Strongly contracted canonical transformation theory
Canonical transformation (CT) theory describes dynamic correlation in multireference systems with large active spaces. Here we discuss CT theory's intruder state problem and why our previous approach of overlap matrix truncation becomes infeasible for sufficiently large active spaces. We propose the use of strongly and weakly contracted excitation operators as alternatives for dealing with intruder states in CT theory. The performance of these operators is evaluated for the H_2O, N_2, and NiO molecules, with comparisons made to complete active space second order perturbation theory and Davidson-corrected multireference configuration interaction theory. Finally, using a combination of strongly contracted CT theory and orbital-optimized density matrix renormalization group theory, we evaluate the singlet-triplet gap of free base porphin using an active space containing all 24 out-of-plane 2p orbitals. Modeling dynamic correlation with an active space of this size is currently only possible using CT theory.
© 2010 American Institute of Physics. Received 3 September 2009; accepted 25 November 2009; published online 13 January 2010. This work was supported by the Department of Energy Office of Science Award No. DE-FG02-07ER46432. Eric Neuscamman would like to acknowledge the support of the National Science Foundation Graduate Research Fellowship Program.
Published - 1_2E3274822.pdf