arXiv subjects

Leonardo Guidoni

Publications and source records attributed to Leonardo Guidoni.

At least 19 recordsLinked to original sources

arXiv2025

Multi-QIDA method for VQE state preparation in molecular systems

The development of quantum algorithms and their application to quantum chemistry has introduced new opportunities for solving complex molecular problems that are computationally infeasible for classical methods. In quantum chemistry, the Variational Quantum Eigensolver (VQE) is a hybrid quantum-classical algorithm designed to estimate ground-state energies of molecular systems. Despite its promise, VQE faces challenges such as scalability issues, high circuit depths, and barren plateaus that make the optimization of the variational wavefunction. To mitigate these challenges, the Quantum Information Driven Ansatz (QIDA) leverages Quantum Mutual Information (QMI) to construct compact, correlation-driven circuits. In this work, we go back to the original field of application of QIDA, by applying the already defined Multi-Threshold Quantum Information Driven Ansatz (Multi-QIDA) methodology on Molecular Systems. to systematically construct shallow, layered quantum circuits starting from approximate QMI matrices obtained by Quantum Chemistry calculations. The Multi-QIDA approach combines efficient creation of the QMI map, reduction of the number of correlators required by exploiting Minimum/Maximum spanning tress, and an iterative layer-wise VQE optimization routine. These enhancements allow the method to recover missing correlations in molecular systems while maintaining computational efficiency. Additionally, the approach incorporates alternative gate constructions, such as SO(4) correlators, to enhance the circuit expressibility without significantly increasing the circuit complexity. We benchmark Multi-QIDA on systems ranging from small molecules like H2O, BeH2, and NH3 in Iterative Natural Orbitals (INOs) basis set, to active-space models such as H2O-6-31G-CAS(4,4) and N2-cc-pVTZ-CAS(6,6), comparing it to traditional hardware-efficient ansatze.

quant-ph↗

arXiv2025

AEGISS -- Atomic orbital and Entropy-based Guided Inference for Space Selection -- A novel semi-automated active space selection workflow for quantum chemistry and quantum computing applications

The selection of a balanced active space is a critical step in multi-reference quantum chemistry calculations, particularly for systems with strong electron correlation. Likewise, active space selection is a key to unlock the potential of contemporary quantum computing in quantum chemistry. Albeit recent progress, there remains a lack of a unified, robust, and fully automated framework for active space selection that performs reliably across a wide range of molecular systems. In this work, we present a novel approach inspired by both the AVAS (Atomic Valence Active Space) and AutoCAS methods. Our method unifies orbital entropy analysis with atomic orbital projections to guide the construction of chemically and physically meaningful active spaces. This integrated scheme enables a more consistent and flexible selection of active orbitals while retaining automation and scalability. We validate our approach on a set of molecular systems relevant to photodynamic therapy, in particular a set of Ru(II)-complexes, selected to span increasing levels of electron correlation and structural complexity. These molecules serve as challenging test cases due to the presence of strong static correlation and the need for highly accurate electronic structure descriptions. Our results demonstrate that the method can reliably identify compact, chemically intuitive active spaces that capture the essential physics, making it suitable for both classical and quantum computational frameworks. Furthermore, we have developed this approach in a package that is intuitive to use for users and can be interfaced with both standard quantum chemistry and quantum computing applications, making it accessible to a broad research community.

physics.chem-ph↗

arXiv2024

Quantum information theory on sparse wavefunctions and applications for Quantum Chemistry

