Quantum Calculations Boosted by Doubling Computational Space for Complex Molecules

Researchers are continually striving to expand the capabilities of quantum computational chemistry to tackle increasingly complex molecular systems. Yoshida, Murokoshi, and Kuroda, from the Center for Quantum Information and Quantum Biology at The University of Osaka, alongside Mizukami et al., present a novel approach to significantly enhance the size of quantum selected configuration interaction (QSCI) calculations. Their work introduces a doubly occupied configuration interaction-selected configuration interaction (DOCI-QSCI) method, effectively doubling the accessible orbital space by sampling from the seniority-zero space. This advancement is crucial because it overcomes a key limitation of conventional QSCI, allowing for more accurate modelling of molecular electronic structure, as demonstrated through successful applications to challenging systems including the H6 chain, N2 dissociation, and the BODIPY-O2 complex, where single-reference methods fail. This breakthrough addresses a critical limitation in current quantum algorithms by effectively doubling the accessible orbital space for calculations. The research, detailed in a recent publication, introduces a technique for sampling from a seniority-zero space, a restricted sector focusing on paired electrons, and then strategically expanding these results to incorporate seniority-breaking determinants. This innovative approach overcomes the qubit limitations inherent in traditional methods, paving the way for more accurate and complex molecular simulations. The core of this advancement lies in a refined quantum selected configuration interaction (QSCI) algorithm. By focusing initially on the seniority-zero space, the number of qubits required is reduced to equal the number of spatial orbitals, a substantial improvement over conventional methods that demand one qubit per spin-orbital. While this restriction initially compromises quantitative accuracy, researchers compensate by expanding the sampled bitstrings into a larger space encompassing seniority-breaking determinants. This expansion, combined with phaseless auxiliary-field Monte Carlo (ph-AFQMC) post-processing, allows for the recovery of dynamical correlations across the full orbital space, resulting in a highly accurate quantum-chemical calculation. Evaluations of DOCI-QSCI-AFQMC on the H6 chain, N2 dissociation, and the addition of singlet O2 to a BODIPY dye demonstrate its effectiveness. For the H6 chain, the method achieves accuracy comparable to complete-active-space calculations using the ibm_kobe quantum device. Notably, for N2 and BODIPY-O2, employing (14e, 28o) and up to (20e, 20o) active spaces, the new method yields reasonable results where single-reference CCSD(T) methods fail qualitatively. These findings confirm that DOCI-QSCI effectively doubles the orbital space accessible to conventional QSCI, and subsequent ph-AFQMC processing delivers a high degree of accuracy for complex molecular systems. This work represents a significant step towards performing practically relevant quantum chemical calculations on existing quantum hardware. The ability to accurately model larger and more complex molecules has implications for diverse fields, including materials science, drug discovery, and fundamental chemical research. By mitigating qubit demands and enhancing computational efficiency, DOCI-QSCI-AFQMC promises to accelerate the development of quantum technologies for tackling previously intractable chemical problems. DOCI-QSCI wave function generation and ph-AFQMC correlation recovery A doubly occupied configuration interaction-selected configuration interaction (DOCI-QSCI) method was developed, initially sampling configurations from the seniority-zero space to reduce the qubit requirements for quantum computation. This restriction to the seniority-zero space, while effectively halving the qubit budget to equal the number of spatial orbitals, potentially compromises quantitative accuracy. To address this, researchers expanded the sampled bitstrings using a Cartesian product, generating a larger space that includes determinants with seniority-breaking characteristics. The resulting wave function then served as a trial state within a phaseless auxiliary-field Monte Carlo (ph-AFQMC) calculation, designed to recover dynamical correlations across the complete orbital space, creating a DOCI-QSCI-AFQMC workflow. The performance of this combined method was evaluated using three distinct chemical systems: the H6 chain, N2 dissociation, and the addition of singlet O2 to a BODIPY dye. For the H6 chain, DOCI-QSCI-AFQMC achieved accuracy comparable to complete-active-space calculations performed on the ibm_kobe