26.2 Getting started

The local correlation treatment is switched on by preceding the command name by an L, i.e., by using the LMP2, LMP3, LMP4, LQCI[[SD](T)], LCCSD[(t)], or LCISD directives. Further options can be given on the same input card or on subsequent LOCAL or MULTP cards. Alternatively, one can also give the LOCAL or the MULTP directive after the normal MP2, MP3, MP4, QCI[[SD](T)], CCSD[(T)], or CISD directives.

Thus, the two input cards

METHOD;
LOCAL,[key1=value],[key2=value2], $\ldots$

are equivalent to

LMETHOD,[key1=value],[key2=value2], $\ldots$

where METHOD is one of MP2, MP3, MP4, QCI[[SD](T)], CCSD[(T)], or CISD.

Similarly,
METHOD;
MULTP,[key1=value],[key2=value2], $\ldots$

is equivalent to

LMETHOD,MULTP,[key1=value],[key2=value2], $\ldots$

(The full set of options is described in section 22.5, and summarized in Table 9.) The LOCAL and MULTP directives only differ in the defaults that they assume for the input keys.

The LOCAL directive requests a traditional local correlation calculation, where all pairs (of occupied orbitals) that are correlated by MP2 are treated equal, regardless of their distance. The MULTP directive turns on additional approximations that depend on the distance between the orbitals: The distant pairs are treated by a multipole approximation as described in Ref. [2], and very distant pairs are neglected. This is a prerequisite to obtain linear scaling for large molecules. Using LOCAL (without choosing appropriate settings manually) will result in $O(N^3)$ scaling. Be sure to read the applicable parts of the next section before starting your own calculations.