Editing ChargeCalculator:Theoretical background (section)

=Full EEM=

The classical EEM formalism estimates atomic charges via a set of coupled linear equations:

[[File:EEM equation.png]]

In order to solve this system of equations and calculate the atomic charges for all atoms, the following terms need to be known:
* distances between all pairs of atoms
* total molecular charge
* empirical parameters (here k,A,B) covering all atom types present in the molecule

ACC calculates the interatomic distances based on the atomic positions it reads from the molecular structure file. The user is required to provide the total charge, or ACC will assume the molecule is neutral (total molecular charge is 0). EEM parameters for each atom type (e.g., carbon, oxygen) present in the molecule are read from a set of EEM parameters suitable for the molecule in question. Many sets of EEM parameters have been published in literature, and are available in ACC as built-in sets. These sets may be used as they are, or with user modifications where necessary.

EEM parameters are generally developed based on reference QM calculations. A QM-based charge calculation approach is characterized by the setup of the wave function calculation (theory level, basis set, environment), as well as by the procedure used to partition the molecular electron density, or to deduce the electrostatic contribution of each atom. We refer to the sum of these characteristics as the "charge definition". 

The maximum accuracy and potential application of any set of EEM parameters is given by the charge definition used during its development. Performance is further influenced by the procedure used when fitting the EEM parameters to the reference data.