Correlated Orbital Theory: An alternative and complement to Kohn-Sham DFT*

Coupled-cluster (CC) theory from ab initio quantum chemistry is known to provide the best answers for most computationally accessible  problems in electronic structure theory. Less appreciated, is that it is possible to develop a new correlated one-particle theory of chemistry, correlated orbital theory (COT), from CC/EOM considerations of the (ionization potential) IP-EOM-CC and (electron affinity) EA-EOM-CC. They provide a new one-particle, effective, frequency independent Hamiltonian whose occupied orbital eigenvalues are the principal IP’s for a molecule, and some of the unoccupied ones, are EA’s. This new model defines the correlated orbitals, that include the homo and lumo frontier MO’s whose eigenvalues are formally exact in the theory. Using this COT as the framework for an effective one-particle theory, one can now appeal to KS-DFT to replace the purely ab initio COT equations in application by using standard  DFT reference density gradient expressions for the exchange-correlation potential and functional, as long as the choice fulfils the requirements of the underlying COT. This route, using CAM-B3LYP as the underlying functional meant to represent COT, though practically any of them could be chosen as long as there is sufficient flexibility to guarantee our IP eigenvalue condition is adequately satisfied for the KS-DFT eigenvalues. Our objective is to create an easily applied DFT-like procedure that can successfully address the ‘Devil’s Triangle of KS DFT.”