Basic Elements of Molecular Similarity

It is worthwhile to introduce molecular similarity as a broader field before starting the discussion on molecular quantum similarity. Molecular similarity is a well-studied area and continues to be a major topic in modern chemical research. It is that area in which we look for methods to identify a degree of similarity between molecules in a dataset or in which we apply these 

Different ways exist to classify molecular descriptors. We will adopt the classification of Downs.Most common descriptor types are largely atom based. These descriptors are based on the individual atoms composing the molecule, but with their description extended to incorporate information about the environment of the atom. To introduce the radically different path taken in quantum similarity as opposed to the more classic procedures in most similarity studies, a small presentation of different types of molecular descriptors first will be given. 

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