m An extended geminal model has been applied to determine the interatomic potential for the X'S: state Be,. By adopting a [lls,9p,6d,4f,2g] contracted Gaussian-type basis, the following potential minimum parameters are obtained: Re = 4.67 a.u. (4.63 a.u.1 and D, = 3.70 mH (3.82 0.05 mH), experimental values in parentheses. A calculation with a nuclei-centered [Ss, 7p, 4d, 2 f, 181 GTO basis plus two sets of bond-type function, each set comprising diffuse (2 s, 2 p , 2 d, 2f, 1 g) GTOs, yielded -3.79 mH as the value of the potential at R = 4.63 a.u. On the basis of an error analysis the best theoretical estimate of the binding energy is determined to be 3.83 k 0.08 mH. The calculated value for the fundamental vibrational frequency is vo

An extended group function model has been applied to determine the three-body potential for the ground state of the helium trimer. The basis set for the calculations is an uncontracted (17s,6p,4d,3f) set of Gaussian type functions contracted to [8s,6p,4d,3f]. Three different types of configurations were considered: (i) equilateral triangles of side R, R∈{3.5,4.0,...,8.5,9.0} a.u., (ii) linear configurations with R12=R23, R12∈{3.5,4.0,...,8.5,9.0} a.u., and (iii) a set of pseudo-random configurations. For the equilateral triangular configurations and the linear configurations, and with equal distances larger than 5.5 a.u., the magnitude of the ratio between the three-body potential and the sum of the corresponding two-body potentials, is less than 0.003. The value of the three-body potential for the triangular and the linear configuration with equal distance R=5.669 289 a.u.=3 Å, is respectively 0.040 μH and −0.045 μH. In the long range region the calculated three-body potential is very close to the Axilrod–Teller triple-dipole energy.

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