Sidechain rotamer flips are observed both deliberately and incidentally by many simulation methods. Depending on the system and the magnitiude of torsional energy barriers, classical MD may sample sidechain rotamer flips and rearrangements. However, obtaining accurate populations requires simulating long enough to obtain many transitions of all relevant sidechain rotamers, which could in some cases be prohibitively long. Monte Carlo (MC) schemes can accelerate sampling across energy barriers by allowing hops over barriers (given suitable move proposals) rather than having barrier-crossing times be limited by normal kinetics. However, MC is not well suited for solvated or other crowded systems (eg. interior of proteins) as the overwhelming majority of proposed moves result in clashes and hence will be rejected. Thus for sidechains, while very small rotational moves may be accepted, larger moves are very likely to result in collisions with the surrounding atoms and be rejected.