arXiv subjects

Achintya Kumar Dutta

Publications and source records attributed to Achintya Kumar Dutta.

At least 19 recordsLinked to original sources

arXiv2026

A reduced-cost two-component relativistic equation-of-motion coupled cluster method for the double electron attachment problem

We present a computationally efficient relativistic formulation of the equation-of-motion coupled-cluster (EOM-CC) method for the double electron attachment (DEA) problem. In this work, the exact two-component Hamiltonian within the atomic mean-field approximation is employed, yielding results that are in close agreement with the corresponding four-component calculations. However, canonical DEA-EOM-CCSD calculations become prohibitively expensive for heavy elements and large basis sets due to the substantial memory requirements associated with the complex-valued 3p1h excitation manifold. To address this limitation, we introduce a new state-specific frozen natural spinor basis that significantly reduces the virtual space through two controllable truncation thresholds. Furthermore, the use of Cholesky decomposition for the two-electron integrals provides an additional reduction in memory requirements. The performance of the proposed approach is demonstrated through calculations of double ionization potentials and excitation energies for group-12 and group-14 heavy elements. Vertical excitation energies for heavy chalcogen dimers are also presented. In addition, a range of diatomic spectroscopic constants is evaluated for group-13 hydrides. Finally, the method is applied to predict the singlet-triplet gaps of dihalocarbenes, indicating that an accurate description of these systems may require excitation manifolds beyond the 3p1h space.

physics.chem-ph↗

arXiv2026

Efficient Implementation of Relativistic Coupled Cluster Linear Response Theory in Combination with Perturbation Sensitive Natural Spinors and Cholesky Decomposition Treatment of Two-electron Integrals

We present an efficient implementation of the low-cost linear-response coupled-cluster singles and doubles (LR-CCSD) method for computing static and frequency-dependent polarizabilities in systems with significant relativistic and electron-correlation effects. The implementation combines X2C-based Hamiltonians (X2CAMF and X2CMP), perturbation-sensitive natural spinors (FNS++), and Cholesky decomposition (CD)- based treatment of two-electron integrals to reduce both the computational and memory demands of relativistic LR-CCSD calculations. Benchmark calculations reveal that X2CMP exhibits more robust behavior than X2CAMF in the presence of highly augmented basis sets. The proposed FNS++CD-X2CMP-LR-CCSD approach reproduces four-component reference values with excellent accuracy across a diverse set of atomic and molecular systems. Additionally, different strategies for constructing the FNS++ basis were assessed, and the averaged-density approach was found to offer a favorable balance between accuracy and computational cost. Across the benchmark systems considered in this work, approximately 70\% of the virtual spinor space can be removed with the FNS++ approach. The present implementation enables accurate and scalable relativistic response calculations for large molecular systems, as demonstrated by the computation of the static and dynamic polarizabilities of uranium hexafluoride using a triple-zeta basis comprising more than 1,400 basis functions.

physics.chem-ph↗

arXiv2026

Role of Native and Zwitterionic Glycine in Electron Attachment to DNA: From Dipole-Bound to Solvent-Bound Doorway States

Electron attachment to DNA is strongly influenced by its molecular environment, yet the role of amino acids under physiologically relevant conditions remains poorly understood. Here, we investigate the effect of native and zwitterionic glycine on electron attachment to thymine using high-level electron-affinity calculations and QM/MM molecular dynamics simulations. Under micro-solvated conditions, electron attachment occurs through a dipole-bound doorway state that evolves into a valence-bound anion via nonadiabatic coupling. The zwitterionic form of glycine strengthens stabilization of the diffuse electron owing to its larger internal charge separation, whereas the stability of the valence-bound anion is determined by the hydrogen-bonding geometry. Barrier-free proton transfer is observed only for specific binding motifs and substantially stabilizes the thymine-centered anion. In bulk solution, the doorway mechanism persists, with a solvent-bound state replacing the dipole-bound state as the initial electron-trapping state. The larger electrostatic field of zwitterionic glycine delays electron localization on thymine, while permanent proton transfer is observed only in selected native glycine trajectories and is absent throughout the present simulations of zwitterionic glycine. Despite these differences in electron-transfer dynamics, both amino acid forms provide similar stabilization of the thymine-centered anion after solvent reorganization. Our results establish the solvent-bound state as the condensed-phase analogue of the dipole-bound doorway state and reveal how amino acid environments modulate electron attachment pathways in realistic DNA systems.

