One shortcoming of presently available fragment-based methods is that electron correlation (if included) is modeled at the level of individual electrons, effectively recomputing local many-body fluctuations for each separate inter-fragment interaction. Without approximation, the ab initio Hamiltonian may be rewritten in terms of subsystem fluctuations between locally correlated states (the exciton basis). Traditional electronic structure methods can then be applied to these renormalized coordinates. In previous work, fragments were internally modeled as groups of strongly coupled harmonic oscillators. Comparing to the exact solution for linearly coupled oscillators, we demonstrated that the basis-set-limit method error for excitonic CCSD was orders of magnitude smaller than for CCSD applied to the primitive oscillator coordinates. This improved accuracy was also obtained an order of magnitude faster. The method has now been extended to real electronic systems, allowing also for charge transfer among them. We use this to model clusters of Be atoms, for which the Be dimer is already a notoriously difficult multiconfigurational, semicovalent problem, necessitating the inclusion of charge transfer terms. Target applications will be outlined, including computation of excited states.

Exciton coupled-cluster theory for large-scale electronic structure calculations: Test application on Ben clusters

254th National Meeting (2017)

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