Collapse transition in polymer models with multiple monomers per site and multiple bonds per edge

We present results from extensive Monte Carlo simulations of polymer models where each lattice site can be visited by up to K monomers and no restriction is imposed on the number of bonds on each lattice edge. These multiple monomer per site (MMS) models are investigated on the square and cubic lattices, for K=2 and 3, by associating Boltzmann weights ω_{0}=1, ω_{1}=e^{β_{1}}, and ω_{2}=e^{β_{2}} to sites visited by 1, 2, and 3 monomers, respectively. Two versions of the MMS models are considered for which immediate reversals of the walks are allowed (RA) or forbidden (RF). In contrast to previous simulations of these models, we find the same thermodynamic behavior for both RA and RF versions. In three dimensions, the phase diagrams, in space β_{2}×β_{1}, are featured by coil and globule phases separated by a line of Θ points, as thoroughly demonstrated by the metric ν_{t}, crossover ϕ_{t}, and entropic γ_{t} exponents. The existence of the Θ lines is also confirmed by the second virial coefficient. This shows that no discontinuous collapse transition exists in these models, in contrast to previous claims based on a weak bimodality observed in some distributions, which indeed exists in a narrow region very close to the Θ line when β_{1}<0. Interestingly, in two dimensions, only a crossover is found between the coil and globule phases.