Hi all,

I would like to calculate the first order electrostatic energy in the monomer-centered basis (MCBS) using Psi4Numpy. I am aware that it is possible to obtain this in the main Psi4 code but I would like to obtain this energy directly without calculating the other components of the SAPT energy.

I have been using the tutorial notebook 7b (AO-SAPT) to do this. In `helper_SAPT.py`

the fragments are obtained in the dimer basis using `dimer.extract_subsets(1, 2)`

i.e. with ghost functions. In the main Psi4 code, the MCBS calculation grabs the fragments without ghost functions using `dimer.extract_subsets(1)`

.

**Are SAPT-AO and MCBS SAPT the same methods?** This difference in how the fragments are obtained and the AO to MO integral transformation in tutorial 7a leads me to think that these methods are not the same. The SAPT-AO method gives essentially the same electrostatic energy as the energy obtained with the main Psi4 code using `energy('sapt0', sapt_basis='dimer')`

. This is mentioned in the tutorial, where the author mentions “we expect exactly the same numerical answers” as SAPT-MO.

Assuming that performing the calculation in the MCBS is different to SAPT-AO, I have been trying to calculate the electrostatic energy in the MCBS using Psi4Numpy, where I obtain the dimer fragments in the monomer basis without ghost functions.

I need the nuclear potential integrals between the nuclei of one system and the basis functions of the other, as well as the ERIs between the basis functions of each system, in the monomer basis. **Is it possible to get these “intermolecular” integrals using MintsHelper i.e. ao_potential() or ao.eri() or some other alternative method in Psi4/Psi4Numpy**?

I would assume that it is possible to get these integrals as they would be used in the MCBS SAPT calculation, but I am unsure of how the main Psi4 code obtains them and whether I could also obtain them in Psi4Numpy.

Thank you for your time.