quantum device. With (14e, 28o) and up to (20e, 20o) active spaces, the method yielded reasonable results for N2 and BODIPY-O2, successfully addressing cases where single-reference CCSD(T) failed to produce qualitatively correct solutions. These findings demonstrate that DOCI-QSCI effectively doubles the accessible orbital space compared to conventional QSCI, and subsequent ph-AFQMC post-processing delivers reasonably high accuracy in the computed results. The seniority-zero space provides a practical foundation for enabling seniority-breaking and compact quantum-circuit designs. DOCI-QSCI-AFQMC achieves high accuracy and expanded orbital access for challenging molecular systems DOCI-QSCI-AFQMC reproduces the accuracy of complete-active-space calculations on the H6 chain using the ibm_kobe device. This achievement demonstrates a level of precision comparable to the complete-active-space counterpart, validating the method’s efficacy for this system. The research successfully applied this approach to N2 and BODIPY-O2, employing (14e, 28o) and up to (20e, 20o) active spaces, yielding reasonable results where single-reference CCSD(T) methods failed qualitatively. These findings highlight the potential of the DOCI-QSCI method to address systems beyond the reach of traditional single-reference techniques. The DOCI-QSCI method effectively doubles the orbital space accessible to conventional QSCI, expanding computational capabilities. Subsequent ph-AFQMC post-processing further enhances accuracy by recovering dynamical correlations across the full orbital space. By sampling from the seniority-zero space, the work circumvents limitations associated with qubit scaling, achieving a balance between computational cost and quantitative precision. This approach utilizes the seniority-zero space, requiring a qubit count equal to the number of spatial orbitals, a significant reduction compared to standard fermion-to-qubit mappings. The study demonstrates the feasibility of utilising the seniority-zero space for quantum computation, offering a promising direction for algorithmic improvements. Expanding sampled bitstrings via their Cartesian product into a larger space, including seniority-breaking determinants, further refines the accuracy of the calculations. The application of this method to the oxygen-addition reaction of a BODIPY dye showcases its relevance to practical molecular systems, extending beyond benchmark examples like the H6 chain and N2 dissociation. This work establishes a foundation for more effective quantum-chemical calculations on current quantum hardware. Enhanced dynamical correlation via seniority-breaking determinants in DOCI-QSCI-AFQMC Doubly occupied configuration interaction-selected configuration interaction (DOCI-QSCI) represents an advancement in quantum chemical calculations by efficiently sampling the seniority-zero space. This approach effectively doubles the accessible orbital space for a given number of qubits, although this restriction can potentially compromise quantitative accuracy. To address this, researchers expanded the sampled bitstrings into a larger space encompassing seniority-breaking determinants, subsequently employing this expanded wave function within phaseless auxiliary-field Monte Carlo (ph-AFQMC) to achieve improved dynamical correlations. Evaluations across several molecular systems, including the H6 chain, N2 dissociation, and the addition of singlet O2 to a BODIPY dye, demonstrate the method’s viability. DOCI-QSCI-AFQMC reproduced results comparable to complete active space methods for the H6 chain and yielded reasonable outcomes for N2 and BODIPY-O2, where conventional single-reference coupled cluster methods failed. These findings indicate that DOCI-QSCI expands the scope of tractable quantum chemical calculations and subsequent ph-AFQMC post-processing delivers reasonably high accuracy. A limitation acknowledged by the researchers is the inaccessibility of open-shell and high-spin states within the seniority-zero space. Future work will likely focus on extending the methodology to accommodate these states and further refine the accuracy of the calculations. Nevertheless, this development offers a promising pathway toward scaling up quantum chemical computations on quantum computers, potentially enabling the study of larger and more complex molecular systems. The inherent advantage of QSCI lies in its deterministic energy evaluation, with errors stemming from model inaccuracies rather than statistical sampling, allowing for independent verification of results. 👉 More information 🗞 Doubling the size of quantum selected configuration interaction based on seniority-zero space and its application to QC-QSCI-AFQMC 🧠 ArXiv: https://arxiv.org/abs/2602.07912 Tags: