arXiv subjects

Paweł Tecmer

Publications and source records attributed to Paweł Tecmer.

At least 19 recordsLinked to original sources

arXiv2026

Efficient and reliable modeling of large $π$-electron systems with the Pariser--Parr--Pople Hamiltonian and pCCD-based methods

Model Hamiltonians offer a cost-effective way to capture the key physics of large $π$-conjugated systems. In this work, we combine the Pariser--Parr--Pople (PPP) model Hamiltonian with pair Coupled Cluster Doubles (pCCD)-based methods to study the excited-state electronic structures of polycyclic aromatic hydrocarbons (PAHs). The model Hamiltonian is implemented in the open-source PyBEST software package, which provides numerous pCCD-type models that have been shown to perform well for the electronic structure properties of large organic molecules when combined with quantum-chemical Hamiltonians and all-electron basis sets. Within the PPP model, we probe canonical Hartree--Fock and natural pCCD-optimized orbitals in predicting excited-state properties using the linear response formalism on top of pCCD. Their performance is compared with configuration-interaction-based methods and the conventional EOM-CCSD approach. Our study demonstrates that pCCD/PPP-based approaches are an efficient, cost-effective, and accurate framework to compute excited-state properties in large $π$-conjugated organic systems, such as extended PAHs or other systems relevant to organic electronics.

physics.chem-ph↗

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

Frontier Orbital Engineering in Heteroatom-Doped Prototypical Organic Dyes for Dye-Sensitized Solar Cells

The computational design of heteroatom-doped organic dyes for dye-sensitized solar cells (DSSCs) remains challenging, as predictive methods must accurately describe long-range charge-transfer (CT) excitations while remaining computationally efficient for systematic materials screening. In this work, we investigate the electronic structure and excited-state properties using the range-separated hybrid functional LC-$ω$PBE in conjunction with linear-response time-dependent density functional theory (TDDFT) within the Tamm-Dancoff approximation (TDA). We employ a simplified, physically motivated, effective tuning protocol ($ω_{eff}$) to enable the rapid and reliable screening of electronic properties of organic dyes. Charge-transfer excitation energies and frontier orbital alignment the key factors governing light absorption and electron injection in DSSCs are analyzed through targeted heteroatom (N, O, and B) incorporation into donor-$π$-acceptor (D-$π$-A) organic dyes. A library of 27 mono-, di-, and tri-doped prototypical organic dyes is designed based on a carbazole donor and a cyanoacrylic acid acceptor through targeted doping at three positions of the $π$-bridge or linker. Distinct design trends emerge: electron-rich nitrogen and oxygen dopants increase the HOMO-LUMO gap and blue-shift CT excitations, with nitrogen exhibiting the strongest effect, whereas electron-deficient boron substitution narrows the gap and induces pronounced red shifts. Notably, the BBN-doped dye exhibits the smallest gap and lowest excitation energy, highlighting boron-rich motifs as promising candidates for enhanced solar light harvesting. Overall, this study establishes transferable heteroatom-doping guidelines and introduces an efficient, reliable, and cost-effective tuned DFT-TDDFT framework for high-throughput computational discovery and optimization of DSSC sensitizers.

physics.chem-ph↗

arXiv2026

EOM-fpCCSD: An Accurate Alternative to EOM-CCSD for Doubly Excited and Charge-Transfer States

We introduce a new equation-of-motion coupled-cluster method based on a pair coupled-cluster doubles (pCCD) reference, termed frozen-pair EOM-CCSD (EOM-fpCCSD). This approach combines the computational efficiency of the pCCD ansatz with a dynamical correlation correction, enabling a reliable description of electronically excited states within the EOM framework. The method has been implemented in the open-source PyBEST software package. Its performance is systematically benchmarked against standard EOM-CCSD and its pair-tailored variant (EOM-ptCCSD), using both canonical Hartree-Fock and pCCD natural orbitals. For charge-transfer (CT) excitations taken from the QUEST database, EOM-fpCCSD yields excitation energies very close to those of EOM-CCSD, outperforming EOM-ptCCSD, as well as to the theoretical best estimates (TBEs). Working within the localized pCCD natural orbital basis allows us to determine the directed CT character, which quantifies the directed charge flow from one molecular domain to another. Numerical results show that EOM-fpCCSD, EOM-CCSD, and EOM-ptCCSD provide nearly identical descriptions of the directed CT character, despite changes in excitation energies. The true advantage of EOM-fpCCSD becomes evident for the challenging QUEST subset of doubly excited states. While EOM-ptCCSD performs similarly to standard EOM-CCSD, EOM-fpCCSD significantly outperforms both methods for these problematic states compared to TBEs. In addition to improving the accuracy of excitation energies, EOM-fpCCSD also converges for several states that standard EOM-CCSD and EOM-ptCCSD fail to converge. These results demonstrate that EOM-fpCCSD offers a promising and computationally efficient route toward a more accurate description of complex electronic excitations.