In recent years Quantum Computing prominently entered in the field of Computational Chemistry, importing and transforming computational methods and ideas originally developed within other disciplines, such as Physics, Mathematics and Computer Science into algorithms able to estimate quantum properties of atoms and molecules on present and future quantum devices. An important role in this contamination process is attributed to Quantum Information techniques, having the twofold role of contributing to the analysis of electron correlation and entanglements and guiding the construction of wavefunction variational ansatzes for the Variational Quantum Eigensolver technique. This paper introduces the tool SparQ (Sparse Quantum state analysis), designed to efficiently compute fundamental quantum information theory observables on post-Hartree-Fock wavefunctions sparse in their definition space. The core methodology involves mapping fermionic wavefunctions to qubit space using fermionic-to-qubits transformations and leveraging the sparse nature of these wavefunctions to evaluate observables and properties of the wavefunction. The effectiveness of SparQ is validated by analyzing the mutual information matrices of wavefunctions for the water molecule and the total entropy of $\sim 10^2$ qubits describing the benzene molecule. This highlights its capability to handle large-scale quantum systems, limited mainly by the capabilities of quantum chemical methods used to retrieve the wavefunctions. The results indicate that quantum information theoretical analysis, so far limited to traditional tensor network methods and study of transition operators, can be applied to all post-Hartree-Fock wavefunctions, extending their applications to larger and more complex chemical systems.

quant-ph↗

arXiv2024

Compact Multi-Threshold Quantum Information Driven Ansatz For Strongly Interactive Lattice Spin Models

Quantum algorithms based on the variational principle have found applications in diverse areas with a huge flexibility. But as the circuit size increases the variational landscapes become flattened, causing the so-called Barren plateau phenomena. This will lead to an increased difficulty in the optimization phase, due to the reduction of the cost function parameters gradient. One of the possible solutions is to employ shallower circuits or adaptive ansätze. We introduce a systematic procedure for ansatz building based on approximate Quantum Mutual Information (QMI) with improvement on each layer based on the previous Quantum Information Driven Ansatz (QIDA) approach. Our approach generates a layered-structured ansatz, where each layer's qubit pairs are selected based on their QMI values, resulting in more efficient state preparation and optimization routines. We benchmarked our approach on various configurations of the Heisenberg model Hamiltonian, demonstrating significant improvements in the accuracy of the ground state energy calculations compared to traditional heuristic ansatz methods. Our results show that the Multi-QIDA method reduces the computational complexity while maintaining high precision, making it a promising tool for quantum simulations in lattice spin models.

quant-ph↗

arXiv2024

Quantum Information reveals that orbital-wise correlation is essentially classical in Natural Orbitals

The intersection of Quantum Chemistry and Quantum Computing has led to significant advancements in understanding the potential of using quantum devices for the efficient calculation of molecular energies. Simultaneously, this intersection is enhancing the comprehension of quantum chemical properties through the use of quantum computing and quantum information tools. This paper tackles a key question in this relationship: Is the nature of the orbital-wise electron correlations in wavefunctions of realistic prototypical cases classical or quantum? We delve into this inquiry with a comprehensive examination of molecular wavefunctions using Shannon and von Neumann entropies, alongside classical and quantum information theory. Our analysis reveals a notable distinction between classical and quantum mutual information in molecular systems when analyzed with Hartree-Fock canonical orbitals. However, this difference decreases dramatically, by approximately 100-fold, when Natural Orbitals are used as reference. This finding suggests that wavefunction correlations, when viewed through the appropriate orbital basis, are predominantly classical. This insight indicates that computational tasks in quantum chemistry could be significantly simplified by employing Natural Orbitals. Consequently, our study underscores the importance of using Natural Orbitals to accurately assess molecular wavefunction correlations and to avoid their overestimation. In summary, our results suggest a promising path for computational simplification in quantum chemistry, advocating for the wider adoption of Natural Orbitals and raising questions about the actual computational complexity of the multi-body problem in quantum chemistry.

quant-ph↗

arXiv2023

Quantum Information Driven Ansatz (QIDA): shallow-depth empirical quantum circuits from Quantum Chemistry

