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Ali Alavi

Publications and source records attributed to Ali Alavi.

At least 19 recordsLinked to original sources
arXiv2026

Chordwise micro-jet placement reveals a trade-off between mean hydrodynamic performance and unsteady loading under cloud cavitation

Prescribed tangential micro-jet injection can modify cloud-cavitation dynamics, but the chordwise location that improves mean hydrodynamic performance may differ from the location that minimizes unsteady loading. This study compares five injection locations (x/c = 0.15, 0.30, 0.45, 0.60, and 0.70) on a Clark-Y hydrofoil at Re = 7 x 10^5 and cavitation number 0.8. Transient large-eddy simulation with a Volume-of-Fluid formulation and the Schnerr-Sauer cavitation model is used in a two-dimensional parametric framework, with one representative three-dimensional case included to illustrate spanwise cavity deformation. The analysis considers vapor topology, turbulent kinetic energy, velocity and pressure fields, cycle-averaged surface pressure, hydrodynamic forces, force fluctuations, spectra, and cavity-thickness histories. Injection at x/c = 0.15 gives the highest cycle-averaged lift-to-drag ratio among the tested cases: drag decreases from 0.137 to 0.107 and the lift-to-drag ratio increases from 5.693 to 6.261, while lift decreases from 0.78 to 0.67. In contrast, x/c = 0.60 gives the lowest recorded force-fluctuation RMS, with lift- and drag-coefficient RMS values of about 0.112 and 0.0165. The preferred injection location is therefore objective-dependent: the location favored by mean hydrodynamic performance does not coincide with the location favored by unsteady-load reduction under the present conditions.

physics.flu-dyn↗
arXiv2026

Compression of virtual spaces in transcorrelated methods via singular value decomposition: application to the G2 set

We introduce a new singular-value-decomposition-based scheme for constructing small virtual spaces out of large basis sets for transcorrelated (TC) calculations, termed SVD-TC. This work builds on the recent finding that the residual basis error in the TC reference energy converges more slowly than that of the correlation energy. Within the new workflow, the post Hartree-Fock TC calculation is performed in a compressed virtual orbital subspace, obtained by projecting the canonical virtual orbitals from a large basis set onto a smaller basis set through singular value decomposition (SVD). This allows us to achieve the high accuracy allowed by the large basis, whilst the bottleneck steps - TC integral calculation and post-HF correlation method such as CCSD(T) - incur the cost of only a small virtual space calculation. The method therefore is highly efficient, whilst avoiding the composite nature of the reference correction method. Using the new scheme, we widen the scope of benchmark-quality TC results into more complex molecules than previously considered: using the G2-1 set of 55 molecules with first- and second-row atoms, we apply SVD-xTC-CCSD(T) to compute atomization energies. We compare our results against the near-exact semistochastic heat-bath configuration interaction (SHCI) reference values and experiment. We find that SVD-xTC-CCSD(T) delivers chemical accuracy already with triple-$\zeta$ basis sets. Finally, we use the quadruple-$\zeta$ results to analyze the accuracy of pseudopotentials within the TC method, and show that pseudopotential TC workflow provides faster basis-set convergence than all-electron TC. We also present timings for computing the atomization energies on G2-1 set, demonstrating the efficiency of our TC workflows.

physics.chem-ph↗
arXiv2026

Interpolative Separable Density-Fitting for Transcorrelated Hamiltonians

The transcorrelated (TC) method dramatically accelerates the convergence of correlated calculations toward the complete-basis-set (CBS) limit by folding a Jastrow correlator into the Hamiltonian via a similarity transformation, incorporating the electron--electron cusp into the effective interaction. We make the TC framework practical for large systems and flexible, multi-center correlators by compressing the grid-evaluated TC integrals with the interpolative separable density-fitting (ISDF) approximation, combined with the effective two-body (xTC) treatment of the three-body operator. This low-rank representation reduces storage and integration costs by orders of magnitude, and a multi-GPU implementation with automatic differentiation of the correlator makes the construction routine for large basis sets. We demonstrate the resulting ISDF-xTC-CCSD method on the linear hydrogen chain, reaching the joint thermodynamic and CBS limits with basis sets up to cc-pV5Z in agreement with state-of-the-art many-body references to within about 1~mHa/atom, and on the benzene ground-state energy with up to 1200 orbitals (cc-pCV5Z), where the method attains state-of-the-art accuracy at the coupled cluster singles and doubles level and its CBS extrapolation is markedly more robust than that of conventional coupled-cluster methods.

