Jure Kokalj and I recently wrote a paper where considered the effect of strong correlations on thermal expansion, all within the framework of a Hubbard model. This is mostly concerned with explaining anomalies in organic charge transfer salts at temperatures less than 100 K, i.e. much less than the Fermi energy.
One referee stated
“However conceptually this Hamiltonian can not capture the free energy of the relevant electrons loyally. Recall the total energy decomposition in density functional theory, the Hamiltonian corresponds only to the band energy part (which is a summation of occupied Khon-Sham states and different from the kinetic energy) plus interaction term. And the remaining Hartree part, exchange-correlated part and also ionic part, which depend on the lattice constants, are totally ignored. It is not known whether the contributions from such terms are trivial or monotonic especially when strong correlation is present. The neglect of such terms in the electronic model in use is not justified. In this sense, even though the parameters are taken from first principles estimations, it is not surprising that the results are not consistent with experimental data quantitatively and sometimes even qualitatively."There are some subtle issues here that I would like to understand.
I am not sure I fully understand the referee's comments.
And, I am not sure I agree.
1. Do I understand that the referee is suggesting that the Hubbard model does not include the effects contained in the Hartree and exchange correlation term? Surely, this is not correct.
2. I agree that the Hubbard model will be missing all effects associated with core electrons and ionic terms. However, surely any effects associated with these will not vary significantly on energy and temperature scales of the order of 100 K?
I welcome any comments and insight.