arXiv subjects

Achintya Kumar Dutta

Publications and source records attributed to Achintya Kumar Dutta.

At least 37 records · Page 2Linked to original sources

arXiv2025

Effect of Protein Environment on the Shape Resonances of RNA Nucleobases: Insights From a Model System

In this work, the effect of amino acid environment on the nucleobase-centered anion radical shape resonances is investigated by employing uracil as a model system for pyrimidine base in RNA. Anionic uracil-glycine complexes have been used to model the RNA-protein interactions. The resonance positions and widths of these complexes have been simulated using the equation of motion coupled cluster method coupled with resonance via Padé approach. Our work shows that in the transient negative ion (TNI, or, the anion radical of glycine:uracil complex), glycine stabilizes the nucleobase-centered resonances through hydrogen bonding, increasing the lifetime of TNI. At the same time, a glycine-centered resonance shows the ability of amino acids to capture the electron density and move it away from the uracil nucleobase. At the micro-solvation level, this modeling indicates that amino acids would have more influence on nucleobase-centered resonances in the TNI than that displayed by the corresponding aqueous environment.

physics.chem-ph↗

arXiv2025

A Reduced Cost Two-component Relativistic Equation-of-Motion Coupled Cluster Method for Ionization Potential

We report an efficient implementation of the ionization potential (IP) variant of the equation-of-motion coupled cluster (IP-EOM-CC) method based on the exact two-component atomic mean field (X2CAMF) framework, utilizing Cholesky decomposition (CD) and frozen natural spinors (FNS). The CD approximation significantly reduces memory demands, whereas the FNS approximation lowers the number of floating-point operations. Together, these techniques make the method computationally efficient for accurate relativistic IP-EOM-CC calculations of molecules containing heavy elements. The calculated IP values are almost identical to those obtained by the four-component relativistic IP-EOM-CC method. Benchmark studies show good agreement with experimental ionization energies and photoelectron spectra, demonstrating the method's reliability. The practical applicability of the approach is demonstrated by IP calculations on the medium-sized [I(H$_{2}$O)$_{12}$]$^{-}$ complex, with 1698 virtual spinors.

physics.chem-ph↗

arXiv2025

A low-cost four-component relativistic coupled cluster linear response theory based on perturbation sensitive natural spinors

We present an efficient implementation of four-component linear response coupled cluster singles and doubles (4c-LRCCSD) theory that enables accurate and computationally efficient calculation of polarizabilities for systems containing heavy elements. We have observed that the frozen natural spinor (FNS)-based truncation scheme is not suitable for linear response properties, as it leads to larger errors in static and dynamic polarizability values. In this work, we have introduced a "perturbation-sensitive" density to construct the natural spinor basis, termed FNS++. Using FNS++, we achieve excellent accuracy when compared to experimental data and other theoretical results, even after truncating nearly 70% of the total virtual spinors. We also present pilot applications of 4c-LRCCSD with a canonical basis to calculate the polarizability spectra of 3d transition metals. By employing the FNS++-based 4c-LRCCSD, we have been able to compute polarizabilities for systems with over 1200 virtual spinors, maintaining low computational cost and excellent accuracy.

physics.chem-ph↗

arXiv2025

Frozen natural spinors for Cholesky decomposition based two-component relativistic coupled cluster method

We present an efficient and cost-effective implementation for the exact two-component atomic mean field (X2CAMF) based coupled cluster (CC) method, which integrates frozen natural spinors (FNS) and the Cholesky decomposition (CD) technique. The use of CD approximation greatly reduces the storage requirement of the calculation without any significant reduction in accuracy. Compared to four-component methods, the FNS and CD-based X2CAMF-CC approach gives similar accuracy as that of the canonical four-component relativistic coupled cluster method at a fraction of the cost. The efficiency of the method is demonstrated by the calculation of a medium-sized uranium complex involving the correlation of over 1000 virtual spinors.

physics.chem-ph↗

arXiv2025

Double Ionization Potential Equation-of-Motion Coupled-Cluster Approach with Full Inclusion of 4-Hole-2-Particle Excitations and Three-Body Clusters

