arXiv subjects

Tamoghna Mukhopadhyay

Publications and source records attributed to Tamoghna Mukhopadhyay.

8 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

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↗

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

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↗

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↗