Methodological Advances in State-Specific Multireference Coupled Cluster Theory

Wesley Allen, Center for Computational Chemistry, Chemistry Department, The University of Georgia

The first production level code (PSIMRCC) for state-specific and rigorously size-extensive Mukherjee multireference coupled cluster singles and doubles (Mk-MRCCSD) computations has been developed.  This breakthrough was aided by our derivation of closed-form expressions for the terms coupling different references in the amplitude equations. Moreover, a hierarchy of Mk-MRCCSDT-n (n = 1a, 1b, 2, 3) methods for the iterative inclusion of connected triple excitations has been formulated and implemented for the first time. The effectiveness of our Mk-MRCC methods is established by extensive computations on benchmark problems, including the dissociation of molecular fluorine and the ozone graveyard for theory.  In chemical applications of Mk-MRCCSD theory with the cc-pVTZ basis set, outstanding results have been obtained for the optimum geometric structures, vibrational frequencies, and adiabatic excitation energies of ortho-, meta-, and para-benzyne, as well the automerization barriers of cyclobutadiene, cyclooctatetraene, and other antiaromatic systems.  Finally, we report Mk-MRCC predictions of UV/Vis spectra of novel carbenes that have led to the identification of these species in matrix isolation experiments.

Oral