The double ionization potential (DIP) equation-of-motion (EOM) coupled-cluster (CC) method with a full treatment of 4-hole-2-particle (4$h$-2$p$) correlations and triply excited clusters, abbreviated as DIP-EOMCCSDT(4$h$-2$p$), and its approximate form called DIP-EOMCCSD(T)(a)(4$h$-2$p$) have been formulated and implemented in the open-source CCpy package available on GitHub. The resulting codes work with both nonrelativistic and spin-free scalar-relativistic Hamiltonians. By examining the DIPs of a few small molecules, for which accurate reference data are available, we demonstrate that the DIP-EOMCCSDT(4$h$-2$p$) and DIP-EOMCCSD(T)(a)(4$h$-2$p$) approaches improve the results obtained using the DIP-EOMCC methods truncated at 3$h$-1$p$ or 4$h$-2$p$ excitations on top of the CC calculations with singles and doubles.

physics.chem-ph↗

arXiv2024

A relativistic third-order algebraic diagrammatic construction theory for electron detachment, attachment and excitation problems

We present the theory and implementation of a highly efficient relativistic third-order algebraic diagrammatic construction [ADC(3)] method based on a four-component (4c) Dirac-Coulomb (DC) Hamiltonian for the calculation of ionization potentials (IP), electron affinities (EA), and excitation energies (EE). Benchmarking calculations for IP, EA, and EE were performed on both atomic and molecular systems to assess the accuracy of the newly developed four-component relativistic ADC(3) method. The results show good agreement with the available experimental data. The Hermitian nature of the 4c-ADC(3) Hamiltonian, combined with the perturbative truncation of the wave function, offers significant computational advantages over the standard equation-of-motion coupled-cluster approach, particularly for property calculations. The method's suitability for property calculations is further demonstrated by computing oscillator strengths and excited-state dipole moments for heavy elements.

physics.chem-ph↗

arXiv2024

Spin-free exact two-component linear response coupled cluster theory for estimation of frequency-dependent second-order property

We have presented the theory, implementation, and benchmark results for the one-electronic variant of spin-free exact two-component (SFX2C1e) linear response coupled cluster (LRCCSD) theory for static and dynamic polarizabilities of atoms and molecules in the spin-adapted formulation. The resolution of identity (RI) approximation for two-electron integrals has been used to reduce the computational cost of the calculation and has been shown to have a negligible effect on accuracy. The calculated static and dynamic polarizability values agree very well with the more expensive X2C-LRCCSD and experimental results. Our calculated results show that accurate predictions of polarizabilities of atoms and molecules containing heavy atoms require the use of a large basis set containing an adequate number of diffuse functions, in addition to accounting for electron correlation and relativistic effects.

physics.chem-ph↗

arXiv2024

Analytic Calculation of Transition dipole moment using four-component relativistic equation-of-motion coupled-cluster expectation value approach

We have developed an efficient scheme for the calculation of transition properties within the four-component relativistic equation-of-motion coupled-cluster (EOM-CC) method using the expectation value approach. The calculation of transition properties within the relativistic EOM-CC framework requires the solution of both right and left eigenvectors using Davidson's iterative diagonalization scheme. The accuracy of the approach has been investigated by calculating low-lying transitions of Xe atom, HI molecule and spin forbidden 1S0 to 3P1 and spin allowed 1S0 to 1P1 transitions in a few closed shell cations. Additionally, applications aimed at evaluating the L-edge X-ray absorption spectrum (XAS) of Ar atom is studied. The calculated results show good agreement with the earlier theoretical studies and experimental values.

physics.chem-ph↗

arXiv2024

A Reduced Cost Four-Component Relativistic Unitary Coupled Cluster Method for Molecules

