  We investigate aggregation mechanism of two proteins in a thermodynamically
unambiguous manner by considering the finite size effect of free energy
landscape of HP lattice protein model. Multi-Self-Overlap-Ensemble Monte Carlo
method is used for numerical calculations. We find that a dimer can be formed
spontaneously as a thermodynamically stable state when the system is small
enough. It implies the possibility that the aggregation of proteins in a cell
is triggered when they are confined in a small region by, for example, being
surrounded by other macromolecules.We also find that the dimer exhibits a
transition between unstable state and metastable state in the infinite system.
