arXiv subjects

Gaurav Harsha

Publications and source records attributed to Gaurav Harsha.

15 recordsLinked to original sources

arXiv2026

Symmetry-resolved topology from interacting Green's functions

Topological phases in narrow-gap materials often arise from a delicate interplay of spin-orbit coupling (SOC) and electron correlation, making their prediction sensitive to the underlying electronic-structure approximation. Electronic topology is commonly characterized using effective one-body Hamiltonians, but this approach becomes delicate near gap closings and does not reveal how a band inversion is realized in the interacting spectrum. Here we introduce a multi-messenger analysis that combines three complementary readouts of a single interacting Green's function: the Fu-Kane invariant of the topological Hamiltonian, the symmetry-resolved spectral function, and the symmetry-resolved orbital occupation. Together, they identify the topological class, resolve band connectivity, and track the orbital character of the inversion. We implement this framework within fully self-consistent relativistic GW, treating SOC and electronic correlation on equal footing while eliminating dependence on a density-functional starting point. Applied to strain-tunable $α$-Sn, the approach distinguishes its competing zero-gap, Dirac-semimetal, and topological-insulator regimes and provides an internally consistent description of their common band-inversion mechanism. More broadly, this framework offers a frequency-resolved route to diagnosing correlated topology across both gapped and near-critical states.

cond-mat.str-el↗

arXiv2026

Hybrid Hamiltonian-diagrammatic quantum impurity solver

Quantum impurity models, which describe the coupling between interacting orbitals and a non-interacting bath, play a central role in the physics of strongly correlated electron systems. Solving a quantum impurity model in general requires the use of non-perturbative numerical methods. Hamiltonian-based approaches, which rely on an explicit bath discretization, are typically limited to a small number of bath sites or small entanglement, and diagrammatic methods suffer from sign problems, slow convergence, or diagram truncation approximations. Here we show that these two classes of methods can be combined: augmenting diagrammatic methods with a small auxiliary bath can reduce the residual problem to a regime where low-order perturbation theory is highly accurate and rapidly converging. In a simple benchmark, the precision of the hybrid approach surpasses bold-line calculations by several orders of magnitude; for a strongly interacting two-orbital model with a severe sign problem, convergence is achieved at three orders of magnitude lower computational cost than competing methods; and convergence to the unknown exact result is rapidly accelerated in a difficult realistic problem. Our results establish a practical route to high-precision quantum impurity solutions in correlated quantum systems.

cond-mat.str-el↗

arXiv2026

Dynamically Corrected Bethe-Salpeter Equation Solver for Self-consistent $GW$ Reference on the Matsubara Frequency Axis

We present a Bethe-Salpeter equation (BSE) solver based on a self-consistent $GW$ reference evaluated on the Matsubara frequency axis, referred to as BSE@sc$GW$. The self-consistent $GW$ starting point provides a robust quasiparticle description and reduces sensitivity to the initial mean-field reference compared to one-shot $GW$-based approaches. We further introduce a dynamical correction to the static Casida formulation via a plasmon-pole model. This scheme incorporates simple dynamical screening effects while retaining the efficiency of an effective eigenvalue problem. The resulting dynamically corrected BSE@sc$GW$ yields excitation energies in close agreement with high-level wavefunction-based benchmarks for both singlet and triplet excitations of small molecules. Overall, the accuracy of the dynamic BSE@sc$GW$ approach arises from the combination of a well-converged single-particle reference and the inclusion of frequency-dependent screening effects.

physics.chem-ph↗

arXiv2025

Performance of wave function and Green's functions based methods for non equilibrium many-body dynamics

Theoretical descriptions of non equilibrium dynamics of quantum many-body systems essentially employ either (i) explicit treatments, relying on truncation of the expansion of the many-body wave function, (ii) compressed representations of the many-body wave function, or (iii) evolution of an effective (downfolded) representation through Green's functions. In this work, we select representative cases of each of the methods and address how these complementary approaches capture the dynamics driven by intense field perturbations to non equilibrium states. Under strong driving, the systems are characterized by strong entanglement of the single particle density matrix and natural populations approaching those of a strongly interacting equilibrium system. We generate a representative set of results that are numerically exact and form a basis for critical comparison of the distinct families of methods. We demonstrate that the compressed formulation based on similarity transformed Hamiltonians (coupled cluster approach) is practically exact in weak fields and, hence, weakly or moderately correlated systems. Coupled cluster, however, struggles for strong driving fields, under which the system exhibits strongly correlated behavior, as measured by the von Neumann entropy of the single particle density matrix. The dynamics predicted by Green's functions in the (widely popular) GW approximation are less accurate by improve significantly upon the mean-field results in the strongly driven regime.

physics.comp-ph↗

arXiv2024

Green/WeakCoupling: Implementation of fully self-consistent finite-temperature many-body perturbation theory for molecules and solids

