elements of ˆ V$_{int}$ between states (6) have the following form 〈 Ψ IM ( n ′ τ ′ I ′ $_{1}$) I ′ 2 | ˆ V$_{int}$ | Ψ IM ( nτI 1 ) I 2 〉 = ( − 1) I 1 + I ′ $_{2}$+ $^{I}$V$_{0}$ β$_{0}$ √ 5 4 π (2 I$_{2}$ + 1)( I$_{2}$ 020 | $_{I}$′ $_{2}$0) $_{×}$  I ′ 2 I ′ 1 I I$_{1}$ I$_{2}$ 2   ( $_{n}$′$_{τ}$ ′$_{I}$ ′ 1 || ( d + + ˜ d ) || nτI$_{1}$ ) , (9) where the reduced matrix elements of the boson operators can be calculated using the boson fractional parentage coefficients Our numerical calculations have shown that with a good accuracy the ground-state band and the first excited negative parity band can be presented with a good accuracy as a superposition of two basis states of the form (6). For the ground state band the two level approximation yields the wave function of the form ( I can have only even values) where and for the energy we obtain In the last expressions Δ = 3 ℏ 2 µR 2 − ℏ ω . For the first excited negative parity bands we have Ψ n.p. I =sin[ γ$_{1}$ ( I )] [ | I − 1 2 I − 1 2 ( I − 1)) × Y$_{1}$ ] ( IM ) + cos[ γ$_{1}$ ( I )] [ | I + 1 2 I + 1 2 ( I + 1)) × Y$_{1}$ ] ( IM ) , (14) ϵ g.s. I = ℏ ω I 2 + Δ 2   1 − √ 1 + ( V$_{0}$β$_{0}$ √$_{2}$$_{π}$$_{Δ}$ ) 2 I   . (13) 2 sin[ γ$_{0}$ ( I )] = 1 √$_{2}$   $^{1 +}$ 1 √ 1 + ( V 0 β 0 √$_{2}$$_{π}$$_{Δ}$ ) 2 I )   $^{1}$$^{/}$$^{2}$ (12) Ψ g.s. I =sin[ γ$_{0}$ ( I )] [ | I 2 I 2 I ) × Y$_{0}$ ] ( IM ) − cos[ γ$_{0}$ ( I )] [ | I − 2 2 I − 2 2 ( I − 2)) × Y$_{2}$ ] ( IM ) , (11) [ d n − $^{1}$( α$_{1}$I$_{1}$ ) dI |} d $^{n}$αI ] = 1 √$_{n}$ 1 √$_{2}$$_{I}$ + 1 ( d $^{n}$αI || d $^{+}$| | d n − $^{1}$α$_{1}$ I$_{1}$ ) d n − $^{1}$( α$_{1}$I$_{1}$ ) dI |} d $^{n}$αI = ( − 1) I − I 1 1 √$_{n}$ 1 √$_{2}$$_{I}$ + 1 ( d n − $^{1}$α$_{1}$ I$_{1}$ || ˜ d || d $^{n}$αI ) . (10) B. Two-level solution 7