A configurational-bias approach for the simulation of inner sections of linear and cyclic molecules

Abstract

A novel Monte Carlo technique, the rebridging configurational-bias (RCB) method, has been developed to rearrange inner sections of chain molecules having strong intramolecular interactions along the backbone. The ability of sampling inner chain segments is important for the simulation of systems with low concentration of chain ends, such as polymers and molecules with cyclic structures. In the RCB method, inner segments are removed and then regrown site-by-site in a configurational-bias fashion. However, a short preliminary simulation is carried out to calculate weighting functions based on histograms of the separation distance between pairs of sites along the chain; these functions are used to bias the trial positions of growing inner sites so as to promote efficient chain closure. A look-ahead search scheme is employed for the sampling of the last two sites to increase the overall acceptance rate of the reconstruction process. The validity and performance of the new algorithm were tested by studying linear alkane systems of various chain lengths. Fast conformational equilibration was observed, from the level of local bond orientations to the level of the chain end-to-end vector orientations. Cyclic alkanes containing from 8 to 100 carbon atoms were also studied by using the RCB method. Our results for the conformational properties of cyclooctane are generally consistent with previous molecular dynamics (MD) results and with experimental data. The vapor–liquid coexistence curve of cyclooctane was also mapped out by using the RCB method in conjunction with a histogram reweighting technique for the analysis of isothermal–isobaric simulations.