Thank you very much for your clarification @ccavender !
Before calculating the energies via SAPT, I optimized the geometry of the pairs by the MP2/aug-cc-pvtz method and obtained a set of bond lengths and angles relatively close to values found in the literature.
I was trying to create a force field for the disyloxane molecule. Then, I obtained the Lennard-Jones parameters for H, O, C and Si through the energy curves of the hydrogen, water, methane and silane pairs, respectively. In order to obtain the Lennard-Jones parameters, I adjusted the SAPT energy curve of induction + dispersion as in the procedure performed on the first link I made available (https://chemistry.stackexchange.com/questions/76708/how-to- calculate-% 20lennard-jones-potential-with-quantum-mechanical-methods)
I found an article here on the forum (https://doi.org/10.1063/1.4867135) commenting on the efficiency and performance of various levels of SAPT. Due to computational limitations, I choosed SAPT2+/aug-cc-pVDZ (second best method according to the article).
For the charges of the atoms of H, O, C, Si, I obtained them via ESP. The energy curves to obtain the intramolecular potential were obtained via HF/6-31g(2d,p) by varying the structure of the disyloxane molecule (this method was the one that best reproduced experimental data of bond lengths and angles)
Using the Lennard-Jones parameters for H, O, C, Si obtained by SAPT2+/aug-cc-pVDZ (induction + dispersion) in psi4, I did a molecular dynamics simulation in LAMMPS and obtained a density of ~ 0.78g/cm^3 vs 0.76 g/cm^3 experimental. I was “surprised” by the quality of this result.
However, I agree with what you said you should test more geometry settings for the pairs (I even saw in some articles the authors doing this). I think this will be the next step I will take!
I will try to apply your advice, if there are new results, I share.