“…We carefully estimated the errors of both critical lines by trying the extrapolation to N → ∞ choosing various sets of system sizes among N = 16, 14, 12, 10 and 8 as shown in gapless transition, the critical points are difficult to determine. Following the procedure proposed by Nomura [3,6,7,8], the critical point is determined by the crossing point of the excitation energy of the lowest excitation ∆E 3 with M z = 4, P = 1, k = 0 and ∆E 0 with M z = 0, P = 1, k = 0 where k is the wave number of the excitation.…”