physics.chem-ph↗
arXiv2026

Full configuration interaction quantum Monte Carlo for accurate $\textit{ab initio}$ nuclear structure calculations: algorithms and calculation details

Full configuration interaction quantum Monte Carlo (FCIQMC) is a stochastic many-body solver that has been widely applied to electronic, molecular, and condensed-matter systems. In this work we apply FCIQMC to $\textit{ab initio}$ nuclear structure calculations using interactions derived from chiral effective field theory. We describe the algorithm in detail, including imaginary-time propagation, excitation generation, estimator choices, the initiator approximation with adaptive shift correction, and reduced-density-matrix (RDM) sampling. Benchmark calculations in small model spaces, where deterministic full configuration interaction (FCI) results are available, validate the stochastic calculation of energies, radii, and RDM-based pure estimators. For large model spaces, we analyze the residual finite-walker bias through systematic walker-number convergence and infinite-walker extrapolations. We also demonstrate that FCIQMC can be extended beyond ground-state calculations by computing the low-lying spectrum of $^6$Li.

nucl-th↗
arXiv2026

An Additive Reference Correction Scheme for the Transcorrelated Method

We introduce an additive reference correction for the transcorrelated (TC) method and its three-body mean-field approximation (xTC), to improve energy differences computed in small orbital basis sets. The correction is motivated by the observation that, for xTC atomization energies, the dominant error in double-{\zeta} bases originates from the reference contribution rather than from the correlation energy. In the proposed reference-corrected scheme (RC-xTC), the small-basis correlation energy is retained, while the corresponding TC reference energy is replaced by its value from a larger basis. Benchmark calculations for the non-relativistic HEAT set with the Dunning basis-set family show that RC-xTC substantially improves both total and atomization energies relative to standard xTC in double-{\zeta} bases. At the CCSD(T) level, RC-xTC yields better atomization energies than CCSD(T)-F12a in the double-{\zeta} regime, while preserving the favorable total-energy accuracy of xTC. At the CCSD level, RC-xTC improves atomization energies relative to F12a throughout the full basis-set sequence. As the basis set is enlarged, xTC and RC-xTC become progressively identical, as expected from the construction of the correction.

physics.chem-ph↗
arXiv2026

Full Configuration Interaction Quantum Monte Carlo for Accurate $\textit{Ab Initio}$ Nuclear Structure Calculations

We introduce novel full configuration interaction quantum Monte Carlo (FCIQMC) as an accurate many-body solver for $\textit{ab initio}$ nuclear structure calculations. This stochastic approach directly samples the exact wave function in the full configuration space, enabling high-fidelity treatment of high-order many-body correlations in strongly interacting nuclear systems. Using interactions from chiral effective field theory, we have computed ground-state energies and charge radii of $^4$He, $^8$Be, $^{12}$C and $^{16}$O with sub-percent-level many-body uncertainties. These results establish FCIQMC as a stochastic full-configuration-space solver capable of treating systems beyond the reach of the conventional no-core shell model, and as an accurate benchmark for truncated many-body expansion methods.

nucl-th↗
arXiv2026

TLG: Temporal-Logic Grounding for Video Question Answering via Source-Annotation Reconstruction and Category-Targeted Reasoning

The TimeLogic Challenge evaluates formal temporal-logic reasoning over video - 16 operators (before, after, until, since, always, co-occur, ordering, ...) in boolean and 4-way multiple-choice form. End-to-end video-language models (VLMs) hover near chance on this task because they treat video as a bag of frames and cannot localize when actions occur. We present TLG (Temporal-Logic Grounding), a three-tier system that (i) reconstructs each video's action timeline from the public source-dataset annotations the benchmark was generated from, parses every question into a temporal-logic program, and executes it deterministically; (ii) falls back to a strong open VLM where no annotation exists; and (iii) routes only the question categories where the VLM is empirically weakest to a frontier reasoning model. TLG raises test accuracy from a 46.9% VLM baseline to 71.37%, a +24.5 absolute gain, reaching within 3 points of the leaderboard top. We report extensive ablations, including three model-based timeline-reconstruction variants that all underperform a holistic VLM, isolating temporal grounding as the irreducible bottleneck and showing that real annotations - not larger models - drive accuracy.