Hardware-efficient empirical variational ansätze for Variational Quantum Eigensolver simulations of Quantum Chemistry suffer from the lack of a direct connection to classical Quantum Chemistry methods. In the present work, we propose a method to fill this gap by introducing a new approach for constructing variational quantum circuits, leveraging quantum mutual information associated with classical Quantum Chemistry states to design simple yet effective heuristic ansätze with a topology that reflects the correlations of the molecular system. As first step, Quantum Chemistry calculations, such as Møller-Plesset (MP2) perturbation theory, firstly provide an approximate Natural Orbitals basis, which has been recently shown to be the best candidate one-electron basis for developing compact empirical wavefunctions (Ratini, et al 2023). Secondly, throughout the evaluation of quantum mutual information matrices, they provide information about the main correlations between qubits of the quantum circuit, enabling the development of a direct design of entangling blocks for the circuit. The resulting ansatz is then utilized with a Variational Quantum Eigensolver (VQE) to obtain a short depth variational groundstate of the electronic Hamiltonian. To validate our approach, we perform a comprehensive statistical analysis by simulations over various molecular systems ($H_2, LiH, H_2O$) and apply it to the more complex $NH_3$ molecule. The reported results demonstrate that the proposed methodology gives rise to highly effective ansätze, surpassing the standard empirical ladder-entangler ansatz in performance. Overall, our approach can be used as effective state preparation providing a promising route for designing efficient variational quantum circuits for large molecular systems.

quant-ph↗

arXiv2023

Pulse variational quantum eigensolver on cross-resonance based hardware

State-of-the-art noisy digital quantum computers can only execute short-depth quantum circuits. Variational algorithms are a promising route to unlock the potential of noisy quantum computers since the depth of the corresponding circuits can be kept well below hardware-imposed limits. Typically, the variational parameters correspond to virtual $R_Z$ gate angles, implemented by phase changes of calibrated pulses. By encoding the variational parameters directly as hardware pulse amplitudes and durations we succeed in further shortening the pulse schedule and overall circuit duration. This decreases the impact of qubit decoherence and gate noise. As a demonstration, we apply our pulse-based variational algorithm to the calculation of the ground state of different hydrogen-based molecules (H$_2$, H$_3$ and H$_4$) using IBM cross-resonance-based hardware. We observe a reduction in schedule duration of up to $5\times$ compared to CNOT-based Ansätze, while also reducing the measured energy. In particular, we observe a sizable improvement of the minimal energy configuration of H$_3$ compared to a CNOT-based variational form. Finally, we discuss possible future developments including error mitigation schemes and schedule optimizations, which will enable further improvements of our approach paving the way towards the simulation of larger systems on noisy quantum devices.

quant-ph↗

arXiv2023

Natural orbitals and sparsity of quantum mutual information

Natural orbitals, defined in electronic structure and quantum chemistry as the (molecular) orbitals diagonalizing the one-particle reduced density matrix of the ground state, have been conjectured for decades to be the perfect reference orbitals to describe electron correlation. In the present work we applied the Wavefunction-Adapted Hamiltonian Through Orbital Rotation (WAHTOR) method to study correlated empirical ansätze for quantum computing. In all representative molecules considered, we show that the converged orbitals are coinciding with natural orbitals. Interestingly, the resulting quantum mutual information matrix built on such orbitals is also maximally sparse, providing a clear picture that such orbital choice is indeed able to provide the optimal basis to describe electron correlation. The correlation is therefore encoded in a smaller number of qubit pairs contributing to the quantum mutual information matrix.

quant-ph↗

arXiv2023

Optimization strategies in WAHTOR algorithm for quantum computing empirical ansatz: a comparative study

