MOLPRO

29 LOCAL CORRELATION TREATMENTS 218
The epart parameter determines the cutoff distance for (intramolecular) bond lengths (in a.u.,
default 3 a.u.) and is used to automatically determine the individual monomer subunits of the
cluster. The iepart parameter enables the energy partitioning, if set to a value larger than zero
(default 1). Additionally, if iepart is set to 2, a list of all intermolecular pair energies and their
components is printed.
The output section produced by the energy partitioning algorithm will look similar to the following
example:
energy partitioning enabled !
centre groups formed for cutoff [au] = 3.00
1 :O1 H11 H12
2 :O2 H21 H22
energy partitioning relative to centre groups:
intramolecular correlation: -.43752663
exchange dispersion : .00000037
dispersion energy : -.00022425
ionic contributions : -.00007637
The centre groups correspond to the individual monomers determined for epart=3. In the
present example, two water monomers were found. The correlation energy is partitioned into
the four components shown above. The exchange dispersion, dispersion and ionic components
reflect directly the related intermolecular components of the complex, while the intramolecular
correlation contribution to the interaction energy has to be determined by a super-molecular
calculation, i.e. by subtracting the (two) corresponding monomer correlation energies from the
intramolecular correlation component of the complex given in the output.
Alternatively, the following form can be used:
ENEPART,RMAX=[r1,r2,r3,. . . ]
and the program will then print the energy contributions of all pairs in the ranges between the
given distances (in bohr, enclosed by square brackets, e.g., enepart,rmax=[0,3,5,7,9,11]).
A second list in which the contributions are given as a function of the number of bonds between
the pair domains will also be printed.
29.9 Doing it right
The local correlation methods in MOLPRO employ localized molecular orbitals (LMOs). Pipek-
Mezey localization is recommended, but Boys localization is also possible. The virtual orbital
space is spanned by non-orthogonal projected atomic orbitals (PAOs). The local character of this
basis makes it possible to introduce two distinct approximations: first, excitations are restricted
to domains, which are subspaces of (PAOs) that are spatially close to the orbitals from which
the electrons are being excited. Secondly, the orbital pairs are classified according to their
importance (based on distance or connectivity criteria), and only strong pairs are treated at
the highest level (e.g. CCSD). The remaining weak and distant pairs are treated at the LMP2
level, and very distant pairs are neglected. These approximations lead to linear scaling of the
computational resources as a function of the molecular size.
Naturally, such approximation can introduce some errors, and therefore the user has to be more
careful than with standard black box methods. On the other hand, the low-order scaling makes
it possible to treat much larger systems at high levels of theory than it was possible so far.
This section summarizes some important points to remember when performing local correlation
calculations.