Open-shell frozen natural orbital approach for quantum eigensolvers
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arXiv
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| Format: | Preprint |
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2025
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| author | Harper, Angela F. Liu, Xiaobing Genin, Scott N. Ryabinkin, Ilya G. |
| author_facet | Harper, Angela F. Liu, Xiaobing Genin, Scott N. Ryabinkin, Ilya G. |
| contents | We present an open-shell frozen natural orbital (FNO) approach, which utilizes the second-order Z-averaged perturbation theory (ZAPT2), to reduce the restricted opten-shell Hartree-Fock virtual space size with controllable accuracy. Our ZAPT2 frozen natural orbital (ZAPT-FNO) selection scheme significantly outperforms the canonical molecular orbital virtual space truncation scheme based on Hartree-Fock orbital energies, especially when using large multiple-polarized and augmented basis sets. We demonstrate that the ZAPT-FNO-selected virtual orbitals lead to a systematic convergence of the correlation energies, but more importantly to the singlet-triplet T$_1$-S$_ 0$ energy gaps with respect to the complete active space (CAS) [occupied + virtual] size. We confirm our findings by simulating T$_1$-S$_ 0$ gaps in H$_2$O$_2$ and O$_2$ molecules using the traditional complete active space configuration interaction (CASCI) approach, as well as in stretched CH$_2$, for which we also employed the iterative qubit coupled cluster (iQCC) method as a quantum eigensolver. Finally, we applied the iQCC method with ZAPT-FNO-selected active space to the phosphorescent Ir(ppy)$_3$ complex with 260 electrons, where extended basis sets are required to achieve chemical (ca. 1 m$E_h$) accuracy. In this case, CASCI results are not available; however, the iQCC-computed T$_1$-S$_ 0$ gaps show robust convergence with enlarging basis set and CAS size, approaching the experimental value. Thus, the ZAPT-FNO method is very promising for improving the accuracy of quantum chemical modelling in a resource-efficient manner, and opens the door to simulating open-shell states of large materials within realistic active space sizes and without compromising on basis-set quality. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2511_13677 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Open-shell frozen natural orbital approach for quantum eigensolvers Harper, Angela F. Liu, Xiaobing Genin, Scott N. Ryabinkin, Ilya G. Chemical Physics Computational Physics We present an open-shell frozen natural orbital (FNO) approach, which utilizes the second-order Z-averaged perturbation theory (ZAPT2), to reduce the restricted opten-shell Hartree-Fock virtual space size with controllable accuracy. Our ZAPT2 frozen natural orbital (ZAPT-FNO) selection scheme significantly outperforms the canonical molecular orbital virtual space truncation scheme based on Hartree-Fock orbital energies, especially when using large multiple-polarized and augmented basis sets. We demonstrate that the ZAPT-FNO-selected virtual orbitals lead to a systematic convergence of the correlation energies, but more importantly to the singlet-triplet T$_1$-S$_ 0$ energy gaps with respect to the complete active space (CAS) [occupied + virtual] size. We confirm our findings by simulating T$_1$-S$_ 0$ gaps in H$_2$O$_2$ and O$_2$ molecules using the traditional complete active space configuration interaction (CASCI) approach, as well as in stretched CH$_2$, for which we also employed the iterative qubit coupled cluster (iQCC) method as a quantum eigensolver. Finally, we applied the iQCC method with ZAPT-FNO-selected active space to the phosphorescent Ir(ppy)$_3$ complex with 260 electrons, where extended basis sets are required to achieve chemical (ca. 1 m$E_h$) accuracy. In this case, CASCI results are not available; however, the iQCC-computed T$_1$-S$_ 0$ gaps show robust convergence with enlarging basis set and CAS size, approaching the experimental value. Thus, the ZAPT-FNO method is very promising for improving the accuracy of quantum chemical modelling in a resource-efficient manner, and opens the door to simulating open-shell states of large materials within realistic active space sizes and without compromising on basis-set quality. |
| title | Open-shell frozen natural orbital approach for quantum eigensolvers |
| topic | Chemical Physics Computational Physics |
| url | https://arxiv.org/abs/2511.13677 |