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ChargeCalculator:Theoretical background

75 bytes added, 00:10, 29 November 2014
EEM Cutoff
Thus, for a molecule with 10000 atoms and a cutoff radius of 10, instead of solving one matrix with 10000 x 10000 elements, ACC will solve 10000 matrices of much smaller size (approximately from 50 x 50 up to 400 x 400). The essence of the ''EEM Cutoff'' method is that, instead of a very large calculation, ACC will run many small calculations, each of them being less memory and time demanding than the original one. ''EEM Cutoff'' is therefore efficient only for large molecules, containing at least several thousands of atoms.
In other words, running ''EEM Cutoff'' is like running ''EEM'' for a set of overlapping fragments of the original molecule. A fragment is generated for each atom. The position and type of the atoms in each fragment are the same as in the original molecule. The only issue is the total charge of the fragment. ''EEM Cutoff'' assigns each fragment a quota of the total molecular charge proportional proportionally to its the number of atomsin the fragment, and irrespective of the nature of these atoms. After solving Then ACC solves the EEM equation for all fragments, the each fragment. The charge on each atom in the molecule is then computed as the sum of its charge contributions from each fragment. Further, each atomic charge is corrected in such a way that the sum of all atomic charges equals the total molecular charge. While this algorithm may not be chemically rigorous, it is has proven both robust and sufficiently accurate (RMSD less than 0.003e compared to the classical EEM) if the cutoff radius is relevant (over 8 angstrom). 
=EEM Cutoff Cover=
To further enhance the time and memory efficiency of EEM

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