Structure of $^{13}$Be was investigated with antisymmetrized moleculardynamics. The variation after spin and parity projections was performed. Anunnatural parity $1/2^-$ state was suggested to be lower than $5/2^+$ stateindicating that vanishing of the N=8 magic number occurs in $^{13}$Be. Alow-lying $3/2^+$ state with a $2\hbar\omega$ configuration was also suggested.Developed cluster structures were found in the intruder states. Loweringmechanism of the intruder states was discussed in terms of molecular orbitalsaround a $2\alpha$ core.
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