We present a four-component relativistic unitary coupled cluster method for molecules. We have used commutator-based non-perturbative approximation using the ''Bernoulli expansion'' to derive an approximation to the relativistic unitary coupled cluster method. The performance of the full quadratic unitary coupled-cluster singles and doubles method \left ( qUCCSD \right ), as well as a perturbative approximation variant \left ( UCC3 \right ), has been reported for both energies and properties. It can be seen that both methods give results comparable to those of the standard relativistic coupled cluster method. The qUCCSD method shows better agreement with experimental results due to better inclusion of the relaxation effects. A natural spinor-based scheme to reduce the computation cost of relativistic UCC3 and qUCCSD methods has been discussed.

physics.chem-ph↗

arXiv2024

Relativistic equation-of-motion coupled-cluster theory analysis of black-body radiation shift in the clock transition of Zn I

We have employed equation-of-motion coupled-cluster (EOM-CC) method in the four-component relativistic theory framework to understand roles of electron correlation effects in the $\textit{ab initio}$ estimations of electric dipole polarizabilities ($α$) of the states engaged in the clock transition ($^{1}$S$_{0}$$\rightarrow$$^{3}$P$_{0}$) of the zinc atom. Roles of basis size, inclusion of higher-level excitations, and higher-order relativistic effects in the evaluation of both excitation energies of a few low-lying excited states and $α$ are analyzed systematically. Our EOM-CC values are compared with the earlier reported theoretical and experimental results. This demonstrates the capability of the EOM-CC method to ascertain the preciseness of the black-body radiation shift in a clock transition, which holds paramount importance for optical clock-based experiments.

physics.atom-ph↗

arXiv2023

Shape resonance induced electron attachment to Cytosine: The effect of aqueous media

We have investigated the impact of microsolvation on the shape resonance states of nucleobases, taking cytosine as a case study. To characterize the resonance position and decay width of the metastable states, we employed the newly developed DLPNO-based EA-EOMCCSD method in conjunction with resonance via Padé approximation. Our calculations show that the presence of water molecules causes a red shift in the resonance position and an increase in the lifetime for all three resonance states. Furthermore, the lowest resonance state in isolated cytosine was converted to a bound state in the presence of an aqueous environment. The calculated results are extremely sensitive to the basis set used for the calculations.

physics.chem-ph↗

arXiv2022

Electron Attachment to Wobble Base Pairs

We have analyzed the low-energy electron attachment to wobble base pairs using the equation motion coupled cluster method and extended basis sets. A doorway mechanism exists for the attachment of the additional electron to the base pairs, where the initially formed dipole-bound anion captures the incoming electron. The doorway dipole-bound anionic state subsequently leads to the formation of a valence-bound state, and the transfer of extra electron occurs by mixing of electronic and nuclear degrees of freedom. The formation of the valence-bound anion is associated with proton transfer in hypoxanthine-cytosine and hypoxanthine-adenine base pairs, which happens through a concerted electron-proton transfer process. The calculated rate constant for the dipole-bound to valence-bound transition in wobble base pairs is slower than that observed in the Watson-Crick guanine-cytosine base pair.

physics.chem-ph↗

arXiv2022

A low-cost four-component relativistic equation of motion coupled cluster method based on frozen natural spinors: Theory, Implementation and Benchmark

We present the theory and the implementation of a low-cost four-component relativistic equation of motion coupled cluster method for ionized states based on frozen natural spinors. A single threshold (natural spinor occupancy) can control the accuracy of the calculated ionization potential values. Frozen natural spinors can significantly reduce the computational cost for valence and core-ionization energies with systematically controllable accuracy. The convergence of the ionization potential values with respect to the natural spinor occupancy threshold becomes slower with the increase in basis set dimension. However, the use of a natural spinor threshold of 10-5 and 10-6 gives excellent agreement with experimental results for valence and core ionization energies, respectively.

