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.