Molecules with competing electronic structures play a key role in many chemical situations such as excited states, dissociating bonds, and reaction pathways. In these so-called strongly correlated systems, electronic structure methods based on a single reference Hartree-Fock (HF) determinant break down, including coupled-cluster (CC) and perturbative approaches. The con- ventional solution to this problem would be a multireference method, for example CASSCF, but such methods are far from black-box and generally scale poorly with system size. Alternatively, we have found that where there is strong correlation there are in fact multiple HF solutions, al- though these multiple solutions may coalesce and disappear as the molecular geometry changes [1].
In this presentation, I will show how a linear combination of these multiple HF solutions in a nonorthogonal configuration interaction (NOCI) approach can provide an effective alter- native approach for multireference systems [2]. To ensure a continuous basis for NOCI can be constructed across all molecular geometries, I will introduce holomorphic HF theory as a complex-analytic extension to the HF equations that extends HF states beyond the coalescence points where solutions vanish [3-5]. These holomorphic HF solutions are smooth, continuous and exist at all molecular geometries, therefore providing a continuous basis for NOCI [4-6]. Finally, using paradigmatic multireference molecular systems, I will illustrate the performance of this new approach and discuss its prospects for future development.
(1) A. J. W. Thom and M Head-Gordon, Phys. Rev. Lett. 101, 193001 (2008)
(2) A. J. W. Thom and M Head-Gordon, J. Chem. Phys. 131, 124113 (2009)
(3) H. G. Hiscock and A. J. W. Thom, J. Chem. Theory Comput. 10, 4795 (2014)
(4) H. G. A. Burton and A. J. W. Thom, J. Chem. Theory Comput. 12, 167 (2016)
(5) H. G. A. Burton, M. Gross and A. J. W. Thom, J. Chem. Theory Comput. 14, 607 (2018) (6) H. G. A. Burton and A. J. W. Thom, J. Chem. Theory Comput. 15, 4851 (2019)