By exploiting the invariance of the molecular Hamiltonian by a unitary transformation of the orbitals it is possible to significantly shorter the depth of the variational circuit in the Variational Quantum Eigensolver (VQE) algorithm by using the Wavefunction Adapted Hamiltonian Through Orbital Rotation (WAHTOR) algorithm. This work introduces a non-adiabatic version of the WAHTOR algorithm and compares its efficiency with three implementations by estimating Quantum Processing Unit (QPU) resources in prototypical benchmarking systems. Calculating first and second-order derivatives of the Hamiltonian at fixed VQE parameters does not introduce a significant QPU overload, leading to results on small molecules that indicate the non-adiabatic Newton-Raphson method as the more convenient choice. On the contrary, we find out that in the case of Hubbard model systems the trust region non-adiabatic optimization is more efficient. The preset work therefore clearly indicates the best optimization strategies for empirical variational ansatzes, facilitating the optimization of larger variational wavefunctions for quantum computing.

quant-ph↗

arXiv2021

Improved accuracy on noisy devices by non-unitary Variational Quantum Eigensolver for chemistry applications

We propose a modification of the Variational Quantum Eigensolver algorithm for electronic structure optimization using quantum computers, named non-unitary Variational Quantum Eigensolver (nu-VQE), in which a non-unitary operator is combined with the original system Hamiltonian leading to a new variational problem with a simplified wavefunction Ansatz. In the present work, we use, as non-unitary operator, the Jastrow factor, inspired from classical Quantum Monte Carlo techniques for simulation of strongly correlated electrons. The method is applied to prototypical molecular Hamiltonians for which we obtain accurate ground state energies with shallower circuits, at the cost of an increased number of measurements. Finally, we also show that this method achieves an important error mitigation effect that drastically improves the quality of the results for VQE optimizations on today's noisy quantum computers. The absolute error in the calculated energy within our scheme is one order of magnitude smaller than the corresponding result using traditional VQE methods, with the same circuit depth.

quant-ph↗

arXiv2018

Theoretical S1 <- S0 absorption energies of the anionic forms of oxyluciferin by Variational Monte Carlo and Many Body Green's Function Theory

The structures of three negatively charged forms (anionic keto-1 and enol-1, dianonic enol-2) of oxyluciferin (OxyLuc), which are the most probable emitters responsible for the firefly bioluminescence, have been fully relaxed at the variational Monte Carlo (VMC) level. Absorption energies of the S1<-S0 vertical transition have been computed using different levels of theory, such as TDDFT, CC2 and many body Green's function Theory (MBGFT). The use of MBGFT, by means of the Bethe-Salpeter (BS) formalism, on VMC structures provides results in excellent agreement with the value (2.26(8) eV) obtained by action spectroscopy experiments for the keto-1 form (2.32 eV). To unravel the role of the quality of the optimized ground state geometry, BS excitation energies have also been computed on CASSCF geometries, inducing a non negligible blue shift (0.08 and 0.07 eV for keto-1 and enol-1 forms, respectively) with respect to the VMC ones. Structural effects have been analyzed in terms of over- or under-correlation along the conjugated bonds of OxyLuc by using different methods for the ground-state optimization. The relative stability of the S$_1$ state for the keto-1 and enol-1 forms depends on the method chosen for the excited state calculation, thus representing a fundamental caveat for any theoretical study on these systems. Finally, Kohn-Sham HOMO and LUMO orbitals of enol-2 are (nearly) bound only when the dianion is embedded into a solvent (water and toluene in the present work); excited state calculations are therefore meaningful only in presence of a dielectric medium which localizes the electronic density. The combination of VMC for the ground state geometry and BS formalism for the absorption spectra clearly outperforms standard TDDFT and quantum chemistry approaches.

physics.chem-ph↗

arXiv2015

Ab-initio molecular dynamics simulation of liquid water by Quantum Monte Carlo

Although liquid water is ubiquitous in chemical reactions at roots of life and climate on the earth, the prediction of its properties by high-level ab initio molecular dynamics simulations still represents a formidable task for quantum chemistry. In this article we present a room temperature simulation of liquid water based on the potential energy surface obtained by a many-body wave function through quantum Monte Carlo (QMC) methods. The simulated properties are in good agreement with recent neutron scattering and X-ray experiments, particularly concerning the position of the oxygen-oxygen peak in the radial distribution function, at variance of previous Density Functional Theory attempts. Given the excellent performances of QMC on large scale supercomputers, this work opens new perspectives for predictive and reliable ab-initio simulations of complex chemical systems.

