Noura Dawass

6.2. M ETHODS 6 117 Table 6.1: Chemical formulas and force fields of the components simulated in this work. Component Chemical formula Force field Monoethylene glycol HO(CH 2 ) 2 OH TraPPE–UA [199] Carbon dioxide CO 2 TraPPE–UA [212] Methane CH 4 TraPPE–UA [198] Hydrogen sulfide H 2 S TraPPE–UA [213] Hydrogen sulfide H 2 S Kristóf and Liszi [207] Nitrogen N 2 TraPPE–UA [212] other four–site model presented by Kristóf and Liszi [207] for H 2 S, here referred to as H 2 S-KL. The main differences between the two force fields are the non– bonded LJ parameters and the atomic charges. Table 6.1 lists the components simulated in this study and the force field used for each component. All force field parameters are listed in the Supporting Information of Ref. [171] . In our simulations, two types of intermolecular interactions are computed: LJ and Coulombic interactions. LJ interactions were truncated at 12 Å and the uncertainty due to truncation is handled by applying analytic tail correc- tions [22, 23] . The Lorentz–Berthelot mixing rules were used for LJ interactions between dissimilar interaction sites [22, 23] . The Ewald summation method was applied to handle electrostatic interactions with a relative precision of 10 − 6 . The real–space part of the electrostatic interactions was truncated at 12 Å. All simula- tions were carried out in the osmotic ensemble (see section 6.2.2) . The PC-SAFT equation of state was used to compute the fugacity of the solutes at the desired temperatures and pressures [214, 215] . 6.2.2. T HE CFCMC METHOD IN THE OSMOTIC ENSEMBLE The osmotic ensemble [22] is used to compute the solubility of small solute molecules in non-volatile solvents. In this open ensemble, the following param- eters are fixed: the temperature ( T ), the imposed hydrostatic pressure ( P ), the number of molecules of MEG ( N MEG ), and the fugacity of the solute ( f ). The number of molecules of the solute ( N s ), and the volume of the system ( V ) are varied to achieve equilibrium. The hydrostatic pressure P inside the simulation box corresponds to the imposed fugacity f of the reservoir. An essential part of the calculations is the insertion and/or deletion of solute molecules in the sim- ulation box. When studying dense solvents, as in the case of MEG, molecule insertions can be challenging [58, 60] . To improve the probability of accepting insertion/deletion trial moves, the CFCMC method was used [59, 60, 216, 217] . The osmotic ensemble was expanded using a so-called fractional molecule [59] .