Active Space Pair 2-Electron Reduced Density Matrix Theory for Strong Correlation

Abstract

An active space variational calculation of the 2-electron reduced density matrix (2-RDM) is derived and implemented where the active orbitals are correlated within the pair approximation. The pair approximation considers only doubly occupied configurations of the wavefunction which enables the calculation of the 2-RDM at a computational cost of $\mathcal{O}(r^3)$. Calculations were performed both with the pair active space configuration interaction (PASCI) method and the pair active space self consistent field (PASSCF) method. The latter includes a mixing of the active and inactive orbitals through unitary transformations. The active-space pair 2-RDM method is applied to the nitrogen molecule, the p-benzyne diradical, a newly synthesized BisCobalt complex, and the nitrogenase cofactor FeMoco. The FeMoco molecule is treated in a [120,120] active space. Fractional occupations are recovered in each of these systems, indicating the detection and recovery of strong electron correlation.

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Cited by in corpus (6)

• Quantum Many-body Theory from a Solution of the $N$-representability Problem
• Dual-Cone Variational Calculation of the 2-Electron Reduced Density Matrix
• Reduced density matrix functional theory from an ab initio seniority-zero wave function: Exact and approximate formulations along adiabatic connection paths
• Seniority-zero Linear Canonical Transformation Theory
• Representability for Quantum Theory beyond Particle-Number Conservation
• Seniority-zero Quadratic Canonical Transformation Theory