MOLPRO

29 LOCAL CORRELATION TREATMENTS 216
29.8.2 The MULTP directive
The MULTP directive turns on the multipole approximations for distant pairs, as described in
Ref. [8]. Further options can be given as described above for the LOCAL directive.
LOCAL,MULTP,options
is equivalent to
MULTP,options
The level of the multipole approximation can be chosen using option DSTMLT (default 3) ( 1
means dipole approximation, 2 quadrupole approximation and so on).
The multipole approximation reduces the computational cost of LMP2 calculations for very
large molecules, but leads to some additional errors, see Ref. [8]. It is normally not recommended
to be used in coupled-cluster calculations and should never be used for computing intermolecular
forces. It can also not be used in geometry optimizations or gradient calculations.
29.8.3 Saving the wavefunction (SAVE)
The wavefunction can be saved for later restart using
SAVE,record
where record has the usual form, e.g., 4000.2 means record 4000 on file 2. If this directive is
given, the domain information as well as the amplitudes are saved (for MPn the amplitudes are
not saved). If just the domain information should be stored, the SAVE option on the LOCAL
directive must be used (cf. section 29.3).
29.8.4 Restarting a calculation (START)
Local CCSD or QCISD calculations can be restarted using
START,record
The record given must have been saved in a previous local calculation using the SAVE directive
(otherwise this directive is ignored). If the START directive is given, the domain information
as well as the amplitudes of the previous calculation are used for restart. It is possible, for
instance, to start a local CCSD calculation with the amplitudes previously saved for a local
QCISD calculation (but of course it is not possible to use a record saved for a non-local CCSD or
QCISD calculation). If it is intended only to use the domain information but not the amplitudes
for a restart, the START option on the command line or LOCAL directive must be used (cf.
section 29.3).
29.8.5 Correlating subsets of electrons (REGION)
In large molecules, it may be sufficient to correlate only the electrons in the vicinity of an
active group, and to treat the rest of the molecule at the SCF level. This approach can even
be extended, different correlation levels may be used for different sections of the system. The
REGION directive allows the specification of a subset of atoms:
REGION,METHOD=method,[DEFAULT=default method], [TYPE=INCLUSIVE|EXCLUSIVE],
atom1, atom2 . . .