MOLPRO

29 LOCAL CORRELATION TREATMENTS 221
3 H2 0.84 1.00
3.1 1 O1 2.02 1.00
4.1 1 O1 1.96 1.00
5.1 1 O1 1.17 0.00
2 H1 0.84 1.00
This tells you that the domains for orbitals 2.1 and 5.1 comprise the basis functions of the
oxygen atom and and one hydrogen atom, while the domains for orbitals 3.1 and 4.1 consist
of the basis function on oxygen only. The latter ones correspond to the oxygen lone pairs, the
former to the two OH bonds, and so this is exactly what one would expect. For each domain of
AOs, corresponding projected atomic orbitals (PAOs) are generated, which span subspaces of
the virtual space and into which excitations are made. Options which affect the domain selection
are described in section 29.6. Improper domains can result from poorly localized orbitals (see
section 29.9.3 or a forgotten NOSYM directive. This does not only negatively affect performance
and memory requirements, but can also lead to unexpected results.
The default for the selection criterion THRBP is 0.98. This works usually well for small basis
sets like cc-pVDZ. For larger basis sets like cc-pVTZ we recommend to use a slightly larger
value of 0.985 to ensure that enough atoms are included in each domain. For cc-pVQZ recommend
THRBP=0.990 is recommended. In cases of doubt, compare the domains you get with a
smaller basis (e.g., cc-pVDZ).
The choice of domains usually has only a weak effect on near-equilibrium properties like equilibrium
geometries and harmonic vibrational frequencies. More critical are energy differences
like reaction energies or barrier heights. In cases where the electronic structure strongly changes,
e.g., when the number of double bonds changes, it is recommended to compare DF-LMP2 and
DF-MP2 results before performing expensive LCCSD(T) calculations. More balanced results
and smooth potentials can be obtained using the MERGEDOM directive, see section 29.8.6.
29.9.5 Freezing domains
In order to obtain smooth potential energy surfaces, domains must be frozen. The domain
information can be stored using the SAVE option and recovered using the START option. Alternatively,
the SAVE and START can be used, see section 29.8.3. In the latter case, also the CCSD
amplitudes are saved/restarted. Freezing domains is particularly important in calculations of intermolecular
interactions, see section 29.9.8. Domains that are appropriate for larger ranges of
geometries, such as reaction pathways, can be generated using the MERGEDOM directive, section
29.8.6. The domains are automatically frozen in geometry optimizations and frequency calculations,
see section 29.9.7. Note, however, that this automatic procedure only works if a single
local calculation is involved in the optimization. In case of optimizations with counterpoise correction
the domains for the complex and each monomer must be frozen individually in different
records using the SAVE and START directives.
29.9.6 Pair Classes
The strong, close, weak and distant pairs are selected using distance or connectivity criteria as
described in more detail in section 29.7. Strong pairs are treated by CCSD, all other pairs by
LMP2. In triples calculations, all orbital triples (i jk) are included for which (i j), (ik), and ( jk)
are close pairs. In addition, one of these pairs is restricted to be strong. The triples energy
depends on the strong and close pair amplitudes. The close pair amplitudes are taken from the
LMP2 calculation. Thus, increasing the distance or connectivity criteria for close and weak
pairs will lead to more accurate triples energies. While for near equilibrium properties like