Calculation of Transport Parameters Using ab initio and AMOEBA Polarizable Force Field Methods

Abstract

The transport properties of chemical species such as coefficients of diffusion, thermal conductivity, and viscosity have been widely used in combustion modeling. Lennard-Jones parameters fitted from the accurate intermolecular potential energy surfaces are crucial to obtain such information. Hence, a fast and accurate energy function is always desired for this purpose. In this study, the quality of a widely used polarizable force field AMOEBA was examined for the interaction between noble gases and nalkanes. First, the intermolecular energy was compared between AMOEBA, MP2/CBS, MP2/aug'-cc-pVDZ, and QCISD(T)/CBS. The root mean squared error of the original AMOEBA was 10.31 cm −1 against QCISD(T)/CBS for all conformations. This was comparable with the errors of 10.84 and 7.75 cm −1 for MP2/aug'-cc-pVDZ and MP2/CBS, respectively. Further optimizing the van der Waals parameters of noble gases, the error of the force field against QCISD(T)/CBS was reduced to 6.24 cm −1 , even better than the MP2/CBS results. Based on the optimized force field parameters, the intermolecular Lennard-Jones parameters were derived using the spherically averaged method and one-dimensional minimization method for a set of (n-alkanes, noble gases) pairs. The discrepancy of the one-dimensional minimization predicted Lennard-Jones collision rates from the tabulated values was typically within 10%, while it could be as large as 20−30% for the spherically averaged method. Additionally, the binary diffusion coefficients were calculated using the present Lennard-Jones parameters. In this case, the parameters derived from the spherically averaged method perform better. The mean unsigned error of the diffusion coefficients is usually within 5%, which is in good agreement with the experimental results. The results demonstrate that the AMOEBA force field can be used to generate the transport parameters systematically.

Transport Parameters for Combustion Species Based on cAMOEBA Polarizable Force Field

“…The collision diameter σ ij in eq 18 can be estimated according to the Lorentz-Berthelot combination rule, 41,42 = + ( )/2 ij ii jj (18) with the ε ij parameter determined by…”

Theoretical Investigations for Kinetics of the Chemical Reactions: H + SiClx (x = 1, 2, 3)

“…Previous studies have demonstrated that N = 2000 directions in the L-J parameters by the ODM method showed good convergence . Hence, N = 2000 was used in the present work.…”

“…Previous studies have demonstrated that N = 2000 directions in the L-J parameters by the ODM method showed good convergence. 36 Hence, N = 2000 was used in the present work. In addition, to make the points uniformly distributed on the sphere, the bath gas positions were derived using the Marsaglia method.…”

“…This method is convenient and cheap, but the accuracy of the L-J parameters needs to be improved. Subsequently, ab initio calculations of L-J parameters were suggested to be determined by using the spherically averaged method (SAM) . This method ignores the anisotropy of molecules and makes a spherical approximation to the intermolecular potential energy, which is not suitable for complicated molecular species.…”

“…Subsequently, ab initio calculations of L-J parameters were suggested to be determined by using the spherically averaged method (SAM). 36 This method ignores the anisotropy of molecules and makes a spherical approximation to the intermolecular potential energy, which is not suitable for complicated molecular species. Recently, Jasper and Miller 37 developed the one-dimensional minimization (ODM) method to obtain accurate L-J parameters.…”