The electrostatics energy term comes in two versions, an inclusion list version (this version ele+) and an exclusion list version (see ele- next). In the inclusion list version, every atom pair that has an electrostatic interaction is explicitly listed. In the exclusion list version, every pair of atoms is assumed to have an electrostatics interaction unless explicitly excluded. The ele+ term is a good choice if there are few electrostatics interactions; using the ele- term would require a very long exclusion list in this case. The ele- term is a better choice if the majority of atom pairs have some kind of electrostatics interaction; using the ele+ term would require a very long inclusion list. 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.

The ele+ energy is the sum of N_ele terms where each term has the form:

Cij T[ rij ] / ( D rijn )

where C_ij is the product of the charges on atoms atom_i and atom_j, 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.

At any one time, the electrostatics term is evaluated only for a small subset of the ele+ list, from a candidate list. The candidates are pairs of atoms 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	Nele	D	rstart	rfinish	rcutoff
	atomi	atomj	Cij
	...	...	...

The record starts with a header line containing the keyword ele+ followed by a code f_type, the number of atom pairs N_ele, the dielectric constant D, the trimming start distance r_start, the trimming completion distance r_finish and the cutoff distance r_cutoff. Following this are N_ele lists, each list contains three numbers: the charged atom pairs atom_i and atom_j and the product of their charges C_ij. 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+   5   3   4.0   5.0   8.0   12.0

3   5    1.0
2   6    1.0
5   6   -2.0

Three charged pairs of atoms. f_type is 5 so the standard dielectric model (n = 1) will be used, and the electrostatics function will be trimmed with the shifting function. The shifting function starts at zero so the value of 5.0 angstroms specified for r_start is not used. r_finish is 8.0 and r_cutoff is 12.0 angstroms.