Noura Dawass

6.2. M ETHODS 6 119 where i and j are the interaction sites. ² i j and σ i j are the LJ parameters. r i j is the distance between i and j . The parameters a , b , c and α are adjusted to achieve an efficient sampling of the λ space. For systems composed of MEG molecules, a number of scaling potentials were tested. Figure 6.1 shows the values of λ of an MEG fractional as a function of the number of MC cycles, for three scaling po- tentials. The commonly used (a–b–c) = 1–2–6 potential results in poor sampling of the λ –space. Figure 6.1 (a) shows that at certain periods, the values of λ are confined to a limited range. Changing the parameter b from 2 to 1 improves the sampling as demonstrated in Figure 6.1 (b). Figure 6.1 (c) shows that the 1–1– 48 potential with α = 0.0025 that was recommended earlier by Pham et al. [218] , results in the best sampling. 6.2.3. S IMULATION DETAILS Molecular simulations were performed using the recently developed open- source software package Brick-CFCMC [196] . The density of pure MEG was com- puted in the NPT ensemble at 1 bar, and at temperatures 333.15 K and 353.15 K. The solubility of CO 2 in MEG was computed at three temperatures, T = 333.15 K, 353.15 K, and 373.15 K. The solubilities of CH 4 , N 2 , and H 2 S in MEG were com- puted at T = 373.15 K. For all gases, solubilities were computed at pressures rang- ing from 1 to 10 bar, but in the case of N 2 pressures up to 100 bar were considered since N 2 has very low solubilities inMEG at low pressures. Simulation boxes were set up with 250 to 350 MEG molecules, depending on the number of absorbed solute molecules. Two fractional molecules were used: a fractional molecule to insert/remove solute molecules into the simulation box, and a MEG fractional molecule. The following MC trial moves were used: translations, rotations, and volume change trial moves. MC trial moves that attempt to change the values of λ were used for both fractional molecules. Simulations in the osmotic ensemble were carried out with the following probabilities for selecting trial moves: 25% translations, 25% rotations, 32% trial moves to change the internal configuration of MEG [196] , 1% volume changes, 17% trial moves to change λ divided equally between the solute and MEG fractional molecules. A minimum of 10 6 cycles was carried out for equilibration. At each MC cycle, the number of the trial moves performed equals the number of molecules of the system. During equilibration, an iterative scheme was used to obtain a weight func- tion W ( λ ) that results in a flat probability distribution of λ . For production runs, 10 6 cycles were carried out. To minimise the statistical error of the computed averages, a number of independent production simulations was performed at a specified T and P . The number of simulations performed was selected such that