physics.chem-ph↗

arXiv2026

Core-excited and shape-type resonances in the micro-solvated Uracil: A CASSCF study

Electronic resonances play an important role in electron attachment-induced processes in biomolecules, and their properties can be significantly influenced by the local molecular environment. Here, we investigate the effect of amino acid micro-solvation on the uracil resonances by employing uracil-glycine as a model system. The resonance spectrum of the uracil-glycine complex consists of four π-type shape resonances and three core-excited resonances, including an additional glycine-centered resonance, as characterized using the CASSCF/Resonance via Padé (RVP) methodology. Comparison with isolated uracil and the uracil(ghostGly) model shows that explicit interaction with glycine stabilizes both the shape and core-excited resonances by lowering their energies and increasing their lifetimes, while the ghost calculations demonstrate that basis-set extension alone cannot account for the observed stabilization. The core-excited resonances exhibit states that retain non-negligible lifetimes despite their much higher energy, suggesting that they may play an important role in electron-induced dissociation pathways. Overall, the present results demonstrate that amino acid micro-solvation significantly modifies the resonance landscape of uracil, highlighting the importance of explicitly accounting for local biomolecular interactions in theoretical studies of electron attachment.

physics.chem-ph↗

arXiv2026

Do Water Molecules Always Stabilize Resonances? Microhydration Effects on Thymine Shape Resonances

We investigate microhydration effects on the three low-lying π* shape resonances of thymine using the Resonance via Padé approach in combination with the DLPNO-EA-EOM-CCSD method. For isolated thymine, the calculated resonance positions are benchmarked against projected CAP-EA-EOM-CCSD calculations and compared with available theoretical and experimental data. Upon hydration, the 1π* and 2π* resonances undergo systematic stabilization accompanied by significant increases in their lifetimes, whereas the 3π* resonance exhibits a more complex behavior. In particular, the lifetime of the lowest resonance increases from 39 fs in isolated thymine to 110 fs in the thymine(H2O)3 cluster. Detailed analysis reveals that the observed resonance shifts arise from competing contributions involving hydrogen bonding, electrostatic interactions, microsolvation-induced geometric distortion, and finite-basis-set effects. Ghost-atom calculations demonstrate that diffuse basis functions associated with nearby water molecules contribute appreciably to the apparent stabilization, while explicit inclusion of water molecules leads to genuine physical stabilization of the resonance states. Furthermore, calculations on multiple conformers of the monohydrated cluster show that resonance positions and lifetimes depend strongly on the local hydrogen-bonding arrangement and microsolvation geometry. These findings demonstrate that resonance stabilization in microhydrated nucleobases is governed by a subtle interplay between geometry, basis-set effects, and intermolecular interactions.

physics.chem-ph↗

arXiv2026

Relativistic Exact-Two-Component Core-Valence-Separated Algebraic Diagrammatic Construction Theory For Near L-edge X-ray Absorption Spectra

We present an efficient implementation of the second-order two-component relativistic core-valence-separated algebraic diagrammatic construction method (CVS-ADC(2)) for core-excitation calculations. The approach employs state-averaged frozen natural spinors (SA-FNS) to reduce the number of floating-point operations, together with the Cholesky decomposition (CD) technique, which lowers the storage requirements associated with two-electron integrals. These reductions make the method particularly well-suited for systems containing heavy elements. Systematic benchmarking against four-component reference calculations confirms the reliability and robustness of the two-component (X2CMP/X2CAMF)-based framework. The close agreement with canonical results further demonstrates that the SA-FNS-based CVS-ADC(2) approach achieves comparable accuracy at only a fraction of the computational cost. Moreover, benchmark studies of L$_{2,3}$-edge spectra for 3$d$ transition-metal compounds demonstrate that CVS-ADC(2) serves as a computationally efficient and reliable alternative to the non-Hermitian EOM-CC method for reproducing experimental spectra. Finally, calculations on a ruthenium complex illustrate the method's applicability to relativistic studies of medium-sized molecular systems.

