To read more about the Distributed Positron Model (DPM) in nonlinear molecules (or to reference the work) please see the following:

1. F.A. Gianturco, T.L. Gibson, P. Nichols, R.R. Lucchese, and T. Nishimura, "Modelling Dynamical Correlation Forces In Low-Energy Positron Scattering From Polyatomic Gases: A Comparison for CH4," Radiat. Phys. Chem. 68, 673-680 (2003). Link to paper. Preprint: [pdf] (217 KB).
2. Robert R. Lucchese, F. A. Gianturco, P. Nichols, and Thomas L. Gibson, "A Test Calculation on SF6 of Model Potentials for Correlation and Polarization Effects in Positron Scattering from Molecules," Chapter 25 in New Directions in Antimatter Physics and Chemistry, edited by Clifford M. Surko and Franco A. Gianturco (Kluwer Academic, Dordrecht, 2001). Preprint: [gzipped ps] (494 KB); [zipped ps] (494 KB); [pdf] (700 KB).