The accurate ab initio simulation of molecules and periodic solids with diagrammatic perturbation theory is an important task in quantum chemistry, condensed matter physics, and materials science. In this article, we present the WeakCoupling module of the open-source software package Green, which implements fully self-consistent diagrammatic weak coupling simulations, capable of dealing with real materials in the finite-temperature formalism. The code is licensed under the permissive MIT license. We provide self-consistent GW (scGW) and self-consistent second-order Green's function perturbation theory (GF2) solvers, analysis tools, and post-processing methods. This paper summarizes the theoretical methods implemented and provides background, tutorials and practical instructions for running simulations.

cond-mat.mtrl-sci↗

arXiv2024

Quasiparticle and fully self-consistent GW methods: an unbiased analysis using Gaussian orbitals

We present a comparison of various approximations to self-consistency in the GW method, including the one-shot G0W0 method, different quasiparticle self-consistency schemes, and the fully self-consistent GW (scGW) approach. To ensure an unbiased and equitable comparison, we have implemented all the schemes with the same underlying Matsubara formalism, while employing Gaussian orbitals to describe the system. Aiming to assess and compare different GW schemes, we analyze band gaps in semiconductors and insulators, as well as ionization potentials in molecules. Our findings reveal that for solids, the different self-consistency schemes perform very similarly. However, for molecules, full self-consistency outperforms all other approximations, i.e., the one-shot and quasiparticle self-consistency GW schemes. Our work highlights the importance of implementation details when comparing different GW methods. By employing state-of-the-art fully self-consistent, finite temperature GW calculations, we have successfully addressed discrepancies in the existing literature regarding its performance. Our results also indicate that when stringent thresholds are imposed, the scGW method consistently yields accurate results.

cond-mat.str-el↗

arXiv2024

Comparing self-consistent GW and vertex corrected G0W0 (G0W0Γ) accuracy for molecular ionization potentials

We test the performance of self-consistent GW and several representative implementations of vertex corrected G0W0 (G0W0Γ). These approaches are tested on benchmark data sets covering full valence spectra (first ionization potentials and some inner valence shell excitations). For small molecules, when comparing against state of the art wave function techniques, our results show that performing full self-consistency in the GW scheme either systematically outperforms vertex corrected G0W0 or gives results of at least the same quality. Moreover, the G0W0Γ results in additional computational cost when compared to G0W0 or self-consistent GW and the G0W0Γ dependency on the starting mean-filed solution is frequently larger than the magnitude of the vertex correction. Consequently, for molecular systems self-consistent GW performed on imaginary axis and then followed by modern analytical continuation techniques offers a more reliable approach to make predictions of IP spectra.

physics.chem-ph↗

arXiv2024

Challenges with relativistic GW calculations in solids and molecules

For molecules and solids containing heavy elements, accurate electronic structure calculations require accounting not only for electronic correlations but also for relativistic effects. In molecules, relativity can lead to severe changes in the ground-state description. In solids, the interplay between both correlation and relativity can change the stability of phases or it can lead to an emergence of completely new phases. Traditionally, the simplest illustration of relativistic effects can be done either by including pseudopotentials in non-relativistic calculations or alternatively by employing large all electron basis sets in relativistic methods. By analyzing different electronic properties (band structure, equilibrium lattice constant and bulk modulus) in semiconductors and insulators, we show that capturing the interplay of relativity and electron correlation can be rather challenging in Green's function methods. For molecular problems with heavy elements, we also observe that similar problems persist. We trace these challenges to three major problems: deficiencies in pseudopotential treatment as applied to Green's function methods, the scarcity of accurate and compact all-electron basis-sets that can be converged with respect to the basis-set size, and linear dependencies arising in all-electron basis-sets particularly when employing Gaussian orbitals. Our analysis provides detailed insight into these problems and opens a discussion about potential approaches to mitigate them.

physics.chem-ph↗

arXiv2024

Relativistic fully self-consistent $GW$ for molecules: Total energies and ionization potentials

The fully self-consistent $GW$ (sc$GW$) method with the iterative solution of Dyson equation provides a consistent approach for describing the ground and excited states without any dependence on the mean-field reference. In this work, we present a relativistic version of sc$GW$ for molecules containing heavy element using the exact two-component (X2C) Coulomb approximation. We benchmark $\texttt{SOC-81}$ dataset containing closed shell heavy elements for the first ionization potential using the fully self-consistent $GW$ as well as one-shot $GW$. The self-consistent $GW$ provides superior result compared to $G_0W_0$ with PBE reference and comparable to $G_0W_0$ with PBE0 while also removing the starting point dependence. The photoelectron spectra obtained at the X2C level demonstrate very good agreement with experimental spectra. We also observe that sc$GW$ provides very good estimation of ionization potential for the inner $d$ shell orbitals. Additionally, using the well conserved total energy, we investigate the equilibrium bond length and harmonic frequencies of few halogen dimers using sc$GW$. Overall, our findings demonstrate the applicability of the fully self-consistent $GW$ method for accurate ionization potential, photoelectron spectra and total energies in finite systems with heavy elements with a reasonable computational scaling.