physics.chem-ph↗

arXiv2026

Electron Attachment Induced Shape Resonances in AT Base Pairs

In this work, we investigated the influence of base pairing and π-π stacking interactions on electron attachment induced shape resonances in the adenine-thymine (AT) base pair. Resonance positions and widths are computed using a DLPNO based equation of motion coupled-cluster approach in conjunction with the Padé analytical continuation method. Seven π* shape resonances are identified for both linear and stacked AT geometries, consistent with the total number of resonances in isolated adenine and thymine. Natural orbital analysis reveals that low-energy resonances exhibit significant electron density delocalization over both nucleobases. This delocalization is enhanced in the stacked geometry, leading to appreciable stabilization and increased lifetimes of the resonance states. These results highlight the important role of intermolecular interactions in modulating electron attachment processes in DNA.

physics.chem-ph↗

arXiv2026

A reduced-cost third-order algebraic diagrammatic construction based on state-specific frozen natural orbitals: Application to the electron-attachment problem

We have developed a reduced-cost non-Dyson third-order algebraic diagrammatic construction theory for the electron-attachment problem based on state-specific frozen natural orbitals. Density fitting and truncated natural auxiliary functions were employed to enhance computational efficiency. The use of state-specific frozen natural orbitals significantly decreases the virtual space and provides a notable speedup over the conventional EA-ADC(3) method with a systematically controllable accuracy. A perturbative correction for the truncated natural orbitals significantly reduces the error in the calculated electron affinity values. The method also shows sufficient accuracy in the case of non-valence correlation-bound anions, where the local approximation-based methods fail. The efficiency of the method is demonstrated by performing an EA-ADC(3) calculation with more than 1300 basis functions.

physics.chem-ph↗

arXiv2026

A Perturbative Super-CI Approach for orbital optimization in Two-Component relativistic CASSCF

In this work, we develop a new orbital optimization approach, perturbative Super-CI (Super-CIPT), for the two-component complete active space self-consistent field (2C-CASSCF) method. By variationally optimizing spinor orbitals and consistently incorporating spin--orbit coupling (SOC) at the orbital level, the 2C-CASSCF method enables a simultaneous treatment of relativistic effects and static correlation. The Super-CIPT approach demonstrates robust convergence behavior and is applicable to systems under strong SOC. The inclusion of Gaunt or Breit term via the atomic mean field approximation yields the most accurate results, with errors dropping below 2% for halogens. We systematically assess the performance of 2C-CASSCF on spin-orbit splittings (SOSs) of selected p-block elements. Results show that 2C-CASSCF outperforms conventional one-component (1C) CASSCF. This work establishes 2C-CASSCF with Super-CIPT as a reliable and efficient approach for multireference relativistic quantum chemistry.

physics.chem-ph↗

arXiv2026

The Effect of Base-Pairing on the Shape Resonances of Nucleobases

In this work, we have studied the effect of base-pairing on the shape resonances of guanine and cytosine nucleobases. Among the seven π* resonances we identified in the guanine-cytosine (GC) anion radical, three were centered on cytosine, and the remaining were guanine-centered. Relative to the isolated bases, upon base pair formation, the cytosine resonances were red shifted, while the guanine-centered states showed an opposite trend - where their energy was blue shifted. In addition to the electronic interactions, geometric distortion and basis set superposition error plays a crucial role in the resonance positions and widths of the GC radical anion. The electronic interaction from the complementary base seems to have a larger effect on the stabilization of the anionic resonances than the surrounding environment.

physics.chem-ph↗

arXiv2026

Relativistic unitary coupled cluster method for ground-state molecular properties

We propose a relativistic unitary coupled cluster (UCC) expectation value approach for computing first-order properties of heavy-element systems. Both perturbative (UCC3) and non-perturbative (qUCC) commutator-based formulations are applied to evaluate ground-state properties, including the permanent dipole moment (PDM), magnetic hyperfine structure (HFS) constant, and electric field gradient (EFG). The results are compared with available experimental data and those from conventional coupled cluster (CC) calculations. The non-perturbative commutator-based approach truncated at the singles and doubles level (qUCCSD) exhibits markedly better agreement with both CCSD and experiment than the perturbative UCC3 method, likely due to its improved treatment of relaxation effects.

