Chemical Potential Perturbation: A Method to Predict Chemical Potential Using Molecular Simulations

• Main Author: Moore, Stan G.
• Format: Others
• Published: BYU ScholarsArchive 2012
• Subjects: surface tension; molecular dynamics; pressure tensor; inhomogeneous fluid; Chemical Engineering
• Online Access: https://scholarsarchive.byu.edu/etd/3248; https://scholarsarchive.byu.edu/cgi/viewcontent.cgi?article=4247&context=etd

A new method, called chemical potential perturbation (CPP), has been developed to predict the chemical potential as a function of composition in molecular simulations. The CPP method applies a spatially varying external potential to the simulation, causing the composition to depend upon position in the simulation cell. Following equilibration, the homogeneous chemical potential as a function of composition can be determined relative to some reference state after correcting for the effects of the inhomogeneity of the system. The CPP method allows one to predict chemical potential for a wide range of composition points using a single simulation and works for dense fluids where other prediction methods become inefficient. For pure-component systems, three different methods of approximating the inhomogeneous correction are compared. The first method uses the van der Waals density gradient theory, the second method uses the local pressure tensor, and the third method uses the Triezenberg-Zwanzig definition of surface tension. If desired, the binodal and spinodal densities of a two-phase fluid region can also be predicted by the new method. The CPP method is tested for pure-component systems using a Lennard-Jones (LJ) fluid at supercritical and subcritical conditions. The CPP method is also compared to Widom's method. In particular, the new method works well for dense fluids where Widom's method starts to fail.The CPP method is also extended to an Ewald lattice sum treatment of intermolecular potentials. When computing the inhomogeneous correction term, one can use the Irving-Kirkwood (IK) or Harasima (H) contours of distributing the pressure. We show that the chemical potential can be approximated with the CPP method using either contour, though with the lattice sum method the H contour has much greater computational efficiency. Results are shown for the LJ fluid and extended simple point-charge (SPC/E) water. We also show preliminary results for solid systems and for a new LJ lattice sum method, which is more efficient than a full lattice sum when the average density varies only in one direction. The CPP method is also extended to activity coefficient prediction of multi-component fluids. For multi-component systems, a separate external potential is applied to each species, and constant normal component pressure is maintained by adjusting the external field of one of the species. Preliminary results are presented for five different binary LJ mixtures. Results from the CPP method show the correct trend but some CPP results show a systematic bias, and we discuss a few possible ways to improve the method.