The full CCSDT model for molecular electronic structure

Abstract

Articles you may be interested inA new parametrizable model of molecular electronic structure J. Chem. Phys. 135, 134120 (2011); 10.1063/1.3646498 Equation-of-motion coupled cluster method with full inclusion of connected triple excitations for electronattached states: EA-EOM-CCSDTThe full coupled-cluster model (CCSDT) single, double, and triple excitation method defined by the wave function exp( T J + T2 + T 3 ) I <1>0) is formulated and computationally implemented for the first time. Explicit computational equations are presented. The method is applied to numerous examples including BH, FH, C 2 H 2 , CO, Ne, F-, and H 2 0 to assess its applicability to the correlation problem. Results from CCSDT agree with full CI, to an average error of less than 1 kcallmol even for difficult bond breaking examples. A series of results for various approximate, but computationally more efficient versions of the full CCSDT model are also studied and shown to give results in excellent agreement with CCSDT. Additional comparisons with fifth-order MBPT are reported.

Rotational spectra of CF+and13CF+: accurate rest frequencies and spectroscopic parameters

“…Most calculations that are reported here were performed at the coupled-cluster (CC) level of theory (Bartlett & Musiał 2007) employing the CC singles and doubles (CCSD) approximation (Purvis & Bartlett 1982) augmented by a perturbative treatment of triple excitations (CCSD(T)) (Raghavachari et al 1989). The full CC singles, doubles and triples (CCSDT) (Noga & Bartlett 1987;Watts & Bartlett 1993), the CC singles, doubles, triples and quadruples (CCSDTQ), as well as the singles, doubles, triples, quadruples, pentuples (CCSDTQP) (Kállay & Surján 2001) models were used in a few additional calculations.…”

Multireference coupled-cluster method using a single-reference formalism

“…Which can be written; (19) This can be done in all of the MRCC equations for all the duplicate amplitudes (or any number of constants preceding the same string of second quantized operators), thus reducing the multi-reference wavefunction to our truncated CCSDTQ wave operator acting on a single reference determinant. …”

Theoretical study on low-lying electronic states of Kr2+, Xe2+, and Rn2+

“…To obtain accurate theoretical results, triple excitations are usually important and the CCSDT approach 34 with a computational scaling of N 8 , where N is a measure of system size, is much too expensive. On the other hand, the CCSD(T) approach is nowadays the "gold standard" of quantum chemistry for ground state or the lowest-energy state of a certain symmetry with a dominant single reference character.…”