arXiv subjects

Seyedehdelaram Jahani

Publications and source records attributed to Seyedehdelaram Jahani.

4 recordsLinked to original sources

arXiv2026

Ionization Potentials at Mean-Field Computational Cost: The Extended Koopmans' Framework for pCCD

We introduce a mean-field-like computational model for calculating ionization potentials (IPs) based on the pair Coupled Cluster Doubles (pCCD) wave function. Specifically, our model combines the extended Koopmans' theorem (EKT) with the advantages of a variationally orbital-optimized (oo)-pCCD ansatz. The computational cost of the EKT(pCCD) method is negligible (O(N^3)) as the response 1- and 2-particle reduced density matrices used to construct the generalized Fock matrix are readily available after an oo-pCCD calculation. We benchmarked our new computational model for IPs of atoms, small molecules, and a set of organic acceptor molecules against experimental and theoretical reference data. The EKT(pCCD) model significantly improves upon the modified Koopmans' approach [J. Chem. Phys. 162, 184110 (2025)], and the obtained IPs are comparable to those of computationally more expensive IP-EOM-pCCD-based models, approaching CCSD(T) reference values (with a mean error of 0.05 eV). Most importantly, the EKT(pCCD) approach is almost independent of the basis set size, and reliable IPs are already obtained with small basis sets.

physics.chem-ph↗

arXiv2026

Simple and efficient computational strategies for calculating orbital energies and pair-orbital energies from pCCD-based methods

We introduce affordable computational strategies for calculating orbital and pair-orbital energies in atomic and molecular systems. Our methods are based on the pair Coupled Cluster Doubles (pCCD) ansatz and its orbital-optimized variant. The computed orbital and pair-orbital energies are then subsequently used to approximate ionization potentials (IPs), electron affinities (EAs), the resulting charge gaps, double ionization potentials (DIPs), and double electron affinities (DEAs). Our methodology builds on the standard Koopmans' theorem and refines it for a pCCD-based wave function. Furthermore, we incorporate pCCD electron correlation effects into the model utilizing canonical Hartree-Fock or natural pCCD-optimized orbitals. The latter represents a diagonal approximation to the (D)IP/D(EA) equation of motion pCCD models. We benchmarked our newly developed models against theoretical and available experimental data for selected atoms in various basis set sizes and a set of 24 organic acceptor molecules. Our numerical results show that the Koopmans' approach based on pCCD natural orbitals provides a balanced treatment of occupied and virtual orbital energies, resulting in reliable predictions of charge gaps at a low computational cost.

physics.chem-ph↗

arXiv2026

Oscillator Strengths and Transition Dipole Moments from a Simplified Equation-of-Motion Coupled Cluster Formalism within the Frozen-Pair Approximation

In this work, we derive the working equations for the transition density matrices within the equation-of-motion frozen-pair coupled-cluster framework. We focus specifically on the recently developed EOM-fpCCSD and EOM-ptCCSD models and present the corresponding transition dipole moments and oscillator strengths obtained via approximate expressions. This formulation avoids the computational overhead of solving the coupled-cluster $Λ$ equations. Furthermore, we employ a matrix inverse approximation to eliminate the need for explicit calculation of the left EOM eigenvectors. The accuracy of the resulting EOM-fpCCD and EOM-ptCCSD excited-state properties is benchmarked against the linear-response (LR)-CCSD method. Our results demonstrate that the description of excited state properties is improved when using EOM-fpCCSD and EOM-ptCCSD models compared to the standard EOM-CCSD variant.

physics.chem-ph↗

arXiv2024

The relationship between structure and excited-state properties in polyanilines from geminal-based methods

We employ state-of-the-art quantum chemistry methods to study the structure-to-property relationship in polyanilines (PANIs) of different lengths and oxidation states. Specifically, we focus on leucoemeraldine, emeraldine, and pernigraniline in their tetramer and octamer forms. We scrutinize their structural properties, HOMO and LUMO energies, HOMO-LUMO gaps, and vibrational and electronic spectroscopy using various Density Functional Approximations (DFAs). Furthermore, the accuracy of DFAs is assessed by comparing them to experimental and wavefunction-based reference data. We perform large-scale orbital-optimized pair-Coupled Cluster Doubles (oo-pCCD) calculations for ground and electronically excited states and conventional Configuration Interaction Singles (CIS) calculations for electronically excited states in all investigated systems. The EOM-pCCD+S approach with pCCD-optimized orbitals allows us to unambiguously identify charge transfer and local transitions across the investigated PANI systems -- an analysis not possible within a delocalized canonical molecular orbital basis obtained, for instance, by DFAs. We show that the low-lying part of the emeraldine and pernigraniline spectrum is dominated by charge transfer excitations and that polymer elongation changes the character of the leading transitions. Furthermore, we augment our study with a quantum informational analysis of orbital correlations in various forms of PANIs.

physics.chem-ph↗