arXiv subjects

Ranjit Thapa

Publications and source records attributed to Ranjit Thapa.

4 recordsLinked to original sources

arXiv2026

QmDFT for Polycyclic Aromatics: Balancing Embedding Ground-State Fidelity and Experimental Gap Estimation

Quantum Embedding density functional theory (QmDFT) embedding offers a highly scalable approach to improve treatment for large highly correlated pi conjugated systems. However, estimating advanced electronic structure properties in polycyclic aromatic hydrocarbons (PAHs) needs advanced exchange correlation functionals that frequently trigger convergence instabilities during the embedding cycle. In this work, we introduce an adaptive damping and direct inversion in the iterative subspace (DIIS) accelerated protocol that stabilizes the embedding procedure, enabling robust integration of hybrid functionals like B3LYP and CAM B3LYP. Using 10 selected PAHs (linear and fused) molecules as a benchmark. We demonstrate a clear functional-dependent ground-state energetics and frontier-orbital gap estimation. While LDA based approaches yield near-quantitative agreement with FCI in DFT reference energies and is further supported by thermochemical isomerization benchmarks, while B3LYP provide significantly improved agreement with experimental E0-0 transition values. This mapping allows us to bypass explicit excited-state calculations for E0 0 values, thereby significantly reducing computational overhead. Among the hybrid functionals screened, CAM B3LYP offers a balanced overall performance. Our results establish a stable QmDFT framework and provide useful guidance for functional selection for quantum embedding studies of PAHs and related low-dimensional pi-conjugated materials.

quant-ph↗

arXiv2026

A comparative first-principles investigation of bilayer NbOX2 (X=Cl, Br, I) for Photocatalytic water splitting applications

Motivated by our previous work on bulk NbOX2 , where we have reported its high 1dielectric polarisation and finite piezoelectric response, this work extends to its 2D homo bilayer system to explore its potential for photocatalytic water splitting. Herein, density functional theory (DFT) were employed in probing the structural, electronic, optical, and photocatalytic properties of 2D homo bilayer NbOX2 (X = Cl, Br, and I). Our results show that structurally, NbOCl2 and NbOBr2 prefer AC bilayer stacking,while AB stacking was preferred by NbOI2 . All the considered bilayers are dynamically, thermally, and mechanically stable. From the analysis of electronic structure we have found a decreasing trend in the energy band gap as X goes down the group from Cl to I, with the position of the valence band maximum shifting upward along the high symmetry points. In terms of carrier mobility, all 2D bilayer systems possess high carrier mobility comparable to known 2D materials. It also exhibits an anisotropic carrier transfer property by which charge carriers are separated efficiently. These materials show similar trends to BiOI and PtSe2 , in which photocatalytic efficiency was increased by forming the multiple layers. The materials under investigation are suitable for photocatalytic water splitting under visible and ultraviolet regions with absorption coefficients of 105 cm-1.

cond-mat.mtrl-sci↗

arXiv2026

Scalable Quantum-Classical DFT Embedding for NISQ Molecular Simulation

Scalable quantum-classical embedding is essential for chemically meaningful simulations on near-term NISQ hardware. Using QDFT, we show systematic recovery of correlation energy relative to the DFT baseline, benchmarked against CCSD in a fixed six-orbital active space across molecules ranging from water to naphthalene. By varying the number of embedded electrons from 2 to 8, aromatic systems saturate near 63-64 percent, while linear molecules such as carbon dioxide reach 68 percent. All systems converge within two embedding iterations under relaxed self-consistency thresholds, highlighting the robustness of the approach. A (4e,6o) active space recovers approximately 60 percent correlation using 10 qubits, providing practical guidelines for NISQ-era simulations.

quant-ph↗

arXiv2015

First principles design of divacancy defected graphene nanoribbon based rectifying and negative differential resistance device

We have elaborately studied the electronic structure of 555-777 divacancy (DV) defected armchair edged graphene nanoribbon (AGNR) and transport properties of AGNR based two-terminal device constructed with one defected electrode and one N doped electrode, by using density functional theory and non-equilibrium Green's function based approach. The introduction of 555-777 DV defect into AGNRs, results in a shifting of the π and π* bands towards the higher energy value which indicates a shifting of the Fermi level towards the lower energy. Formation of a potential barrier, very similar to that of conventional p-n junction, has been observed across the junction of defected and N doped AGNR. The prominent asymmetric feature of the current in the positive and negative bias indicates the diode like property of the device with high rectifying efficiency within wide range of bias voltages. The device also shows robust negative differential resistance (NDR) with very high peak-to-valley ratio. The analysis of the shifting of the energy states of the electrodes and the modification of the transmission function with applied bias provides an insight into the nonlinearity and asymmetry observed in the I-V characteristics. Variation of the transport properties on the width of the ribbon has also been discussed.

cond-mat.mtrl-sci↗