An open-shell spin-restricted coupled cluster method: application to ionization potentials in nitrogen

Abstract

In order to circumvent the problem of spin contamination in unrestricted Hartree-Fock based coupled cluster (CC) calculations, we present a new method of calculation for certain classes of open-shell systems. The approach ensures that the proper spin component of the resulting correlated wave function is projected out in the energy evaluation by the use of a reference function constructed from suitably chosen restricted open-shell Hartree-Fock or other orbitals. This single-reference open-shell spin-restricted CC method is applied to the calculation of ionization potentials in the N2 molecule, and it is shown that highly accurate results can be obtained in a 5s4pld basis. The mean error for all the principal ionization potentials of N2 compared to experiment is 0.45%.

Remarkably High Stability of Late Actinide Dioxide Cations: Extending Chemistry to Pentavalent Berkelium and Californium

“…The openshell calculations were done with the R/UCCSD(T) approach where ar estricted open shell Hartree-Fock (ROHF) calculation was initially performed and the spin constraint was then relaxed in the coupled cluster calculation. [53,[61][62][63] Only the geometries were optimized at the CCSD(T) level. The CCSD(T) method with the chosen basis set combination was chosen as acompromise between reliable predictions and computational efficiency.A sn oted above, the available multi-reference calculations [23] show that the electronic state of the AnO 2 + is dominated by as ingle reference so the CCSD(T) method is an acceptable computational approach.…”

Influence of molecular symmetry on strong-field ionization: Studies on ethylene, benzene, fluorobenzene, and chlorofluorobenzene

“…For example, in N 2 the ordering of the orbitals obtained from a simple Hartree-Fock (HF) calculation is not consistent with the observed single-photon photoelectron spectrum: The calculated highest occupied molecular orbital (HOMO) of N 2 is a π u orbital while the HOMO inferred from experiments is a σ g orbital [25]. When including electron correlations in the calculations, the correct ordering and ionization energies are obtained [26]. Another example where the effects of electron-electron correlation are important is F 2 which is not even bound in a HF treatment.…”

Communication: Spin densities within a unitary group based spin-adapted open-shell coupled-cluster theory: Analytic evaluation of isotropic hyperfine-coupling constants for the combinatoric open-shell coupled-cluster scheme

“…The disadvantage of these CC schemes, however, is the spin contamination introduced into the wave function. 14 Whether this might lead to significant errors in the calculation of spin-dependent properties, e.g., isotropic hfcc's, is not known till date. This issue can only be investigated in a meaningful way by comparing spin-orbital based results with those obtained by using a spin-adapted CC theory.…”