cs.CV↗
arXiv2026

Perception First: A Frontier Native-Video Model with Self-Consistency for Implicit Video Question Answering

We describe our submission to the VRR Challenge @ CVPR 2026, built on the \emph{ImplicitQA} / \emph{VRR-QA} benchmark~\cite{implicitqa}: multiple-choice video question answering in which answers are deliberately \emph{not} observable in any single frame and must be inferred from spatial layout, motion, depth, viewpoint, causality, and social context across discontinuous frames of creative video. We conduct a systematic, training-free study spanning open-source Video-LMMs (Qwen2.5-VL~\cite{qwen25vl}, Qwen3-VL~\cite{qwen3vl}, InternVL3, Gemma-3, and the RL-tuned video reasoners Video-R1~\cite{videor1} and VideoChat-R1.5~\cite{videochatr15}) and a battery of inference-time strategies (chain-of-thought, question decomposition, describe-then-reason cascades, audio transcripts, spatial state prompting, self-consistency~\cite{selfconsistency}, multi-model ensembling, and category routing). Our central finding is that this benchmark is \emph{perception-bound rather than reasoning-bound}: reasoning-side augmentations are neutral-to-harmful, whereas base-model perceptual capability and lightweight test-time denoising are the only reliable levers. A per-category error analysis localizes the difficulty to low-level perception -- relative depth, viewpoint, and counting are the hardest categories, while causal and social reasoning are nearly solved -- and a prompt that explicitly injects monocular depth cues to attack the weakest category \emph{lowers} test accuracy by $5.8$ points, confirming that the model needs a better \emph{percept}, not a better \emph{procedure}.

cs.CV↗
arXiv2026

Reason, Retrieve, Re-rank: A Zero-Shot Reasoning-Aware Framework for Composed Video Retrieval

Composed Video Retrieval (CoVR) seeks the target video that results from applying a free-form textual modification to a reference video. We address the \emph{Reason-Aware} CoVR (CoVR-R) challenge at the CVPR~2026 VidLLMs workshop, where retrieval is strictly zero-shot. We present \textbf{R3-CoVR} (\emph{Reason, Retrieve, Re-rank}), a training-free pipeline built entirely from frozen foundation models. A multimodal large language model (Qwen3-VL-8B) reasons about the \emph{after-effects} an edit implies -- state transitions, action phases, scene, camera and tempo -- and verbalises a concise post-edit description; a contrastive video--text encoder (SigLIP-2) embeds this description and the gallery for first-stage retrieval; finally a constraint-aware re-ranking stage uses the same multimodal model as a judge that scores each shortlisted candidate against the intended edited result. On the challenge test set, R3-CoVR attains \textbf{91.9\% R@1} and \textbf{98.2\% R@10}. Two findings drive these results: (i)~matching the description length to the contrastive encoder's text window lifts \Rk{1} from $67.5$ to $72.7$; and (ii)~the constraint-aware re-ranker, which reorders only the shortlist, lifts \Rk{1} from $72.7$ to $91.9$ -- the single largest gain. We analyse the re-ranker's behaviour, the retrieve/re-rank blend, and the shortlist depth, and we release a clean three-layer implementation.

cs.CV↗
arXiv2025

A Transcorrelated Wave-Function Framework for Solids: An Application to Bulk and Defected Silicon

