IL3. Electron correlation methods for near-degenerate electronic states based on the driven similarity renormalization group

We have recently demonstrated that it is possible to combine conventional Quantum Chemistry many-body approaches with flow renormalization group methods to produce a new family of electronic structure theories. These methods have several advantages: (i) they are numerically-robust and avoid intruder states, (ii) they lead to equation that are relatively simple, and (iii) they are easily generalized to multi-determinantal reference states. An important aspect of our work has been to reformulate the similarity renormalization group (SRG) of Głazek and Wilson and Wegner [1,2] into a practical quantum chemistry approach, the driven SRG (DSRG) [3]. Recently, we examined new approximation schemes for the DSRG that are both accurate and economical. This talk will focus on second- and third-order perturbative approximations of the DSRG [4,5] equations and their generalization to treat near-degenerate ground and excited states. Francesco A. Evangelista, Emory University Chenyang Li, Emory University Kevin P. Hannon, Emory University Prakash Verma, Emory Universit