physics.chem-ph↗

arXiv2022

Electron Attachment to DNA: The Protective Role of Amino Acids

We have studied the effect of amino acids on electron attachment properties of DNA nucleobases, taking cytosine as a model system. The equation of motion coupled cluster theory with an extended basis set has been used to simulate the electron-attached state of the DNA model system. Four selected amino acids, Arginine, Alanine, Lysine, and Glycine which form a major component of histone proteins are considered to investigate their role in electron attachment to DNA nucleobase. The electron attachment to cytosine in all the cytosine-amino acid dimer complexes follows a doorway mechanism, where the electron gets transferred from initial dipole-bound doorway state to the final nucleobase-bound state through the mixing of electronic and nuclear degrees of freedom. In higher amino acid concentration, amino acid-bound state acts as the doorway state, where the initial electron density is localized on the amino acid, away from the nucleobase. This leads to the physical shielding of nucleobase from the incoming extra electron. At the same time, the presence of amino acids can increase the stability of nucleobase-bound anionic state, which can suppress the dissociative electron attachment induced sugar-phosphate bond breaking.

physics.chem-ph↗

arXiv2022

A lower scaling four-component relativistic coupled cluster method based on natural spinors

We present the theory, implementation, and benchmark results for a frozen natural spinors-based lower scaling four-component relativistic coupled cluster method. The natural spinors are obtained by diagonalizing the one-body reduced density matrix from a relativistic MP2 calculation based on four-component Dirac-Coulomb Hamiltonian. The correlation energy in the coupled cluster method converges more rapidly with respect to the size of the virtual space in the frozen natural spinor basis than that observed in the standard canonical spinors obtained from the Dirac-Hartree-Fock calculation. The convergence of properties is not smooth in the frozen natural spinor basis. However, the inclusion of the perturbative correction smoothens the convergence of the properties with respect to the size of the virtual space in the frozen natural spinor basis and greatly reduces the truncation errors for both energy and properties calculations. The accuracy of the frozen natural spinor based coupled cluster methods can be controlled by a single threshold and is a black box to use.

physics.chem-ph↗

arXiv2021

A Doorway mechanism for Electron Attachment Induced DNA Strand Break

We report a new doorway mechanism for the dissociative electron attachment to genetic materials. The dipole-bound state of the nucleotide anion acts as the doorway for electron capture in the genetic material. The electron gets subsequently transferred to a dissociative σ* type anionic state localized on a sugar-phosphate or a sugar-nucleobase bond, leading to their cleavage. The electron transfer is mediated by the mixing of electronic and nuclear degrees of freedom. The cleavage rate of the sugar-phosphate bond predicted by this new mechanism is higher than that of the sugar-nucleobase bond breaking, and both processes are considerably slower than the formation of a stable valence-bound anion. The new mechanism explains the relative rates of electron attachment induced bond cleavages in genetic materials.

physics.chem-ph↗

arXiv2021

A Similarity Transformed Second-order Approximate Coupled Cluster Method for the Excited States: Theory, Implementation, and Benchmark

We present a novel and cost-effective approach of using a second similarity transformation of the Hamiltonian to include the missing higher-order terms in the second-order approximate coupled cluster singles and doubles (CC2) model. The performance of the newly developed ST-CC2 model has been investigated for the calculation of excitation energies of valence, Rydberg, and charge-transfer excited states. The method shows significant improvement in the excitation energies of Rydberg and charge-transfer excited states as compared to the conventional CC2 method while retaining the good performance of the latter for the valence excited state. The method retains the charge-transfer separability of the CT excited states, which is a significant advantage over the traditional CC2 method. An MBPT2 variant of the new method is also proposed.

physics.chem-ph↗

arXiv2021

An Efficient Fock Space Multi-reference Coupled Cluster Method based on Natural Orbitals: Theory, Implementation, and Benchmark

We present a natural orbital-based implementation of the intermediate Hamiltonian Fock space coupled-cluster method for (1,1) sector of Fock space. The use of natural orbital significantly reduces the computational cost and can automatically choose an appropriate active space. The new method retains the charge transfer separability of the original intermediate Hamiltonian Fock space coupled-cluster method and gives excellent performance for valence, Rydberg, and charge-transfer excited states. It offers significant computational advantages over the popular equation of motion coupled cluster method for excitation energy calculations, especially when one is interested in the calculation of large no excited states.

physics.chem-ph↗