Extension of the applicability of the closed-shell coupled-cluster ͑CC͒ theory with doubly ͑CCD͒ or singly and doubly ͑CCSD͒ excited clusters to quasidegenerate situations requires the inclusion of the connected triply and quadruply excited cluster components T 3 and T 4 . Since an explicit consideration of these clusters for larger systems is computationally very demanding, we explore the possibility of estimating the T 4 contribution using the projected unrestricted Hartree-Fock ͑PUHF͒ wave function. The resulting CCDQЈ and CCSDQЈ approaches are shown to correctly approximate the effect of the quadruply (QЈ) excited T 4 clusters, even when the PUHF wave function itself cannot serve as a good source of the lower-order pair-cluster components. It is important that the results of the cluster analysis of the PUHF solution are used directly and no further approximations are made. Only when both pair-cluster and T 4 cluster components are reasonably well approximated by the PUHF wave function, the T 4 cluster contributions cancel out certain CCSD diagrams, and good CC results can be obtained with the approximate coupled-pair approaches. It is also demonstrated that it may be more difficult to balance T 3 and T 4 clusters relying on simple perturbative approaches, such as CCSDQЈϩT͑CCSDQЈ͒. The results of formal considerations, including a thorough investigation of the cluster structure of the PUHF wave function, are illustrated on several examples. These include a few model systems composed of four and eight hydrogen atoms in various geometrical arrangements, as described using minimum and double zeta plus polarization basis sets, for which the exact full configuration interaction data are available. The orthogonally spin-adapted formulation of CC theory is used throughout. ͓S1050-2947͑96͒07407-0͔ PACS number͑s͒: 31.10.ϩz, 31.15.Dv, 31.15.Ar, 31.25.Ϫv
Context. Over the next few years, the ALMA and Herschel missions will perform high spatial and spectral resolution studies at infrared and sub-millimeter wavelengths. Modeling of molecular emission requires excitation calculations using radiative, as well as collisional rates, with the most abundant species. In the interstellar medium, the dominant collision partner is H 2 , but little data is available for collisions with H 2 . If data for collisions with He are available, it has often been proposed to use the more available rate coefficients for collision with He, with the appropriate reduced mass correction, as a first estimate of rate coefficients with H 2 ( j = 0). The validity of this approximation is not known. Aims. The present paper focuses on the calculation of rate coefficients among the first rotational levels of the SiS molecule in its ground vibrational state in collision with para-H 2 and compares these new data with recently published He ones to investigate the validity of using He rate coefficients to estimate H 2 ( j = 0) rate coefficients. Methods. A new potential energy surface for the SiS-para-H 2 system was obtained using highly correlated ab initio calculations. Dynamical calculations of pure rotational (de)excitation of SiS by para-H 2 were performed for the first rotational levels within the coupled-states approximation. Results. Collisional cross sections among the 51 first rotational levels of SiS were calculated for kinetic energies up to 2500 cm −1 . State-to-state rate coefficients are calculated for temperatures ranging from 5 K up to 300 K. A propensity rule that favors even ∆ j transitions is found and is explained by the near homonuclear symmetry of the SiS-para-H 2 potential energy surface. A detailed comparison with recent SiS-He rate coefficients is also presented. We demonstrate that collision with He is a reasonable model for collisions with para-H 2 , although this approximation must be used with caution.
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