Multireference perturbation theory with nonorthogonal determinants
Abstract
We introduce NOCI-RSPT, a framework for constructing perturb-then-diagonalize multireference perturbation theories for model spaces defined by nonorthogonal determinants. NOCI-RSPT realizes order-by-order size consistency by orthogonalizing the effective Hamiltonian before perturbatively truncating it and removes problematic internal excitations through model-space projection. The resulting second- and third-order theories, NOCI-RS2 and NOCI-RS3, recover MP2 and MP3 in the single-reference limit and exhibit the same respective cost scaling at fixed model-space size. Calculations on assemblies of multireference H2 fragments demonstrate size consistency, while calculations of the low-energy states of H2 along its dissociation curve demonstrate improved numerical stability and characterize the influence of intruder states. The latter calculations demonstrate the exceptional accuracy achieved by NOCI-RS3 and illustrate how model-space expansion and amplitude regularization can mitigate the effects of large perturbative corrections. Altogether, NOCI-RSPT provides a systematic route to low-scaling multireference perturbation theories built from single-reference methods.
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BibTeX
@misc{krotz2026multireference,
title = {{Multireference perturbation theory with nonorthogonal determinants}},
author = {Alex Krotz and K. Birgitta Whaley and Martin Head-Gordon},
howpublished = {arXiv},
year = {2026},
month = nov,
doi = {10.48550/arxiv.2611.07244},
eprint = {2611.07244},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2611.07244},
}
RIS
TY - UNPB
TI - Multireference perturbation theory with nonorthogonal determinants
AU - Alex Krotz
AU - K. Birgitta Whaley
AU - Martin Head-Gordon
PY - 2026
DA - 2026/11/04
PB - arXiv
DO - 10.48550/arxiv.2611.07244
UR - https://arxiv.org/abs/2611.07244
AB - We introduce NOCI-RSPT, a framework for constructing perturb-then-diagonalize multireference perturbation theories for model spaces defined by nonorthogonal determinants. NOCI-RSPT realizes order-by-order size consistency by orthogonalizing the effective Hamiltonian before perturbatively truncating it and removes problematic internal excitations through model-space projection. The resulting second- and third-order theories, NOCI-RS2 and NOCI-RS3, recover MP2 and MP3 in the single-reference limit and exhibit the same respective cost scaling at fixed model-space size. Calculations on assemblies of multireference H_2 fragments demonstrate size consistency, while calculations of the low-energy states of H_2 along its dissociation curve demonstrate improved numerical stability and characterize the influence of intruder states. The latter calculations demonstrate the exceptional accuracy achieved by NOCI-RS3 and illustrate how model-space expansion and amplitude regularization can mitigate the effects of large perturbative corrections. Altogether, NOCI-RSPT provides a systematic route to low-scaling multireference perturbation theories built from single-reference methods.
ER -