physics.chem-ph↗

arXiv2014

Static and dynamical correlation in diradical molecules by Quantum Monte Carlo using the Jastrow Antisymmetrized Geminal Power ansatz

Diradical molecules are essential species involved in many organic and inorganic chemical reactions. The computational study of their electronic structure is often challenging, because a reliable description of the correlation, and in particular of the static one, requires multi-reference techniques. The Jastrow correlated Antisymmetrized Geminal Power (JAGP) is a compact and efficient wave function ansatz, based on the valence-bond representation, which can be used within Quantum Monte Carlo (QMC) approaches. The AGP part can be rewritten in terms of molecular orbitals, obtaining a multi-determinant expansion with zero-seniority number. In the present work we demonstrate the capability of the JAGP ansatz to correctly describe the electronic structure of two diradical prototypes: the orthogonally twisted ethylene, C2H4, and the methylene, CH2, representing respectively a homosymmetric and heterosymmetric system. On the other hand, we show that the simple ansatz of a Jastrow correlated Single Determinant (JSD) wave function is unable to provide an accurate description of the electronic structure in these diradical molecules, both at variational level and, more remarkably, in the fixed-nodes projection schemes showing that a poor description of the static correlation yields an inaccurate nodal surface. The suitability of JAGP to correctly describe diradicals with a computational cost comparable with that of a JSD calculation, in combination with a favorable scalability of QMC algorithms with the system size, opens new perspectives in the ab initio study of large diradical systems, like the transition states in cycloaddition reactions and the thermal isomerization of biological chromophores.

physics.chem-ph↗

arXiv2014

Properties of Reactive Oxygen Species by Quantum Monte Carlo

The electronic properties of the oxygen molecule, in its singlet and triplet states, and of many small oxygen-containing radicals and anions have important roles in different fields of Chemistry, Biology and Atmospheric Science. Nevertheless, the electronic structure of such species is a challenge for ab-initio computational approaches because of the difficulties to correctly describe the statical and dynamical correlation effects in presence of one or more unpaired electrons. Only the highest-level quantum chemical approaches can yield reliable characterizations of their molecular properties, such as binding energies, equilibrium structures, molecular vibrations, charge distribution and polarizabilities. In this work we use the variational Monte Carlo (VMC) and the lattice regularized Monte Carlo (LRDMC) methods to investigate the equilibrium geometries and molecular properties of oxygen and oxygen reactive species. Quantum Monte Carlo methods are used in combination with the Jastrow Antisymmetrized Geminal Power (JAGP) wave function ansatz, which has been recently shown to effectively describe the statical and dynamical correlation of different molecular systems. In particular we have studied the oxygen molecule, the superoxide anion, the nitric oxide radical and anion, the hydroxyl and hydroperoxyl radicals and their corresponding anions, and the hydrotrioxyl radical. Overall, the methodology was able to correctly describe the geometrical and electronic properties of these systems, through compact but fully-optimised basis sets and with a computational cost which scales as $N^3-N^4$, where $N$ is the number of electrons. This work is therefore opening the way to the accurate study of the energetics and of the reactivity of large and complex oxygen species by first principles.

physics.chem-ph↗

arXiv2014

Ab initio geometry and bright excitation of carotenoids: Quantum Monte Carlo and Many Body Green's Function Theory calculations on peridinin