Accurate wave-function descriptions of pristine and defected solids remain challenging due to the simultaneous presence of finite-size, basis-set, and correlation errors. While embedding techniques alleviate finite-size effects and correlated wave-function approaches systematically improve correlation, basis-set incompleteness continues to limit practical accuracy. Here we present a study of transcorrelated (TC) many-body wave-function methods on properties of solid state systems. We augment the existing xTC theory to periodic systems, and establish an unified transcorrelated embedding framework that integrates periodic TC theory with fragment-based correlated solvers. Using silicon as a test case, we validate the method against coupled-cluster, FCIQMC, and diffusion Monte Carlo benchmarks for bulk. Then we apply TC embedding to calculation of formation energies of two silicon self-interstitials. The TC Hamiltonian yields rapid basis convergence and quantitatively reliable defect formation energies at the triple-$\zeta$ level, substantially reducing the basis-set bottleneck for wave-function treatments of crystalline defects.

cond-mat.mtrl-sci↗
arXiv2025

Modular Construction of Jastrow Factors for the Transcorrelated Method

In this work, we explore the reuse of terms in the Jastrow factor between systems for use in the transcorrelated method, to reduce the number of optimisable parameters for a given system. In particular, we propose a workflow in which atom-specific parts of Jastrow factors, optimised in atoms, may be reused in the molecule, with only a few parameters in the electron-electron part of the Jastrow left to optimise, while maintaining performance. We find that the modified workflow not only reduces the number of terms needing to be optimised, but also improves the accuracy of xTC-CCSD(T) energies.

physics.chem-ph↗
arXiv2025

Nodal error behind discrepancies between coupled cluster and diffusion Monte Carlo in hydrogen-bonded systems

The small magnitude and long-range character of non-covalent interactions pose a significant challenge for computational quantum chemical and electronic-structure methods alike. State-of-the-art coupled cluster (CC) theory and benchmark-grade diffusion Monte Carlo (DMC) are ideally positioned to tackle these problems, but concerning differences between both methods have been reported in numerous studies of the interaction energy of non-covalently bound dimers. Given that the basic theoretical frameworks underpinning both methods are exact in principle, the error must arise from one or several of the approximations required to make the calculations computationally tractable. Here, we carry out a rigorous and systematic examination of the effect of each of these approximations using the acetic acid dimer and water-peptide systems as convenient testing grounds. Thanks to the use of stringently optimized backflow wave functions we are able to find that the significant discrepancies are dominated by the fixed-node error incurred by the Slater-Jastrow DMC result, while errors in the CC calculations do not significantly alter the result. This finding, likely applicable to other hydrogen-bonded systems, helps establish that CC should be regarded as the benchmark for these systems, and can potentially guide the search for pragmatic solutions to the fixed-node problem in the future.

physics.chem-ph↗
arXiv2025

Quantum chemistry with provable convergence via randomized sample-based Krylov quantum diagonalization

Quantum algorithms based on classical processing of individual samples have recently emerged as the most effective and robust methods to approximate ground-state wave functions of many-body quantum systems on pre-fault-tolerant and early-fault-tolerant quantum devices. In these algorithms, the quantum computer acts as a sampling engine that generates the subspace in which the Hamiltonian is classically diagonalized. The recently proposed Sample-based Krylov Quantum Diagonalization (SKQD), uses quantum Krylov states as circuits from which samples are collected. Convergence guarantees can be derived for SKQD under similar assumptions to those of quantum phase estimation, provided that the ground-state wave function is well approximated by a polynomial subset of the full Hilbert space. However, implementations of SKQD for complex many-body Hamiltonians, such as quantum chemistry ones, are limited by the depths of time-evolution circuits needed to generate Krylov vectors. In this work, we introduce a method that combines SKQD with a qDRIFT randomized compilation of the Hamiltonian propagator. The resulting algorithm, termed SqDRIFT, enables quantum chemistry experiments on quantum processors, while preserving the convergence guarantees similar to the phase estimation algorithm. We demonstrate its viability by applying SqDRIFT to calculate the electronic ground-state energy of several polycyclic aromatic hydrocarbons, up to system sizes beyond the reach of exact diagonalization.

