Volume 2 Chapter 2 2D-Constrain Sub-menu SIMIL

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TANI

Function

TANI is used to select the Tanimoto Coefficient for similarity searching.

Remarks

Similarity searching involves the comparison of an input connectivity record, referred to as the query, with the connectivity records of every entry in the database, the targets.

A quantitative measure of the similarity between the query and each of the targets can be calculated. This measure is known as a similarity coefficient. The procedure implemented within QUEST and QUEST3D is based on the work reported by Willett et al. (J.Chem.Inf.Comput.Sci. 26,36,1986).

The similarity coefficient is derived from the chemical connectivity bit screens (bits 249-682 in Appendix 1) of the query structure and each of the target structures.

A variety of similarity coefficients can be used to express the similarity between two bit-maps of equal length and a full discussion is presented by B.Everitt (Cluster Analysis; Halsted-Heinemann:London, 1980).

Two similarity coefficients are available in QUEST and QUEST3D:

(i)
the Tanimoto coefficient (also known as the Jaccard coefficient) - this is the default.
(ii)
the Dice coefficient

Suppose
Nq
= the number of bit screens set in the query structure
Nt
= the number of bit screens set in the target structure
Nc
= the number of bit screens which are common to both the query and target structures.

The Tanimoto coefficient is: T= Nc/(Nq+Nt-Nc)

The Dice coefficient is: D= 2Nc/(Nq+Nt)

Related Commands

DICE, SIMIL

See also Vol. 1, Chapter 6.

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Volume 2 Chapter 2 2D-Constrain Sub-menu TOTAL-COORD-NO.