I and III can be used as a basis for more accurate calculations of bound state properties in the future. Note that the expectation values of the two-particle cusps cannot be evaluated with the use of the KT-expansion, Eq.(\[gauss\]). Formally, for these values one always finds zero, if the variational KT-expansion is used in the cusp definitions, Eqs.(\[cusp\]) - (\[cusp3\]). Theoretically, these expectation values can be corrected, e.g., by using different representations of the wave functions at the short interparticle distances. The linear coefficients in such a ‘new’ representation of the wave function are determined from the known KT-expansions in some ‘intermediate’ areas. In reality, very often this procedure does not lead to the accurate cusp values. Furthermore, the answer substantially depends upon the size of this ‘intermediate’ area. Therefore, the comparison of the ‘expected’ and actual cusp values can be made only approximately, i.e. the original sense of the cusp condition as a simple (but effective) criterion for the wave functions is essentially lost.

To conclude this Section we note that there is another fundamental question about accurate calculations of bound state properties. In general, the physical meaning of each property is clear only for those few-body systems where 