There are two alternative modes for defining the pair classes: either distance criteria RCLOSE, RWEAK, RDIST, RVDIST can be used. These are in Bohr and refer for a given orbital pair to the minimum distance between any atom in the standard orbital domains and any atom in the standard orbital domains . Alternatively, the connectivity criteria ICLOSE, IWEAK, IDIST, IVDIST can be used. These refer to the minimum number of bonds between any atom contained in the standard domain and any atom contained in the standard domain The advantage of using connectivity criteria is the independence of the bond lengths, while the advantage of distance criteria (default) is that they are also effective in non-bonding situations. Only one of the two possibilities can be used, i.e., they are mutually exclusive. The use of distance criteria is the default. Using connectivity criteria for pair selection requires to set the option USE_DIST=0.
Setting RCLOSE or RWEAK to zero means that all pairs up to the corresponding class are treated as strong pairs (RWEAK=0 implies RCLOSE=0). For instance, RCLOSE=0 means that strong and close pairs are fully included in the LCCSD (in this case KEEPCL=1 has no effect). Note, however, that setting RCLOSE=0 increases the length of the triples list. Setting RDIST=0 means that all distant pairs are treated as weak pairs. This does not affect RWEAK and RCLOSE and has no effect unless multipole approximations are used for distant pairs. Setting RVDIST=0 means that no very distant pairs are neglected. Again, this has no effect on the other distance parameters.