We develop a generalized pseudofermion functional renormalization group
(PFFRG) approach that can be applied to arbitrary Heisenberg models with spins
ranging from the quantum case S=1/2 to the classical limit S→∞. Within this
framework, spins of magnitude S are realized by implementing M=2S copies of
spin-1/2 degrees of freedom on each lattice site. We confirm that even without
explicitly projecting onto the highest spin sector of the Hilbert space,
ground states tend to select the largest possible local spin magnitude. This
justifies the average treatment of the pseudofermion constraint in previous
spin-1/2 PFFRG studies. We apply this method to the antiferromagnetic J1−J2
honeycomb Heisenberg model with nearest-neighbor J1>0 and second-neighbor J2>0
interactions. Mapping out the phase diagram in the J2/J1−S plane, we find that
upon increasing S, quantum fluctuations are rapidly decreasing. In particular,
already at S=1 we find no indication for a magnetically disordered phase. In
the limit S→∞, the known phase diagram of the classical system is exactly
reproduced. More generally, we prove that for S→∞ the PFFRG approach is
identical to the Luttinger-Tisza method
