The electrostatics energy term is available in two versions, an exclusion list version (this version ele-) and an inclusion list version (see ele+ above). In the exclusion list version, every pair of atoms is assumed to have an electrostatics interaction unless explicitly excluded. In the inclusion list version, every atom pair that has an electrostatic interaction is explicitly listed. The ele- term is a good choice if the majority of atom pairs have some kind of electrostatics interaction; using the ele+ term would require a very long inclusion list. The ele+ term is a better choice if there are few electrostatics interactions; using the ele- term would require a very long exclusion list in this case.Only one version or the other should be used in a descriptor; it is unlikely to be be useful (but it is not impossible) to use both versions in a single descriptor.

For a system of N_atom atoms, there are a maximum of N_atom (N_atom-1)/2 pairs of atoms for which electrostatics interactions are to be calculated. A large number of these can be eliminated from consideration as they involve uncharged atoms. Pairs of atoms that are much further apart than r_finish can also be eliminated at least while they remain beyond a cutoff distance r_cutoff. The ele- energy is the sum of terms where each term has the form:

Ci Cj T[ rij ] / ( D rijn )

where C_i is the charge on atom atom_i, r_ij is the distance between atom_i and atom_j, D the dielectric constant, n a power which is either one or two depending on the dielectric model and T[r] is a trimming function. The trimming function may be a simple truncation, the shifting or the switching function; these functions are triggered either at zero or r_start, and byr_finish the trimming function would have completely transformed the original function to a zero value. Trimming Function Details [this will open in a separate window]. The electrostatics term need to be evaluated only up to this limit. This is an approximation and in many cases it is a poor approximation but nevertheless it is necessary to reduce the computational burden.

The electrostatics term is evaluated only for a small subset of potential interactions, from a candidate list. The candidates are pairs of atoms, that are not explicitly excluded, that are within a distance r_cutoff which is a distance larger than r_finish. The candidate list is updated at regular intervals. In between updates, some of the pairs in the candidate list move apart and some of the pairs that are not in the candidate list move closer together. The difference between r_finish and r_cutoff provides a buffer distance and the user has to choose the difference and the candidate list update interval to minimize the difference and to prolong the update interval. These are mutually incompatible goals; a small difference between r_finish and r_cutoff reduces the number of computations but requires more frequent candidate list updates which increases the number of computations. The candidate list must be updated frequently enough so that most of the atom pairs will actually contribute to the electrostatics energy and none of the atom pairs that are not in the candidate list are within a distance r_finish.

The ele- record in the descriptor has the form:

ele-	ftype	Nexclude	D	rstart	rfinish	rcutoff
	Ci
	...
	atomi	atomj
	...	...

The record starts with a header line containing the keyword ele- followed by a code f_type, the number of excluded atom pairs N_exclude, the dielectric constant D, the trimming start distance r_start, the trimming completion distance r_finish and the cutoff distance r_cutoff. Immediately following are N_atom atomic charges C_i.

The record is completed by a listing of N_exclude pairs of atoms atom_i and atom_j. The exclusion list follow a strictly increasing order. This means that atom_j must be larger (not equal and not smaller) than atom_i and each atom_i must be larger than the previous atom_i.

The value of the code f_type specifies the power n and the trimming function according to the following table:

ftype	1	2	3	4	5	6
n	1	2	1	2	1	2
Trimming	Truncation	Truncation	Switching	Switching	Shifting	Shifting

Example:

ele-   6   6   2.0   8.0   12.0   15.0

0.0
1.0
0.0
-2.0
1.0
 
1   2
1   3
1   4
1   5
3   4
3   5

---

An example for a system with five atoms. f_type is 6 so the distance dielectric model will be used and the Shifting function will be applied to trim the electrostatics function so that it terminates at a distance of 12.0Å. The cutoff distance is set to 15.0Å The dielectric constant is 2.0. Following the header line are the five atomic charges, atoms 1 and 3 are uncharged. Following the atomic charge listing are the six excluded pairs. These eliminate all pairs involving the uncharged atoms.