62 WF-in-DFT EMBEDDING

Embedding methods allow a system to be divided into two smaller subsystems, each of which can be treated using a different level of theory. For example, in WF-in-DFT embedding, a WF-level (e.g. HF, MP2, CCSD(T), CASSCF, etc.) calculation is performed on one subsystem, while a DFT-level calculation is performed on the other subsystem. The interactions between the two subsystems are calculated at the DFT level. The primary advantage of WF-in-DFT embedding is that it facilitates the application of an accurate, systematically improvable WF method to regions where such accuracy is required, while a more efficient DFT method is applied to the remainder of the system. The overall strategy for WF-in-DFT embedding is described as follows:

(1) A DFT calculation is performed on the full system.

(2) The system is partitioned into two subsystems, labeled the active subsystem and the frozen subsystem.

(3) The interaction potential between the two subsystems is calculated at the DFT level.

(4) A WF-level calculation is performed on the electrons in the active subsystem, embedded in the DFT-level interaction potential produced by the electrons in the frozen subsystem.

Note: WF-in-DFT embedding is not implemented with symmetry.

It is possible to replace the WF-level calculation on subsystem A with a DFT-level calculation, which corresponds to DFT-in-DFT embedding. Because the interaction potential between the subsystems is calculated at the DFT level, the result of a DFT-in-DFT embedding calculation is numerically identical to the results of a DFT calculation on the full system. Confirming that the DFT-in-DFT energy is numerically equivalent to the energy of a DFT calculation on the full system can thus be a useful sanity check. Similarly, it is possible to replace the DFT-level calculation with a HF-level calculation, which corresponds to WF-in-HF embedding.

Molpro implements the numerically exact projection-based WF-in-DFT embedding method for open and closed shell systems developed in the following papers:

F. R. Manby, M. Stella, J. D. Goodpaster, and T. F. Miller III, J. Chem. Theory Comput., 8, 2564 (2012)

T. A. Barnes, J. D. Goodpaster, F. R. Manby, and T. F. Miller III, J. Chem. Phys., 139, 024103 (2013)

J. D. Goodpaster, T. A. Barnes, F. R. Manby, and T. F. Miller III, J. Chem. Phys., 140, 18A507 (2014)

S. J. Bennie, M. Stella, T. F. Miller III and F. R. Manby, J. Chem. Phys., 143, 024105 (2015)

All publications resulting from the use of this method must acknowledge the above.