KT Scientific Publications – Kimika Teorikoa

@article{Lew-Yee2025,

title = {Efficient Energy Measurement of Chemical Systems via One-Particle Reduced Density Matrix: A NOF-VQE Approach for Optimized Sampling},

author = {Juan Felipe Huan Lew-Yee and Mario Piris},

doi = {10.1021/acs.jctc.4c01734},

issn = {1549-9626},

year  = {2025},

date = {2025-03-11},

urldate = {2025-03-11},

journal = {J. Chem. Theory Comput.},

volume = {21},

issue = {5},

pages = {2402-2413},

publisher = {American Chemical Society (ACS)},

abstract = {In this work, we explore the use of the one-particle reduced density matrix (1RDM) to streamline energy measurements of chemical systems on quantum computers, particularly within the variational quantum eigensolver (VQE) framework. This approach leverages the existence of an exact energy functional of the 1RDM, enabling a reduction in both the number of expectation values to measure and the number of circuits to execute, thereby optimizing quantum resource usage. Specifically, sampling the 1RDM involves measuring only  elements, which is significantly fewer than the  required for the Hamiltonian’s expectation value ⟨Ĥ⟩. We demonstrate our approach by harnessing the well-established natural orbital functional (NOF) theory, using the natural orbitals and occupation numbers derived from the diagonalization of the 1RDM measured from the quantum computer. Starting with the H2 system, we validate the accuracy of our method by comparing the energy derived from NOF approximations applied to the exact wave function with the value obtained from ⟨Ĥ⟩. This is followed by an optimization of the gate parameters by minimizing the energy using the NOF approximations as the objective function. The analysis is extended to larger systems, such as LiH, Li2, OH–, FH, NeH+, and F2 using a wave function ansatz with single and double excitation gates. This NOF-based method reduces the scaling cost of circuit executions compared to standard VQE implementations, achieving around 90% savings in the systems used in this work. Overall, by using a well-performing NOF as the objective function, the proposed NOF-VQE demonstrates the viability of NOF approximations for obtaining accurate energies in the noisy intermediate-scale quantum era and underscores the potential for developing new functionals tailored to quantum computing applications.},

keywords = {NOFT},

pubstate = {published},

tppubtype = {article}

}

82.

Boutou, Élodie; Lew-Yee, Juan Felipe Huan; Mercero, Jose M.; Piris, Mario

Enhancing the computational efficiency of the DoNOF program through a new orbital sorting scheme Journal Article

Advances in Quantum Chemistry, 2025, 91, 169--189

Abstract | Links | BibTeX | Tags: NOFT

@article{2025,

title = {Enhancing the computational efficiency of the DoNOF program through a new orbital sorting scheme},

author = {Élodie Boutou and Juan Felipe Huan Lew-Yee and Jose M. Mercero and Mario Piris},

doi = {10.1016/bs.aiq.2025.03.001},

isbn = {9780443343575},

year  = {2025},

date = {2025-02-04},

urldate = {2025-02-04},

booktitle = {Advances in Quantum Chemistry},

journal = {Advances in Quantum Chemistry},

volume = {91},

pages = {169--189},

publisher = {Elsevier},

abstract = {This work presents a novel approach to distribute orbitals into subspaces within electron-pairing-based natural orbital functionals (NOFs). This approach modifies the coupling between weakly and strongly occupied orbitals by applying an alternating orbital sorting strategy. In contrast to the previous orbital sorting that enforced electron pairing within subspaces of contiguous orbitals, the new approach provides greater flexibility, enabling a calculation scheme where the size of the subspaces can be gradually expanded. As a consequence, one can start using subspaces of only one weakly occupied orbital (perfect pairing) and progressively enlarge their size by incorporating more weakly occupied orbitals (extended pairing) up to the maximum size allowed by the basis set. In this way, the alternate orbital sorting allows solving first a simpler problem with small subspaces and leverage its orbital solution for the more intensive problem with larger subspaces, thereby reducing the overall computational cost and improving convergence, as we observed in the DoNOF program. The efficiency provided by the new sorting approach has been validated through benchmark calculations in H2O, H2O2, and NH3. In particular, we compared three strategies: (i) solving directly the calculation with the largest subspaces (one-shot strategy), as was usually done before this work, (ii) starting with perfect pairing and stepwise increasing the number of orbitals in the subspaces one by one until reaching the maximum size (incremental strategy), and (iii) starting with perfect pairing and transitioning directly to the maximum subspace size (two-step strategy). Our results show that the two-step approach emerges as the most effective strategy, achieving the lowest computational cost while maintaining high accuracy. These results confirm that the alternating orbital sorting scheme provides a robust and scalable framework for improving NOF calculations and could be particularly advantageous for extending these methods to larger and strongly correlated systems.},

keywords = {NOFT},

pubstate = {published},

tppubtype = {article}

}

This work presents a novel approach to distribute orbitals into subspaces within electron-pairing-based natural orbital functionals (NOFs). This approach modifies the coupling between weakly and strongly occupied orbitals by applying an alternating orbital sorting strategy. In contrast to the previous orbital sorting that enforced electron pairing within subspaces of contiguous orbitals, the new approach provides greater flexibility, enabling a calculation scheme where the size of the subspaces can be gradually expanded. As a consequence, one can start using subspaces of only one weakly occupied orbital (perfect pairing) and progressively enlarge their size by incorporating more weakly occupied orbitals (extended pairing) up to the maximum size allowed by the basis set. In this way, the alternate orbital sorting allows solving first a simpler problem with small subspaces and leverage its orbital solution for the more intensive problem with larger subspaces, thereby reducing the overall computational cost and improving convergence, as we observed in the DoNOF program. The efficiency provided by the new sorting approach has been validated through benchmark calculations in H2O, H2O2, and NH3. In particular, we compared three strategies: (i) solving directly the calculation with the largest subspaces (one-shot strategy), as was usually done before this work, (ii) starting with perfect pairing and stepwise increasing the number of orbitals in the subspaces one by one until reaching the maximum size (incremental strategy), and (iii) starting with perfect pairing and transitioning directly to the maximum subspace size (two-step strategy). Our results show that the two-step approach emerges as the most effective strategy, achieving the lowest computational cost while maintaining high accuracy. These results confirm that the alternating orbital sorting scheme provides a robust and scalable framework for improving NOF calculations and could be particularly advantageous for extending these methods to larger and strongly correlated systems.

81.

Piris, Mario; Lopez, Xabier; Ugalde, Jesus M.

Time-Resolved Chemical Bonding Structure Evolution by Direct-Dynamics Chemical Simulations Journal Article

J. Phys. Chem. Lett., 2024, 15, 12138-12143

Abstract | Links | BibTeX | Tags: NOFT