Elucidating Many-Body Effects in Molecular Core Spectra through Real-Time Approaches: Efficient Classical Approximations and a Quantum Perspective
ORAL
Abstract
Accurate modeling of many-body satellite features in core-level spectra requires methods capable of efficiently capturing many-body effects. The real-time equation-of-motion coupled-cluster Green's function framework offers a cost-effective route but overlooks the \(N\)-electron ground state correlations, limiting its accuracy.
The recently proposed time-dependent double coupled-cluster (TD-dCC) ansatz remedies this by incorporating correlations from both \(N\)- and \((N-1)\)-electron sectors, achieving systematic improvability at a significant computational expense.
Here, we introduce efficient approximations to the TD-dCC framework that preserve the key correlation effects while substantially reducing computational cost through a truncated Baker-Campbell-Hausdorff expansion, maintaining a single-similarity-transformation structure. These approximations systematically recover additional diagrams from commutators involving the amplitude operators from both \(N\)- and \((N-1)\)-electron sectors. Applications to molecular systems such as water and stretched methane demonstrate that the approximate TD-dCC methods accurately reproduce many-body satellite features. In parallel, we apply a fault-tolerant quantum algorithm for core-level Green's function evaluation based on quantum signal processing (QSP), guided by how the time-dependent $(N-1)$ correlation can be simulated differently would impact the performance of the Green's function calculations. Benchmark studies show that approximate TD-dCC methods achieves an optimal balance between accuracy and efficiency, while the QSP-based quantum approach provides a scalable path toward future large-scale fault-tolerant quantum simulations. Together, these developments establish complementary classical and quantum frameworks for systematically improving the description of many-body features in X-ray and photoelectron spectra.
The recently proposed time-dependent double coupled-cluster (TD-dCC) ansatz remedies this by incorporating correlations from both \(N\)- and \((N-1)\)-electron sectors, achieving systematic improvability at a significant computational expense.
Here, we introduce efficient approximations to the TD-dCC framework that preserve the key correlation effects while substantially reducing computational cost through a truncated Baker-Campbell-Hausdorff expansion, maintaining a single-similarity-transformation structure. These approximations systematically recover additional diagrams from commutators involving the amplitude operators from both \(N\)- and \((N-1)\)-electron sectors. Applications to molecular systems such as water and stretched methane demonstrate that the approximate TD-dCC methods accurately reproduce many-body satellite features. In parallel, we apply a fault-tolerant quantum algorithm for core-level Green's function evaluation based on quantum signal processing (QSP), guided by how the time-dependent $(N-1)$ correlation can be simulated differently would impact the performance of the Green's function calculations. Benchmark studies show that approximate TD-dCC methods achieves an optimal balance between accuracy and efficiency, while the QSP-based quantum approach provides a scalable path toward future large-scale fault-tolerant quantum simulations. Together, these developments establish complementary classical and quantum frameworks for systematically improving the description of many-body features in X-ray and photoelectron spectra.
*We gratefully acknowledge support from the U.S. Department of Energy, Office of Science, Early Career Research Program under Grant No. FWP 83466.
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Presenters
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Vibin Abraham
- Pacific Northwest National Lab