25.2 Getting started

The local correlation treatment is switched on by giving either the LOCAL or the MULTP directive after the MP2, MP3, MP4, QCI[SD], CCSD, or CISD directives.

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

(The full set of options is described in section 21.5, and summarized in Table 9.) The two 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 (whithout choosing appropriate settings manually) will result in $O(N^3)$ scaling. Be sure to read the applicaple parts of the next section before starting your own calculations.