Tests

In this section, we discuss results of two numerical tests. First, by switching off the Local Scaling Transformation (LST) of THO, we run HFBTHO in the axial HO basis and test it against HFODD. For a given Skyrme interaction and zero-range, density-dependent pairing force, both codes should give exactly the same results. Since technical details of the inner structure of both codes are completely different, such calculations constitute an extremely stringent test of both codes.

Second, by switching the LST on, we could test the code HFBTHO
against the spherical code HFBRAD [5]. Here, results of both codes
cannot be *exactly* identical, because the phase spaces in which the
solutions are obtained are significantly different.

Table 1 displays the results of test calculations performed for the SLy4 Skyrme interaction [6] and for the mixed zero-range pairing force [8]: = for =0.32fm. The cutoff energy of =60MeV was used for summing up contributions of the HFB quasiparticle states to density matrices [2]. For a given phase space, the strength of the pairing force was adjusted so as to reproduce the experimental neutron pairing gap in Sn. The resulting values are =285.88, 284.10, and 284.36MeVfm for the HO (THO) bases of 680 and 3276 states, and for the radial box of =30fm, respectively. The radial HFBRAD calculations were performed with 300 points (i.e., the =0.1fm grid spacing), and the wave functions were included up to =39/2. We checked that even with =33/2, all energies were stable within 1eV. The nucleon-mass and elementary-charge parameters were fixed at =20.73553MeVfm and =1.439978MeVfm, respectively.

Table 1 displays the following quantities: is the maximum number of the HO oscillator quanta included in the basis (for the deformed basis we give the numbers of quanta in the perpendicular () and axial () directions); is the number of the lowest deformed HO states included in the basis; and are the numbers of (doubly degenerate) neutron and proton quasiparticle states with equivalent single-particle energies [2] below the cutoff energy ; and are the oscillator constants in the perpendicular and axial directions; and are the neutron and proton Fermi energies, which, for vanishing pairing correlations, are taken as the s.p. energies of the last occupied states; and are the average pairing gaps [7]; and are the rms radii; and are the quadrupole moments ; and are the s.p. energies of the most bound neutron and proton states; and are sums of the canonical energies weighted by the corresponding occupation probabilities; and are the pairing energies; and are the kinetic energies; and are the energies corresponding to the central and spin-orbit parts of the Skyrme energy density functional; and are the direct and exchange parts of the Coulomb energy; and is the stability energy characterizing the level of self-consistency. In the code HFODD, is estimated from the sum of s.p. energies [9]; in the code HFBTHO is estimated from the maximum difference of all matrix elements of s.p. potentials calculated in two consecutive iterations; and in the code HFBRAD it is calculated as a variance of the total binding energy, , over the last five iterations.

Calculations for Pb yield a spherical solution with vanishing pairing gaps. HFBTHO and HFODD give the total binding energies that differ by 627eV, and this difference can be (primarily) traced back to the direct Coulomb energy. We have checked that without the Coulomb interaction, this difference decreases to 202eV. The axial-basis HFBTHO calculation gives a very small total quadrupole moment of 39b. This suggests that the THO basis generates a slight deviation from the spherical symmetry due to a different numerical treatment of - and -direction. In this respect, HFODD calculations should be considered more accurate.

Calculations for Er performed within a spherical HO basis, =, yield a well-deformed and weakly paired prolate ground state. Here, the total binding energies and quadrupole moments obtained within HFBTHO and HFODD differ only by 32eV and 5b, respectively. When the same calculation is performed in a deformed HO basis, , the differences grow to 5.1keV and 207b, respectively. Again, without the Coulomb interaction, the difference in the total binding energy is only 96eV. It is seen that by employing the deformed basis, the binding energy decreases, as expected.

Comparison with the coordinate-space code HFBRAD for Sn shows that in HFBTHO is correct up to 14keV for =25. However, the kinetic energy still differs by as much as 221keV, which is compensated by a similar difference in the interaction energy. Within the HO basis and =25, the corresponding differences are larger: 41 and 337keV. The analogous differences obtained for =20 are 142 and 1103keV (THO), and 152 and 964keV (HO), respectively. Nevertheless, the above comparison shows that the =20 calculations yield with a precision of a couple of hundred keV.