F opt

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The f_opt keyword determines whether a "self-consistent sigma" calculation is performed to optimize the effective translational hard-sphere diameters of the different components (f_opt = .true., which is also the default) as presented in Ref. [1], or instead the simpler treatment of Lai et al.[2], which follows the monocomponent formalism, is used (f_opt = .false.). For the effective rotational hard-sphere diameters, see f_rot_opt.

Monocomponent hard-sphere diameter

In the original 2PT paper by Lin et al.[3], the effective hard-sphere diameter of a monocomponent system (all molecules/groups are the same) is not explicitly calculated. Instead, the fluidicity is uniquely defined from the thermodynamic state of the system, $(N,V,T)$, and the (translational) zero-frequency density of states, $S_\text{trn}(0)$.

References

1. M.A. Caro, T. Laurila, and O. Lopez-Acevedo. Accurate schemes for calculation of thermodynamic properties of liquid mixtures from molecular dynamics simulations. J. Chem. Phys. 145, 244504 (2016).
2. P.-K. Lai, C.-M. Hsieh and S.-T. Lin. Rapid determination of entropy and free energy of mixtures from molecular dynamics simulations with the two-phase thermodynamic model. Phys. Chem. Chem. Phys. 14, 15206 (2012).
3. S.-T. Lin, M. Blanco and W.A. Goddard III. The two-phase model for calculating thermodynamic properties of liquids from molecular dynamics: Validation for the phase diagram of Lennard-Jones fluids. J. Chem. Phys. 119, 11792 (2003).