quant-ph↗
arXiv2025

Transcorrelated Theory for Transition Metal Atoms

We benchmark ionisation and excitation energies of transition-metal atoms Sc-Zn with a transcorrelated Hamiltonian combined with pseudopotentials. The similarity transformed Hamiltonian provides compact TC wave functions in affordable aug-cc-pVTZ and aug-cc-pVQZ Gaussian bases and eliminates the need for complete basis set extrapolations. The use of Douglas-Kroll-Hess theory is omitted because scalar relativistic effects are included in the pseudopotentials. Treating the full semicore (3s 3p) valence and freezing only 1s-2p shells, we reach chemical accuracy for all atoms and properties with coupled cluster and full configuration interaction quantum Monte Carlo. Consistent total energies across disparate orbital sets and correlation solvers highlights the robustness of the TC workflow. Our study pushes benchmark-quality quantum chemistry into the 3d block without large-scale basis sets and opens a practical route for transcorrelation to strongly correlated molecules and materials hosting heavier transition metals.

physics.chem-ph↗
arXiv2025

Deterministic Optimisation of Jastrow Factors

Highly flexible Jastrow factors have found significant use in stochastic electronic structure methods such as variational Monte Carlo (VMC) and diffusion Monte Carlo, as well as in quantum chemical transcorrelated (TC) approaches, which have recently seen great success in generating highly accurate electronic energies using moderately sized basis sets. In particular for the latter, the intrinsic noise in the Jastrow factor due to its optimisation by VMC can pose a problem, especially when targeting weak (non-covalent) interactions. In this paper, we propose a deterministic alternative to VMC Jastrow optimisation, based on minimising the "variance of the TC reference energy" in a standard basis set. Analytic expressions for the derivatives of the TC Hamiltonian matrix elements are derived and implemented. This approach can be used to optimise the parameters in the Jastrow functions, either from scratch or to refine an initial VMC-based guess, to produce noise-free Jastrows in a reproducible manner. Applied to the first row atoms and molecules, the results show that the method yields Slater-Jastrow wavefunctions whose variances are almost as low as those obtained from standard VMC variance optimisation, but whose energies are lower, and comparable to those obtained from energy-minimisation VMC. We propose that the method can be used both in the context of the transcorrelated method or in standard VMC as a new way to optimise Jastrow functions.

physics.chem-ph↗
arXiv2025

Transcorrelated Methods for Multireference Problems

We apply the transcorrelated method to problems of multireference character. For this, we show that the choice of reference wavefunction during the Jastrow optimisation procedure is vital, and we propose a workflow wherein we use conventional multi-configurational methods to provide a reference wavefunction for Jastrow factor optimisation. This Jastrow function is subsequently used with transcorrelated-full configuration interaction quantum Monte Carlo within the xTC approximation (TC-FCIQMC) to yield highly accurate transcorrelated energies. This is demonstrated for N$_2$ using the aug-cc-pVTZ basis set, achieving chemical accuracy across the entire binding curve compared with experiment. We also apply the method to compute excitation energies of dinitrogen, CO and the ammonia molecule, where accurate results, comparable to the best available theoretical predictions, are obtained with modest basis sets.

physics.chem-ph↗
arXiv2025

Individual and cooperative superexchange enhancement in cuprates

It is now widely accepted that the antiferromagnetic coupling within high temperature superconductors strongly exhibits a profound correlation with the upper limit of superconducting transition temperature these materials can reach. Thus, accurately calculating the positive and negative mechanisms that influence magnetic coupling in specific materials is crucial for the exploration of superconductivity at higher temperatures. Nevertheless, it is notoriously difficult to establish a complete description of electron correlations employing ab initio theories because of the large number of orbitals involved. In this study, we tackle the challenge of achieving high-level ab initio wave function theory calculations, which allow an explicit treatment of electron correlations associated with a large number of high-energy orbitals. We elucidate the atomic-shell-wise contributions to the superexchange coupling in the lanthanum cuprate, including individual effects of high-energy orbitals (Cu 4d, 5d, 4f, 5p) and cooperative effects between the core and these high-energy orbitals. Specifically, the prominent contributions from Cu 4d, 5d, 4f and 5p give rise to a rich collection of previously unexamined superexchange channels. We propose a p-d-f model to universally account for the contributions of high-energy orbitals at copper sites. Our calculations and physical rationalizations offer a more robust theoretical foundation for investigating cuprate-type high-temperature superconductors.

cond-mat.str-el↗