MONTE-CARLO VS MOLECULAR-DYNAMICS FOR CONFORMATIONAL SAMPLING

A comparison study has been carried out to test the relative efficienc y of Metropolis Monte Carlo and molecular dynamics simulations for con formational sampling. The test case that has been examined is liquid h exane. OPLS potential functions have been used with sampling of extern al motions and internal angle bending and torsions. The BOSS 3.6 and A MBER 4.0 programs were used to perform the internal-coordinate Monte C arlo and Cartesian molecular dynamics simulations, respectively, for s amples of 267 molecules in the NPT ensemble at 25 degrees C and 1 atm with periodic boundary conditions. The initial configurations of the s ystem were equilibrated with the-hexane molecules in the all trans con figuration. The major findings are (1) the Monte Carlo and molecular d ynamics results for thermodynamic properties and conformer populations are in accord, (2) the conformer populations reach equilibrium in ca. 10(7) Monte Carlo configurations or 100 ps of molecular dynamics, and (3) the molecular dynamics calculations require 1.6-3.8 times more co mputer time to achieve the same level of convergence as the Monte Carl o simulations. The viability of internal-coordinate Monte Carlo method ology contrasts with earlier notions on the inefficiency of Cartesian Monte Carlo methods. Monte Carlo sampling in related contexts such as for side-chain torsional motion in proteins appears promising.