physics.chem-ph↗

arXiv2026

A Low Cost Relativistic Algebraic Diagrammatic Construction Method Based on Cholesky Decomposition and Frozen Natural Spinors for Electronic Ionization, Attachment and Excitation Energy Problem

We present an efficient relativistic implementation of algebraic diagrammatic construction (ADC) theory up to third order for the treatment of electronic ionization potentials (IP), electron affinities (EA), and excitation energies (EE) in heavy-element systems using an exact two-component atomic mean-field (X2CAMF) Hamiltonian. The approach combines Cholesky decomposition (CD) of two-electron integrals with frozen natural spinors (FNS) to significantly reduce the computational cost without compromising accuracy. To improve the description of excited states, we have implemented a state-specific frozen natural spinor (SS-FNS) framework and applied it to both electron affinity and excitation energy calculations. In addition to the standard relativistic ADC(3) method, we investigate a semi-empirically scaled variant in which the third-order contribution to the ADC secular matrix is multiplied by a scaling factor (x), denoted as FNS/SS-FNS-[ADC(2)+(x)(3)]. This [ADC(2)+(x)(3)] approach shows systematic improvements over conventional ADC(3) in a variety of cases. Substantial computational savings are achieved through the use of FNS and SS-FNS schemes when compared to canonical calculations, resulting in significant speedups for ionization, attachment, and excitation energy computations. The current implementation accurately reproduces the canonical four-component ADC(3) results while significantly reducing computational cost. The efficiency and robustness of the method are demonstrated through applications to medium and large-sized molecular systems, including systems with 70 atoms and over 2600 basis functions.

physics.chem-ph↗

arXiv2025

A Third-Order Relativistic Algebraic Diagrammatic Construction Method for Double Ionization Potentials: Theory, Implementation, and Benchmark

We present a relativistic third-order algebraic diagrammatic construction (ADC(3)) approach for calculating double ionization potentials (DIPs). By employing the exact two-component atomic mean-field (X2CAMF) Hamiltonian in combination with a Cholesky decomposition (CD) representation of two-electron integrals and the frozen natural spinor (FNS) framework for virtual space truncation, we achieve a significant reduction in both memory requirements and computational cost. The DIPs obtained using the X2CAMF Hamiltonian show excellent agreement with results from fully relativistic four-component calculations. We have validated the accuracy of our implementation through comparisons with available experimental and theoretical data for inert gas atoms and diatomic species. The effect of higher-order relativistic corrections is also explored.

physics.chem-ph↗

arXiv2025

A perturbative triples correction to relativistic Quadratic Unitary Coupled Cluster Method: Theory, Implementation and Benchmarking

We present a perturbative triples correction to the relativistic quadratic unitary coupled cluster singles and doubles (qUCCSD) method, denoted as qUCCSD[T]. The method builds upon the Hermitian structure of the unitary ansatz and employs a many-body perturbation theory framework to consistently include the effects of triple excitations without the need for computationally intensive iterative procedures. Relativistic effects are incorporated using the exact two-component atomic mean-field (X2CAMF) Hamiltonian, and the computational cost is further reduced through the frozen natural spinor (FNS) and Cholesky decomposition (CD) approximations. Benchmark results demonstrate that qUCCSD[T] outperforms previously proposed triples corrections to the unitary coupled cluster method in the clasical computing regime and yields excellent agreement with experimental data and Full CI benchmarks. Specifically, the method shows high accuracy in computing bond dissociation enthalpies, molecular geometries, vibrational frequencies, ionization potentials, and electron affinities of heavy-element-containing systems.

physics.chem-ph↗

arXiv2025

Reduced-Cost Four-Component Relativistic Double Ionization Potential Equation-of-Motion Coupled-Cluster Approaches with 4-Hole--2-Particle Excitations and Three-Body Clusters

