Helium SAPT calculations

Moszynski et al. calculated the interaction-induced polarizability of the helium dimer for the internuclear separations 3computational approach relied on spin-adapted perturbation theory (SAPT) calculations with a large basis set of [5s4p3d2f] size. Subsequently, they determined the polarized and depolarized Raman spectrum of the dimer. The computed polarized spectrum displays fair agreement with experiment. Most important, the computed intensities of the depolarized spectrum agree quite well with the experimental data reported by Proffitt et al7 ... 

To determine the second virial coefficients to the required acciu acy of about 100 ppm, one needs to know the helium dimer potential to a few mK at the minimum. Consider first the interaction of two hehum atoms in the nonrelativistic Born-Oppenheimer (BO) approximation. The best published calculations using the supermolecular method have estimated error bars of 8 mK [ 150,151]. More recently, improved calculations of this type reached an accuracy of 5 mK [ 158]. Also, the SAPT calculations have been repeated with increased accuracy, resulting in an agreement to 5 mK with the supermolecular method. Furthermore, the upper bound from four-electron exphcitly correlated calculations is 5 mK above the new supermolecular value [159]. All this evidence from three different theoretical models seems to show convinc-... 

A symmetry-adapted perturbation theory approach for the calculation of the Hartree-Fock interaction energies has been proposed by Jeziorska et al.105 for the helium dimer, and generalized to the many-electron case in Ref. (106). The authors of Refs. (105-106) developed a basis-set independent perturbation scheme to solve the Hartree-Fock equations for the dimer, and analyzed the Hartree-Fock interaction energy in terms of contributions related to many-electron SAPT reviewed in Section 7. Specifically, they proposed to replace the Hartree-Fock equations for the... 

The SAPT potential for the He-C02 complex was also used in the calculations of the rovibrational spectra of the He -CC clusters 366. High resolution experimental data were also reported in this paper. Comparison of the theoretical and experimental effective rotational constants B and other spectroscopic characteristics as functions of the cluster size N is shown on Figure 1-9. Again, the agreement between the theory and experiment is impressive showing that theory can describe with trust spectroscopic characteristics of small clusters He -CO This especially true for the effective rotational constant and the frequency shift of the C02 vibration due to the solvation by the helium atoms. One may note in passing that the clusters HeA,-C02 with the number of helium atoms N around 20 do not exhibit all the properties of the C02 molecule in the first solvation shell of the (quantum) liquid helium at very low temperatures. 

Recently, the sapt approach has been applied (32) to compute the interaction-induced polarizability for the helium diatom. The computed polarizability invariants have been analytically fitted, and used in quantum-dynamical calculations of the binary collision-induced Raman spectra. The results of the dynamical calculations are summarized in Fig. 1. 

The many-body SAPT approach was used in Ref. [87] to compute the nonadditive interactions in the helium trimer. The computed points were fitted by a global potential function. The SAPT potential agrees well with accurate literature calculations at selected... 

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