ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
Besides unavoidable discontinuities of the finite-order potential curves derivatives at the symmetry-breaking point, the UHH Möller–Plesset perturbation series show very slow convergence at intermediate and large interatomic distances. This bad behavior has two origins: (i) the MP denominators tend towards a constant value instead of vanishing so that each contribution is decreasing too rapidly with the interatomic distance; this remark condemns the use of MP zeroth-order Hamiltonian and forces to adopt an Epstein–Nesbet definition; (ii) the coupling between the singly and doubly excited determinants is very strong at intermediate distances, and the effect of the singly excited configurations, which only appears at the fourth order, slows down the convergence rate. These trends are both analytically demonstrated and numerically illustrated in the model minimal basis sets He++2 and N2 problems. A proposal to include the effect of singly excited determinants through a low-dimensional effective intermediate-type Hamiltonian has significantly improved the second-order results. By the way it is demonstrated that the perturbation from a singlet symmetry-broken HF solution (charge instability) necessarily diverges at intermediate distance.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.455170
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