arXiv subjects

Sudipta Chakraborty

Publications and source records attributed to Sudipta Chakraborty.

7 recordsLinked to original sources

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

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

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 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↗

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↗