The GEM Functional Form

The idea for GEM is to employ the fitted Hermite Gaussians to evaluate each term in 

The exchange-repulsion term is calculated by means of the charge density overlap following the Wheatley-Price overlap model (Wheatley and Price, 1990 Domene et al., 2001)  

The polarization term is approximated by the use of dipole polarizabilities, which yield a very good results for the polarization energies (if the electric fields are not large) (Bottcher, 1993). To this end, the electric fields are calculated with the fitted densities and interacted with distributed dipolar polarizabilities with Garmer and Steven s approach (Day etal., 1996)  

Finally, the charge transfer term is evaluated using the semiem-pirical formalism implemented in the SIBFA force field (Gresh et al., 1979 Piquemal etal., 2007)  

In our initial implementations of GEM-0 and GEM we have not introduced an explicit term for the dispersion interactions. This is because these force fields have been originally parametrized using the CSOV method at the DFT level, which, by definition, does not include a dispersion contribution. However, it is possible to include this term in a similar way to the SIBFA potential (Gresh et al., 1979 Piquemal et al., 2007). 

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