The Exponential Ansatz in the Context of Density Functional Theory: Elimination of Fractional Charges and Implications for Optical Excitations

The exponential-ansatz operator is traditionally applied within coupled cluster theory to compute relatively accurate wave functions and related electronic properties of molecular systems. One of the most advantageous properties of this operator is its ability to offer size-consistent quantum methods. Motivated by such property, we discuss the application of this operator to density functional calculations. We show the adapted (non-Hermitian) coupled-cluster variational method and the exponential ansatz can eliminate (spurious) fractional charges for spin-symmetry-broken heterogenous molecular systems, whereas preserving a strong consistency with Kohn-Sham LDA calculations in cases where these perform reasonably well. For linear-response TDDFT calculations we show the exponential operator can also assist in the description of avoided-crossing regions, and the study of quantum states where multiple electrons are optically excited. Finally, we discuss the role of the self-interaction error in our methods involving the exponential operator.