The double ionization potential (DIP) equation-of-motion (EOM) coupled-cluster (CC) method with 4-hole--2-particle (4$h$-2$p$) excitations on top of the CC with singles, doubles, and triples calculation, abbreviated as DIP-EOMCCSDT(4$h$-2$p$), along with its perturbative DIP-EOMCCSD(T)(a)(4$h$-2$p$) approximation, are extended to a relativistic four-component (4c) framework. In addition, we introduce and test a new computationally practical DIP-EOMCC approach, which we call DIP-EOMCCSD(T)($\tilde{a}$)(4$h$-2$p$), that approximates the treatment of 4$h$-2$p$ correlations within the DIP-EOMCCSD(T)(a) (4$h$-2$p$) method and reduces the $\mathscr{N}^8$ scaling characterizing DIP-EOMCCSDT(4$h$-2$p$) and DIP-EOMCCSD(T)(a)(4$h$-2$p$) to $\mathscr{N}^7$ with the system size $\mathscr{N}$. Further improvements in computational efficiency are obtained using the frozen natural spinor (FNS) approximation to reduce the numbers of unoccupied spinors entering the correlated steps of the DIP-EOMCC calculations according to a well-defined occupation-number-based threshold. The resulting 4c-FNS-DIP-EOMCC approaches are used to compute DIPs for the series of inert gas atoms from argon to radon as well as the vertical DIPs in \Cltwo{}, \Brtwo{}, HBr, and HI, which have been experimentally examined in the past. We demonstrate that, when using complete basis set extrapolations and FNS truncation thresholds of $10^{-4.5}$, the 4c-FNS-DIP-EOMCCSD(T)($\tilde{a}$)(4$h$-2$p$) calculations are capable of predicting DIPs in agreement with experimental data, improving upon their nonrelativistic and spin-free scalar-relativistic counterparts, particularly when examining DIPs characterized by stronger spin-orbit coupling effects.

physics.chem-ph↗

arXiv2025

The Effect of Aqueous Medium on Nucleobase Shape Resonances: Insights from Microsolvation

We have studied the effect of microhydration on the shape resonances of uracil nucleobase. The resonance parameters were determined using the resonance via Padé approach along with the efficient wave function-based EA-EOM-DLPNO-CCSD method. Our results showed that the uracil resonances become stabilized with an increase in the extent of microsolvation. The energy of the resonances decreased, and the lifetime increased as the number of water molecules surrounding uracil was increased. It showed that ten water molecules are sufficient to make the lowest shape resonance of uracil a bound radical anionic state. Our results also indicate that the lowest energy resonance state may become a bound state under bulk solvation.

physics.chem-ph↗

arXiv2025

A reduced cost equation of motion coupled cluster method for excited states based on state-specific natural orbitals: Theory, Implementation, Benchmark

We present a reduced-cost equation-of-motion coupled-cluster method for excited states, built on a new state-specific frozen natural orbital (SS-FNO) framework. This approach enables systematic and controllable truncation of the virtual spaces, significantly reducing computational demands while maintaining high accuracy. The method allows black-box application via two adjustable thresholds and includes a perturbative correction that compensates for truncation errors. We have tested the performance of both CIS(D) and ADC(2) methods in generating appropriate natural orbitals for excited states. Benchmarking on valence, Rydberg, and charge-transfer excited states demonstrates excellent agreement with canonical EE-EOM-CCSD results, with mean absolute deviations typically below 0.02 eV when ADC(2) natural orbitals with perturbative corrections are applied.

physics.chem-ph↗

arXiv2025

Reduced-cost Relativistic Equation-of-Motion Coupled Cluster Method based on Frozen Natural Spinors: A State-Specific Approach

We present the theoretical framework, implementation, and benchmark results for a reduced-cost relativistic equation-of-motion coupled cluster singles and doubles (EOM-CCSD) method based on state-specific frozen natural spinors (SS-FNS). In this approach, the state-specific frozen natural spinors are derived from the second-order algebraic diagrammatic construction (ADC(2)) method, providing a compact virtual space for excited-state calculations. The excitation energies computed with the SS-FNS-EE-EOM-CCSD method exhibit smooth convergence with respect to the truncation threshold and demonstrate significant improvements over those obtained using the conventional MP2-based FNS approach. We have implemented the relativistic SS-FNS-EE-EOM-CCSD method using both the four-component Dirac-Coulomb and the exact two-component atomic mean-field (X2CAMF) Hamiltonians to compute excitation energies and transition properties. The X2CAMF-based relativistic EOM-CCSD method emerges as a promising approach for large-scale excited-state calculations, achieving excellent agreement with the standard relativistic EOM-CCSD method based on the untruncated canonical spinor basis, but at a significantly reduced computational cost.

physics.chem-ph↗