bartlett et al. [J. Chem. Phys. 80, 4371 (1984), 81, 5906 (1984), 82, 5761 (1985)]. Similar modification of an approximate coupled pair theory corrected for connected quadruply excited clusters (ACPQ) yields a new approach called ACPTQ. Both the CCDT-1 and ACPTQ methods can be formulated in terms of effective interaction matrix elements between the orthogonally spin-adapted biexcited singlet configurations. The same matrix elements also appear in the orthogonally spin-adapted form of the CCD + T(CCD) perturbative estimate of triply excited contributions due to Raghavachari [J. Chem. Phys. 82, 4607 (1985)] and Urban et al. [J. Chem. Phys. 83, 4041 (1985)], and in the OIP method when applied to the Pariser-Parr-Pople (PPP) model Hamiltonians. We use the diagrammatic approach based on the graphical methods of spin algebras to derive the explicit form of these interaction matrix elements. Finally, the relationship between different diagrammatic spin-adaptation procedures and their relative advantages are discussed in detail. Springer-Verlag, 1990 Many-electron correlation problem nationallicence Spin-adaptation nationallicence Coupled cluster approach nationallicence Optimized inner projection nationallicence Graphical methods of spin algebras nationallicence Piecuch Piotr Department of Applied Mathematics, University of Waterloo, N2L 3G1, Waterloo, Ontario, Canada aut Paldus Josef Department of Applied Mathematics, University of Waterloo, N2L 3G1, Waterloo, Ontario, Canada aut Theoretica chimica acta Springer-Verlag 78/2(1990-03-01), 65-128 0040-5744 78:2<65 1990 78 214 https://doi.org/10.1007/BF01119191 text/html Onlinezugriff via DOI 1 research-article jats NATIONALLICENCE 856 40 https://doi.org/10.1007/BF01119191 text/html Onlinezugriff via DOI NATIONALLICENCE 700 1- Piecuch Piotr Department of Applied Mathematics, University of Waterloo, N2L 3G1, Waterloo, Ontario, Canada aut NATIONALLICENCE 700 1- Paldus Josef Department of Applied Mathematics, University of Waterloo, N2L 3G1, Waterloo, Ontario, Canada aut NATIONALLICENCE 773 0- Theoretica chimica acta Springer-Verlag 78/2(1990-03-01), 65-128 0040-5744 78:2<65 1990 78 214 Metadata rights reserved Springer special CC-BY-NC licence nationallicence BK010053 XK010053 XK010000 NATIONALLICENCE NATIONALLICENCE NL-springer