A path is considered to reach a dissociation channel (DC) when the nearest interatomic distance between fragments becomes larger than a suitable value. Recognition of fragments can be made from an interatomic connection matrix composed of 1 for bonding or 0 for nonbonding which can be constructed from the interatomic distance matrix and the sum of covalent radii Rb defined for each atom. Values of Rb are taken from B. Cordero, et al., Dalton Trans. 2008, 0, 2832–2838, where an averaged value is adopted for atoms in which two or more values are available, and values for H and He are scaled 2.0 times. When a distance between an atom A and another atom B exceeds 0.1×n×2×(RbA+RbB), they are recognized to be nonbonding.
In uphill walking by the ADDF method, the default n value is 10 which can be changed by the UpDC=n option. In downhill walking by the IRC following or the EQ optimization, the default n value is 8 which can be changed by the DownDC=n option.
The default with different n values for up and down implies that an application of the ADDF method will find pathways leading to weak bonding clusters but they will be assigned to be DCs. Since 20 kJ/mol attractions are comparable with or larger than typical hydrogen-bonding ones, these default values are for calculations on neutral covalent bonding systems. One should adjust UpDC and DownDC values for other types of bonding systems, although larger values will increase computation demands to reach a DC.