  Single particles moving in a reflection-asymmetric potential are investigated
by solving the Schr\"{o}dinger equation of the reflection-asymmetric Nilsson
Hamiltonian with the imaginary time method in 3D lattice space and the harmonic
oscillator basis expansion method. In the 3D lattice calculation, the
$\bm{l}^2$ divergence problem is avoided by introducing a damping function, and
the$\langle \bm{l^2}\rangle_N$ term in the non-spherical case is calculated by
introducing an equivalent $N$-independent operator. The efficiency of these
numerical techniques is demonstrated by solving the spherical Nilsson
Hamiltonian in 3D lattice space. The evolution of the single-particle levels in
a reflection-asymmetric potential is obtained and discussed by the above two
numerical methods, and their consistency is shown in the obtained
single-particle energies with the differences smaller than
10$^{-4}~[\hbar\omega_0]$.
