32.7 Options for selection of pair classes

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 $(ij)$ to the minimum distance $R^{(ij)}$ between any atom in the standard orbital domains $[i]$ and any atom in the standard orbital domains $[j]$ . 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 $[i]$ and any atom contained in the standard domain $[j]$ 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.

USE_DIST

(default 1) If nonzero, use distance criteria, otherwise connectivity criteria.

CHGMIN_PAIRS

Only atoms in the primary domains are considered for the pair classification if the atomic Löwdin charge is larger than CHGMIN_PAIRS (default value 0.2). This criterion was introduced in order to reduce the dependence of the pair selection on localization tails.

RCLOSE

(default 1) Strong pairs are defined by $0 \le R^{(ij)} \lt {\tt RCLOSE}$. Close pairs are defined by ${\tt RCLOSE} \le R^{(ij)} \lt {\tt RWEAK}$.

RWEAK

(default 3) Weak pairs are defined by ${\tt RWEAK} \le R^{(ij)} \lt {\tt RDIST}$.

RDIST

(default 8) Distant pairs are defined by ${\tt RDIST} \le R^{(ij)} \lt {\tt RVDIST}$.

RVDIST

(default 15) Very distant pairs for which $R^{(ij)} \ge$RVDIST are neglected.

ICLOSE

(default 1) Strong pairs are separated by less that ICLOSE bonds. Close orbital pairs are separated by at least ICLOSE bonds but less than IWEAK bonds.

IWEAK

(default 2) Weak orbital pairs are separated by at least IWEAK bonds but less than IDIST bonds.

IDIST

(default 5) Distant orbital pairs are separated by at least IDIST bonds but less than IVDIST bonds.

IVDIST

(default 8) Very distant orbital pairs (neglected) are separated by at least IVDIST bonds.

KEEPCL

(default 0) If KEEPCL=1, the LMP2 amplitudes of close pairs are included in the computation of the strong pair LCCSD residuals. If KEEPCL=2 all close pairs are fully included in the LCCSD (this does not affect the triples list). This option is not yet implemented as efficiently as it could, and can therefore lead to a significant increase of the CPU time.

Setting a distance criterion to zero means that all pairs up to the corresponding class are treated as strong pairs. 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.