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A popular choice are Hylleraas type
functions [106,107,108,109], which for J=0 states read

(34) 
and are expressed in the interparticle coordinates, where
r_{xy} denotes the distance between the particle x and y. The
exponential parameters
are often
taken to be the same for all n to reduce the number of parameters to be
optimized. For loosely bound states (1,1) of
and ,
similar functions with
k_{i}=l_{i}=m_{i}, known as Slater gemials were
found to be more useful in representing the large physical size of the
states [110,84]. The disadvantage with Slater gemials is that
one has to optimize a large number of exponential parameters, which is very
time consuming. Another disadvantage is its ``linear dependencies''
problem. This is due to the fact that the basis in this set is nearly
linearly dependent,
i.e. rather nonorthogonal. Since the functions differ only by their
exponents, an optimized basis set sometimes has several functions with
close values of the exponents [85]. Thus, the use of extended
arithmetic precision (
decimal digits) is necessary.
Next: Coupled rearrangement channel method
Up: Variational approaches
Previous: Variational approaches