physics.chem-ph↗

arXiv2023

Thermofield theory for finite-temperature electronic structure

Wave-function methods have offered a robust, systematically improvable means to study ground-state properties in quantum many-body systems. Theories like coupled cluster and their derivatives provide highly accurate approximations to the energy landscape at a reasonable computational cost. Analogs of such methods to study thermal properties, though highly desirable, have been lacking because evaluating thermal properties involve a trace over the entire Hilbert space, which is a formidable task. Besides, excited-state theories are generally not as well studied as ground-state ones. In this mini-review, we present an overview of a finite-temperature wave function formalism based on thermofield dynamics to overcome these difficulties. Thermofield dynamics allows us to map the equilibrium thermal density matrix to a pure state, i.e., a single wave function, albeit in an expanded Hilbert space. Ensemble averages become expectation values over this so-called thermal state. Around this thermal state, we have developed a procedure to generalize ground-state wave function theories to finite temperatures. As explicit examples, we highlight formulations of mean-field, configuration interaction, and coupled cluster theories for thermal properties of fermions in the grand-canonical ensemble. To assess the quality of these approximations, we also show benchmark studies for the one-dimensional Hubbard model, while comparing against exact results. We will see that the thermal methods perform similarly to their ground-state counterparts, while merely adding a pre-factor to the asymptotic computational cost. They also inherit all the properties, good or bad, from the ground-state methods, signifying the robustness of our formalism and the scope for future development.

cond-mat.str-el↗

arXiv2022

Thermal coupled cluster theory for SU(2) systems

Coupled cluster (CC) has established itself as a powerful theory to study correlated quantum many-body systems. Finite-temperature generalizations of CC theory have attracted considerable interest and have been shown to work as nicely as the ground-state theory. However, most of these recent developments address only fermionic or bosonic systems. The distinct structure of the $su(2)$ algebra requires the development of a similar thermal CC theory for spin degrees of freedom. In this paper, we provide a formulation of our thermofield-inspired thermal CC for SU(2) systems. We apply the thermal CC to the Lipkin-Meshkov-Glick system as well as the one-dimensional transverse field Ising model as benchmark applications to highlight the accuracy of thermal CC in the study of finite-temperature phase diagrams in SU(2) systems.

cond-mat.str-el↗

arXiv2020

Wave function methods for canonical ensemble thermal averages in correlated many-fermion systems

We present a wave function representation for the canonical ensemble thermal density matrix by projecting the thermofield double state against the desired number of particles. The resulting canonical thermal state obeys an imaginary time-evolution equation. Starting with the mean-field approximation, where the canonical thermal state becomes an antisymmetrized geminal power wave function, we explore two different schemes to add correlation: by number-projecting a correlated grand-canonical thermal state, and by adding correlation to the number-projected mean-field state. As benchmark examples, we use number-projected configuration interaction and an AGP-based perturbation theory to study the Hydrogen molecule in a minimal basis and the six-site Hubbard model.

physics.chem-ph↗

arXiv2019

Thermofield theory for finite-temperature coupled cluster

We present a coupled cluster and linear response theory to compute properties of many-electron systems at non-zero temperatures. For this purpose, we make use of the thermofield dynamics, which allows for a compact wavefunction representation of the thermal density matrix, and extend our recently developed framework [J. Chem. Phys. 150, 154109 (2019)] to parameterize the so-called thermal state using an exponential ansatz with cluster operators that create thermal quasiparticle excitations on a mean-field reference. As benchmark examples, we apply this method to both model (one-dimensional Hubbard and Pairing) as well as ab-initio (atomic Beryllium and molecular Hydrogen) systems, while comparing with exact results.

physics.chem-ph↗

arXiv2019

Thermofield Theory for Finite-Temperature Quantum Chemistry

Thermofield dynamics has proven to be a very useful theory in high-energy physics, particularly since it permits the treatment of both time- and temperature-dependence on an equal footing. We here show that it also has an excellent potential for studying thermal properties of electronic systems in physics and chemistry. We describe a general framework for constructing finite temperature correlated wave function methods typical of ground state methods. We then introduce two distinct approaches to the resulting imaginary time Schrodinger equation, which we refer to as fixed-reference and covariant methods. As an example, we derive the two corresponding versions of thermal configuration interaction theory, and apply them to the Hubbard model, while comparing with exact benchmark results.

physics.chem-ph↗

arXiv2018

On the difference between variational and unitary coupled cluster theories

There have been assertions in the literature that the variational and unitary forms of coupled cluster theory lead to the same energy functional. Numerical evidence from previous authors was inconsistent with this claim, yet the small energy differences found between the two methods and the relatively large number of variational parameters precluded an unequivocal conclusion. Using the Lipkin Hamiltonian, we here present conclusive numerical evidence that the two theories yield different energies. The ambiguities arising from the size of the cluster parameter space are absent in the Lipkin model, particularly when truncating to double excitations. We show that in the symmetry adapted basis under strong correlation the differences between the variational and unitary models are large, whereas they yield quite similar energies in the weakly correlated regime previously explored. We also provide a qualitative argument rationalizing why these two models cannot be the same. Additionally, we study a generalized non-unitary and non-hermitian variant that contains excitation, de-excitation and mixed operators with different amplitudes and show that it works best when compared to the traditional, variational, unitary, and extended forms of coupled cluster doubles theories.

cond-mat.str-el↗