Higher excitations in coupled-cluster theory

Abstract

Articles you may be interested inInclusion of selected higher excitations involving active orbitals in the state-specific multireference coupledcluster theory J. Chem. Phys. 133, 234110 (2010); 10.1063/1.3515478Second-and third-order triples and quadruples corrections to coupled-cluster singles and doubles in the ground and excited states Extended benchmark studies of coupled cluster theory through triple excitations The viability of treating higher excitations in coupled-cluster theory is discussed. An algorithm is presented for solving coupled-cluster ͑CC͒ equations which can handle any excitation. Our method combines the formalism of diagrammatic many-body perturbation theory and string-based configuration interaction ͑CI͒. CC equations are explicitly put down in terms of antisymmetrized diagrams and a general method is proposed for the factorization of the corresponding algebraic expressions. Contractions between cluster amplitudes and intermediates are evaluated by a string-based algorithm. In contrast to our previous developments ͓J. Chem. Phys. 113, 1359 ͑2000͔͒ the operation count of this new method scales roughly as the (2nϩ2)nd power of the basis set size where n is the highest excitation in the cluster operator. As a by-product we get a completely new CI formalism which is effective for solving both truncated and full CI problems. Generalization for approximate CC models as well as multireference cases is also discussed.

Quasi-variational coupled cluster theory

“…Again, OQVCCD does better than OAVCCD also, which performs no better than CCSD in this case. We have also compared CCSD and OQVCCD with QCCSD, for which results were obtained from Kállay's MRCC program, 67 in Table V. These results are as expected; QCCSD is more accurate than OQVCCD overall and does not collapse in a non-variational fashion as the bond is stretched.…”

Theoretical 57Fe Mössbauer spectroscopy: isomer shifts of [Fe]-hydrogenase intermediates

“…5,6 Mössbauer spectroscopy relies on the Mössbauer effect, which is the recoilless emission or absorption of γ radiation from a nucleus in a (solid) sample. A common source to create excited state iron nuclei is the radioactive 57 Co isotope, which decays by electron capture to the nuclear isomer 57m Fe. The emitted γ-ray from the relaxation of the spin I = 3/2 to the nuclear ground I = 1/2 state of iron is then used to probe the sample (absorber).…”

“…In older literature, the Mössbauer isomer shifts were often reported with respect to the source material in which 57 Co was embedded, whereas today it is more common to use a well-defined reference, which thereby takes the place of the source in the above expression. Typical examples of Mössbauer references are iron foil (α-Fe) and sodium nitroprusside Na 2 [Fe(NO)(CN) 5 ]·2 H 2 O.…”

Collisional properties of cold spin-polarized nitrogen gas: Theory, experiment, and prospects as a sympathetic coolant for trapped atoms and molecules

“…The inclusion of full connected triple excitations makes the ab initio calculations much more computationally demanding, and we employed a single correlation-consistent AVTZ basis set with an additional set of 3s3p2d2f 1g bond functions (BF) placed at the middle of the N 2 bond to reduce computational costs. To perform the full multireference CCSDT (MRCCSDT) calculations, we used the MRCC program [49] by Kállay et al [50] interfaced with the MOLPRO code [42]. The MRCCSDT calculation used a single-determinant RHF wave function as a reference.…”