arXiv subjects

G. Andrés Cisneros

Publications and source records attributed to G. Andrés Cisneros.

5 recordsLinked to original sources

arXiv2026

Mitigating Noise-Induced Barren Plateaus Using a Non-Unitary Ansatz: Application to Molecular Electronic Transport

Variational quantum algorithms (VQAs) offer a promising route toward simulating many-body quantum systems on noisy intermediate-scale quantum (NISQ) hardware. However, their scalability is severely limited by noise-induced barren plateaus (NIBPs), where hardware noise causes the gradients of the cost function to vanish exponentially with circuit depth, rendering optimization impossible. In this work, we demonstrate that introducing nonunitary elements into the variational ansatz can mitigate NIBPs in open-quantum systems. Using an analytically tractable infinite-range dissipative Ising model, we show that a nonunitary ansatz restores finite gradients in the presence of depolarizing noise, enabling convergence to the correct symmetry-broken steady state. We also develop a Floquet-type variational ansatz in which each layer repeats the same parameters, reducing the deep variational circuit to an effective quantum channel whose fixed points can be analyzed directly. We then extend these ideas to a realistic quantum-chemistry system by simulating electron transport through Oligophenylethynylene-sulfurmethyl (OPE-SMe) using Hamiltonians and jump operators of the model derived from first-principles polarizable QM/MM calculations. Our results show that nonunitary variational ansätze provide a scalable and physically grounded route for simulating open-system steady states on NISQ hardware, offering a pathway to overcoming one of the limitations of current quantum hardware.

quant-ph↗

arXiv2024

Theoretical Study of Reactivity Indices and Rough Potential Energy Curves for the Dissociation of 59 Fullerendiols in Gas-Phase and in Aqueous Solution with an Implicit Solvent Model

Buckminsterfullerene, C$_{60}$, has not only a beautiful truncated icosahedral (soccerball) shape, but simple Hückel calculations predict a three-fold degenerate lowest unoccupied molecular orbital (LUMO) which can accomodate up to six electrons making it a good electron acceptor. Experiments have confirmed that C60 is a radical sponge and it is now sold for use in topical cosmetics. Further medical uses require functionalization of C60 to make it soluble and one of the simplest functionalization is to make C60(OH)n fullerenols. A previous article [Adv. Quant. Chem. 8, 351 (2023)] studied reactivity indices for the successive addition of the $^\bullet$OH radical to ($^\bullet$)C$_{60}$(OH)$_n$ in gas phase. [($^\bullet$)C$_{60}$(OH)$_n$ is only a radical when n is an odd number.] This present article extends this previous work by examining various aspects of how the reaction, changes in aqueous solution. One obvious difference between C$_{60}$ and their various isomers of C$_{60}$(OH)$_2$ is the presence of a dipole. As fullerendiols are nearly spherical, their change in dipole moment in going from gas to aqueous phase may be estimated using back-of-the-envellope calculations with the Onsager model. The result is remarkably similar to what is obtained using density-functional theory (DFT) and the more sophisticated solvation model based upon the quantum mechanical density (SMD). Calculation of fullerendiol C-O bond energies and reactivity indices using with the SMD approach confirm that the general conclusions from the earlier work regarding gas-phase reactivity still hold in the aqueous phase.

physics.chem-ph↗

arXiv2022

Development of the Quantum Inspired SIBFA Many-Body Polarizable Force Field: Enabling Condensed Phase Molecular Dynamics Simulations

We present the extension of the SIBFA (Sum of Interactions Between Fragments Ab initio Computed many-body polarizable force field to condensed phase Molecular Dynamics (MD) simulations. The Quantum-Inspired SIBFA procedure is grounded on simplified integrals obtained from localized molecular orbital theory and achieves full separability of its intermolecular potential. It embodies long-range multipolar electrostatics (up to quadrupole) coupled to a short-range penetration correction (up to charge-quadrupole), exchange-repulsion, many-body polarization, many-body charge transfer/delocalization, exchange-dispersion and dispersion (up to C10). This enables the reproduction of all energy contributions of ab initio Symmetry-Adapted Perturbation Theory (SAPT(DFT)) gas phase reference computations. The SIBFA approach has been integrated within the Tinker-HP massively parallel MD package. To do so all SIBFA energy gradients have been derived and the approach has been extended to enable periodic boundary conditions simulations using Smooth Particle Mesh Ewald. This novel implementation also notably includes a computationally tractable simplification of the many-body charge transfer/delocalization contribution. As a proof of concept, we perform a first computational experiment defining a water model fitted on a limited set of (SAPT(DFT)) data. SIBFA is shown to enable a satisfactory reproduction of both gas phase energetic contributions and condensed phase properties highlighting the importance of its physically-motivated functional form.

physics.chem-ph↗

arXiv2021

Computational Investigations of Selected Enzymes From Two Iron and α-ketoglutarate-Dependent Families

DNA alkylation is used as the key epigenetic mark in eukaryotes, however, most alkylation in DNA can result in deleterious effects. Therefore, this process needs to be tightly regulated. AlkB and TET are families within the Fe and $α$-kg-dependent superfamily of enzymes that are tasked with dealkylating DNA and RNA in cells. Members of these families span all species and are an integral part of transcriptional regulation. While both families catalyze oxidative dealkylation of various bases, each has specific preference for alkylated base type as well as distinct catalytic mechanisms. This perspective aims to provide an overview of computational work carried out to investigate several members of these enzyme families including AlkB, ALKBH2, ALKBH3 and TET2. Insights into structural details, mutagenesis studies, reaction path analysis, electronic structure features in the active site, and substrate preferences are presented and discussed

physics.bio-ph↗

arXiv2020

Development and Application of QM/MM Methods with Advanced Polarizable Potentials

Quantum Mechanics/Molecular Mechanics (QM/MM) simulations are a popular approach to study various features of large systems. A common application of QM/MM calculations is in the investigation of reaction mechanisms in condensed-phase and biological systems. The combination of QM and MM methods to represent a system gives rise to several challenges that need to be addressed. The increase in computational speed has allowed the expanded use of more complicated and accurate methods for both QM and MM simulations. Here, we review some approaches that address several common challenges encountered in QM/MM simulations with advanced polarizable potentials, from methods to account for boundary across covalent bonds and long-range effects, to polarization and advanced embedding potentials.

physics.chem-ph↗