Accuracy of two-particle N-representability conditions for describing different spin states and the singlet–triplet gap in the linear acene series

ABSTRACT

Variational two-electron reduced-density-matrix (2-RDM) methods can provide a reference-independent description of the electronic structure of strongly correlated molecules and materials. These methods represent one of few ways of performing large active-space-based computations that are beyond the scope of conventional configuration-interaction-based approaches. However, implementations of the method are quite rare, making it difficult for the quantum chemistry community to assess the utility of the approach. Here, we discuss an open-shell implementation of the variational 2-RDM method and explore its ability to describe different spin states in several model systems, including linear hydrogen chains and linear acenes. The largest calculations considered are comparable to complete-active-space computations with 50 electrons in 50 orbitals.

KEYWORDS:

• Variational 2-RDM
• electronic structure
• spin gap
• static electron correlation

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by an Army Research Office 385 Small Business Technology Transfer (STTR) Phase I award (65925CHST1) and by the American Chemical Society Petroleum Research Fund (54668-DNI6).