In this letter we report the singlet ground state structure of the full carotenoid peridinin by means of variational Monte Carlo (VMC) calculations. The VMC relaxed geometry has an average bond length alternation of 0.1165(10) Å, larger than the values obtained by DFT (PBE, B3LYP and CAM-B3LYP) and shorter than that calculated at the Hartree-Fock (HF) level. TDDFT and EOM-CCSD calculations on a reduced peridinin model confirm the HOMO-LUMO major contribution of the Bu+-like (S2) bright excited state. Many Body Green's Function Theory (MBGFT) calculations of the vertical excitation energy of the Bu+-like state for the VMC structure (VMC/MBGFT) provide excitation energy of 2.62 eV, in agreement with experimental results in n-hexane (2.72 eV). The dependence of the excitation energy on the bond length alternation in the MBGFT and TDDFT calculations with different functionals is discussed.

physics.chem-ph↗

arXiv2013

Molecular properties by Quantum Monte Carlo: an investigation on the role of the wave function ansatz and the basis set in the water molecule

Quantum Monte Carlo methods are accurate and promising many body techniques for electronic structure calculations which, in the last years, are encountering a growing interest thanks to their favorable scaling with the system size and their efficient parallelization, particularly suited for the modern high performance computing facilities. The ansatz of the wave function and its variational flexibility are crucial points for both the accurate description of molecular properties and the capabilities of the method to tackle large systems. In this paper, we extensively analyze, using different variational ansatzes, several properties of the water molecule, namely: the total energy, the dipole and quadrupole momenta, the ionization and atomization energies, the equilibrium configuration, and the harmonic and fundamental frequencies of vibration. The investigation mainly focuses on variational Monte Carlo calculations, although several lattice regularized diffusion Monte Carlo calculations are also reported. Through a systematic study, we provide a useful guide to the choice of the wave function, the pseudo potential, and the basis set for QMC calculations. We also introduce a new strategy for the definition of the atomic orbitals involved in the Jastrow - Antisymmetrised Geminal power wave function, in order to drastically reduce the number of variational parameters. This scheme significantly improves the efficiency of QMC energy minimization in case of large basis sets.

physics.chem-ph↗

arXiv2013

Optimized structure and vibrational properties by error affected potential energy surfaces

The precise theoretical determination of the geometrical parameters of molecules at the minima of their potential energy surface and of the corresponding vibrational properties are of fundamental importance for the interpretation of vibrational spectroscopy experiments. Quantum Monte Carlo techniques are correlated electronic structure methods promising for large molecules, which are intrinsically affected by stochastic errors on both energy and force calculations, making the mentioned calculations more challenging with respect to other more traditional quantum chemistry tools. To circumvent this drawback in the present work we formulate the general problem of evaluating the molecular equilibrium structures, the harmonic frequencies and the anharmonic coefficients of an error affected potential energy surface. The proposed approach, based on a multidimensional fitting procedure, is illustrated together with a critical evaluation of systematic and statistical errors. We observe that the use of forces instead of energies in the fitting procedure reduces the the statistical uncertainty of the vibrational parameters by one order of magnitude. Preliminary results based on Variational Monte Carlo calculations on the water molecule demonstrate the possibility to evaluate geometrical parameters, harmonic and anharmonic coefficients at this level of theory with an affordable computational cost and a small stochastic uncertainty (<0.07% for geometries and <0.7% for vibrational properties).

cond-mat.str-el↗

arXiv2013

Tautomerism in liquid 1,2,3-triazole: a combined Energy-Dispersive X-Ray Diffraction, Molecular Dynamics and FTIR study

In this work, we report a multitechnique (energy-dispersive X-Ray diffraction, computational methods and FT-IR spectroscopy) study of the tautomeric equilibrium of 1,2,3-triazole, one of the few small nitrogen-containing eterocycles liquid at room temperature. The T-2H form (C2v symmetry) is found to be strongly favored in gas and solid phases, whereas the neat liquid gives diffraction patterns that can be interpreted satisfactorily with the structure functions calculated from some molecular dynamics results for both T-2H and T-1H tautomers, although the T-2H form gives a slightly better agreement.

physics.chem-ph↗