Frozen-Pair-Type pCCD-Based Methods and Their Double Ionization Variants to Predict Properties of Prototypical BN-Doped Light Emitters.

Novel, robust, computationally efficient, and reliable theoretical methods are indispensable for the large-scale modeling of desired molecular properties. One such example is the orbital optimized pair coupled-cluster doubles (oo-pCCD) ansatz and its various CC extensions, which range from closed-shell ground- and excited-state models to open-shell variants. Specifically, the ionization-potential equation-of-motion frozen-pair (IP-EOM-fp)CC methods proved to be competitive with standard CC-type methods for modeling the ionization potentials of organic electronics. In this work, we extend the existing IP-EOM-pCCD-based methods to their double ionization potential (DIP) variants, resulting in various DIP-EOM-fpCC models, including up to double excitations. These methods open the way to reach open-shell singlet, triplet, and quintet states using various pCCD reference functions. Their accuracy is tested for the singlet-triplet gaps of the ortho-, meta-, and para-benzynes. Then, the most accurate models are applied to study the effects of boron and nitrogen doping on designing prototypical naphthalene-based donors and acceptors. Our results demonstrate consistent and reliable outcomes with standard methods and available experimental data. Most importantly, fpCC-type methods show slightly better performance than DIP-EOM-CCSD for strongly-correlated cases and similar performance for systems dominated by dynamical correlation when determining singlet-triplet gaps.