I used this original EPM2model to describe TraPPECO2 model
the two models are different in parameters. For TraPPE, there is no k(theta) term.
the TraPPE CO2 is from Stubbs, Drake-Wilhelm and Siepmann 'Partial Molar Volume and Solvation structure of Na in SCCO2', 2005
Fields inherited from class etomica.potential.P2CO2EMP