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- Xiangzhu Li, Josef Paldus
- Physical chemistry chemical physics : PCCP
- 2009

We examine a spin-preserving stability of restricted Hartree-Fock (RHF) and open-shell RHF (ROHF) solutions for homonuclear diatomic species, namely the molecule C(2) and the N(2)(+) and F(2)(+)cations, in the entire relevant range of internuclear separations. In the presence of respective singlet, doublet, or triplet instabilities we explore the implied… (More)

Starting from the simplest possible building blocks—a ket, a bra, a time-reversed ket, and a time-reversed bra— a diagrammatic formalism is developed for angular momentum coupling problems. The formalism comprises Clebsch–Gordan coefficients as well as 3jm-symbols. The idea of constructing invariants (internal lines) by contracting contragredient pairs of… (More)

- Xiangzhu Li, Josef Paldus
- The Journal of chemical physics
- 2008

Relying on a 56-dimensional reference space and using up to the correlation-consistent, polarized, valence-quadruple-zeta (cc-pVQZ) basis sets, the reduced multireference (RMR) coupled-cluster method with singles and doubles (CCSD), as well as its perturbatively corrected version for secondary triples [RMR CCSD(T)], is employed to generate the full… (More)

- Xiangzhu Li, Josef Paldus
- The Journal of chemical physics
- 2008

We describe a fully size-extensive alternative of the reduced multireference (RMR) coupled-cluster (CC) method with singles (S) and doubles (D) that generates a subset of higher-than-pair cluster amplitudes, using linearized CC equations from the full CC chain, projected onto the corresponding higher-than-doubly excited configurations. This approach is… (More)

- Xiangzhu Li, Josef Paldus
- The Journal of chemical physics
- 2008

The partially linearized (pl), fully size-extensive multireference (MR) coupled-cluster (CC) method, fully accounting for singles (S) and doubles (D) and approximately for a subset of primary higher than doubles, referred to as plMR CCSD, as well as its plMR CCSD(T) version corrected for secondary triples, as described in Part I of this paper [X. Li and J.… (More)

The positions of the αand β-bands in the electronic absorption spectra of twenty aromatic benzenoid hydrocarbons were calculated by the semiempirical method of limited configuration interaction in the πelectron approximation using the Huckel molecular orbitals. The agreement of the experimental and calculated values is good for the β-band whereas a… (More)

- Xiangzhu Li, Josef Paldus
- The journal of physical chemistry. A
- 2010

A multireference (MR), general-model-space (GMS), state-universal (SU), coupled-cluster (CC) method that considers singly (S) and doubly (D) excited cluster amplitudes relative to the reference configurations spanning the model space (GMS SU CCSD) is employed to investigate a number of low-lying excited states of two five-membered, six pi -electron aromatic… (More)

- Xiangzhu Li, Josef Paldus
- The Journal of chemical physics
- 2009

The automerization of cyclobutadiene (CBD) is employed to test the performance of the reduced multireference (RMR) coupled-cluster (CC) method with singles and doubles (RMR CCSD) that employs a modest-size MR CISD wave function as an external source for the most important (primary) triples and quadruples in order to account for the nondynamic correlation… (More)

- Xiangzhu Li, Josef Paldus
- The Journal of chemical physics
- 2009

We explore spin-preserving, singlet stability of restricted Hartree-Fock (RHF) solutions for a number of closed-shell, homonuclear diatomics in the entire relevant range of internuclear separations. In the presence of such instabilities we explore the implied broken-symmetry (bs) solutions and check their stability. We also address the occurrence of… (More)

- Xiangzhu Li, Josef Paldus
- The Journal of chemical physics
- 2009

The instabilities of Hartree-Fock (HF) solutions at or near the equilibrium geometry of symmetric molecular species imply the existence of broken-symmetry solutions having a lower energy than the corresponding symmetry-adapted ones. Moreover, the distortion of the nuclear framework along the normal modes that are implied by such broken-symmetry solutions… (More)