12.7.1 ALMO-EDA for the MP2 Method

The previously described EDA methods are limited to SCF methods such as HF and DFT. However, for many systems, it is preferable to use a wave function based correlation method. For this reason, the ALMO-EDA has been extended to MP2. ¹²⁶² Thirman J., Head-Gordon M.
J. Chem. Phys.
(2015), 143, pp. 084124. Link ^{, 1263} Thirman J., Head-Gordon M.
J. Phys. Chem. A
(2017), 121, pp. 717. Link The MP2 ALMO-EDA is based on the first-generation ALMO-EDA. It provides an MP2 correction to the FRZ, POL, and CT terms defined by the ALMO-EDA and also adds in a term corresponding to the London dispersion force. This is done by defining a constrained intermediate MP2 wave function corresponding to each HF intermediate.

The current implementation is limited to only RI-MP2 rather than full MP2, and only works in the closed shell, spin restricted case. Frozen core and spin scaling are also not yet supported. Attempting to use the EDA with a correlation method other then RI-MP2 or with unrestricted orbitals will result in a crash. Frozen core and spin scaling settings will be ignored with a warning by the EDA, but not by the final energy, leading to inconsistent results.

Though the MP2 EDA is based on the first generation ALMO-EDA, the code path and REM settings are shared with the second generation ALMO-EDA. The MP2 ALMO-EDA does not define any new REM variables of its own. Rather, running an EDA job with EDA2 and GEN_SCFMAN will trigger an MP2 ALMO-EDA when the correlation method is RI-MP2. The correlation setting causes the SCF portion of the EDA to be switched back to the original scheme and will also decompose the correlation energy. Most settings intended for the second generation ALMO-EDA are not supported, but EDA_NO_CT and EDA_BSSE are. An example appears below.