Calculations at various coupled-cluster (CC) levels with and without the inclusion of linear r ijdependent terms are performed for the HF molecule in its ground state with a systematic variation of basis sets. The main emphasis is on spectroscopic properties such as the equilibrium distance r e and the harmonic vibration frequency x e . Especially with the R12 methods (including linear r ij -dependent terms), convergence to the basis set limit is reached. However, the results (at the basis set limit) are rather sensitive to the level of the treatment of electron correlation. The best results are found for the CCSDT1-R12 and CCSD[T]-R12 methods (CCSD[T] was previously called CCSD T(CCSD)), while CCSD(T) overestimates x e by %6 cm À1 . The good agreement of conventional CCSD(T) with experiment for basis sets far from saturation (e.g. truncated at gfunctions) is probably the result of a compensation of errors. The contribution of core-correlation is nonnegligible and must be included (eect on x e %5 cm À1 ). Relativistic eects are also important (2±3 cm À1 ), while adiabatic eects are much smaller (`1cm À1 ) and nonadiabatic eects on x e can be simulated in replacing nuclear by atomic masses; for rotation nuclear masses appear to be the better choice, at least for hydrides. From a potential curve based on calculations with the CCSDT1-R12 method with relativistic corrections, the IR spectrum is computed quantum-mechanically. Both the band heads and the rotational structures of the observed spectra are reproduced with a relative error of %10 À4 for the three isotopomers HF, DF, and TF.
mExtremal pair functions for an n-electron wave function of a closed-shell state are defined as linear combinations of spin-orbital-product pair functions that make some functionals (e.g., r& or r;;) extremal. They are related to the natural spin geminals in the uncorrelated limit and are useful both for an analysis of wave functions in view of an understanding of the chemical bond and for the treatment of electron correlation. Numerical examples are shown and discussed
mExtremal pair functions for an n-electron wave function of a closed-shell state are defined as linear combinations of spin-orbital-product pair functions that make some functionals (e.g., r& or r;;) extremal. They are related to the natural spin geminals in the uncorrelated limit and are useful both for an analysis of wave functions in view of an understanding of the chemical bond and for the treatment of electron correlation. Numerical examples are shown and discussed
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