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24.14 Dual basis set calculations

Dual basis set calculations are possible with the closed-shell MP2 and CCSD codes
(conventional and local, also with density fitting where available). Normally this
means that the Hartree-Fock calculation is done with a smaller basis set than the
correlation calculation. In MOLPRO, two possibilities exist: the recommended one
is to perform the HF and MP2 or CCSD calculations using the same basis set, and
only omit higher angular momentum functions in the HF. This means that the resulting
HF orbitals can be used directly in the correlation calculation. Alternatively,
one can use entirely different basis sets in HF and the correlation calculation; in
this case the orbitals in the correlation calculation are determined by a least
square fit to the HF orbitals. This is less efficient (in particular in fully direct
calculations) and somewhat less accurate. In any case, a new Fock matrix is
computed in the MP2/CCSD program and block diagonalized in the occupied and virtual
orbital subspaces. A perturbative singles correction is applied in the MP2 in order
to reduce the HF basis set error.

Typically, the input is as follows:

basis=vtz(d/p) !triple zeta basis set without f on heavy atoms and without d on hydrogen
hf !Hartree-Fock in the small basis
basis=vtz !full cc-pVTZ basis set to be used in ccsd
ccsd(t),dual=1 !ccsd calculation

The option `dual=1` is required, otherwise the program will stop with an
error message saying that the basis set of the reference orbitals is inconsistent.
This is a precaution in order to avoid unexpected results.

Similarly, this works for other closed-shell single reference methods such as
`MP2, QCISD, MP2-F12, CCSD-F12`, and for the local variants `LMP2, LQCISD, LCCSD` in either
conventional or direct mode. Furthermore, dual basis set `DF-LMP2, DF-LCCSD, DF-LMP2-F12`
calculations are possible.

molpro@molpro.net 2019-03-20