Coarse Grained Molecular Mechanics (MM)/Molecular Dynamics (MD) Force Field for Petroleum Chemistry: I. Coarse Grained Model for Alkanes in Petroleum

The general coarse grained (CG) model for alkanes in petroleum was described in this paper. According to the structural characteristic of alkanes in petroleum, seven coarse grained beads were mapped from A1 to A7. Almost all of alkane molecules in petroleum can be coarse-grained by the combination of these seven beads which contain 3 to 6 carbon atoms. The seven coarse grained building blocks individually correspond to normal alkanes and isoparaffins. In order to obtain precise force field parameters, density functional theory (DFT) was employed to optimize the molecular structure corresponding to the single coarse grained bead. The non-bonded Lennard-Jones potential parameters were calculated by application of modified Flory-Huggins method. And the bonded potential parameters were obtained by all atom molecular dynamics (AA-MD) simulation. In order to test the accuracy of CUP 1.0 coarse molecular mechanism/molecular dynamics force field (CUP 1.0 CG-MM/MD) which is established on the basis of non-bonded and bonded potential parameters of seven alkane coarse grained beads, density of thirty seven model alkane compounds from C-3 to C-40 in petroleum was computed through coarse grained molecular dynamics (CG-MD) in NPT ensemble. The CG-MD simulation results demonstrated that none of the simulating error was beyond 1.0% than that of experimental data, which is more accurate than that calculated by MARTINI force field which is specific for biomolecules in water. In addition, the solubility between propane (A1) and neohexane (A7) was simulated by using CG molecular dynamics, which are the most distinctive in non-bond interaction strength among seven coarse grained beads. The simulating result showed that coarse grained alkane beads of A1 and A7 are completely soluble, which demonstrated the rationality of CUP 1.0 force field.