physics.chem-ph↗

arXiv2026

Frozen density embedding with pCCD electron densities

The pair-coupled-cluster doubles (pCCD) method has emerged as a viable approach for quantum-chemical studies of strongly correlated systems. Despite its lower formal scaling (O(N$^4$)) compared to other versions of coupled cluster (CC) theory, applications to large chemical structures are still expensive. Fragmentation and embedding strategies offer a viable approach in such cases. In this work, we present a simple and efficient density-embedding scheme based on pCCD electron densities. The main computational benefit arises from the fact that pCCD response $Λ$-equations are much cheaper to compute than those of standard CC methods, providing easy access to one-electron properties. The pCCD densities of the individual subsystems are used to generate static embedding potentials that capture the environment's effect on the embedded system. The individual fragment energies are then iteratively converged in a self-consistent fashion. We demonstrate the reliable performance of this scheme with the estimation of dipole moments of the weakly bound CO2$\cdots$Rg (Rg = He, Ne, Ar, and Kr) complexes and with the modeling of vertical excitations of some microsolvated molecules.

physics.chem-ph↗

arXiv2026

Efficient Coupled-Cluster Python Frameworks for Next-Generation GPUs: A Comparative Study of CuPy and PyTorch on the Hopper and Grace Hopper Architecture

In this work, we introduce new batching algorithms to effectively handle large contractions encountered in coupled-cluster singles and doubles (CCSD) implementations in Python on the Video Random Access Memory (VRAM) of graphical processing units (GPUs), thereby improving performance. Specifically, we benchmark the performance of the CuPy and PyTorch libraries on a single NVIDIA Hopper (H100) and the Grace Hopper (GH200) architectures. We begin by optimizing the particle-particle ladder bottleneck contraction in CCSD using an asymmetric and dynamic splitting recipe, and then move toward a generic tensor contraction protocol that enables tensor contractions to be performed almost exclusively on GPUs. We benchmark our new, fully generic GPU-accelerated coupled-cluster implementations for various molecular systems and basis-set sizes, using both the CuPy and PyTorch libraries. While PyTorch outperforms CuPy on H100 by approximately 20\%, both perform similarly on the GH200 architecture. Compared to our initial GPU implementation [J. Chem. Theory Comput. 2024, 20, 3, 1130--1142], we achieve a 10-fold speedup. In molecular CCSD calculations, we report additional speedups between 3 and 16 for a single CCSD iteration using Cholesky-decomposed electron repulsion integrals compared to our original GPU-CPU hybrid implementation.

physics.chem-ph↗

arXiv2024

Linear Response pCCD-Based Methods: LR-pCCD and LR-pCCD+S Approaches for the Efficient and Reliable Modeling of Excited state Properties

In this work, we derive working equations for the Linear Response pair Coupled Cluster Doubles (LR-pCCD) ansatz and its extension to singles (S), LR-pCCD+S. These methods allow us to compute electronic excitation energies and transition dipole moments based on a pCCD reference function. We benchmark the LR-pCCD+S model against the {linear response} coupled-cluster singles and doubles method for modeling electronic spectra (excitation energies and transition dipole moments) of the BH, \ce{H2O}, \ce{H2CO}, and furan molecules. We also analyze the effect of orbital optimization within pCCD on the resulting LR-pCCD+S transition dipole moments {and oscillator strengths} and perform a statistical error analysis. We show that the LR-pCCD+S method can correctly reproduce the transition dipole moments features, thus representing a reliable and cost-effective alternative to standard, more expensive electronic structure methods for modeling electronic spectra of simple molecules. Specifically, the proposed models require only mean-field-like computational cost, while excited-state properties may approach the CCSD level of accuracy. Moreover, we demonstrate the capability of our model to simulate electronic transitions with non-negligible contributions of double excitations and the electronic spectra of polyenes of various chain lengths, for which standard electronic structure methods perform purely.

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↗

arXiv2024

Changing the paradigm in f-containing cold molecules: the impact of spin-orbit coupling and f-d transitions on quasi-bound vibrational states

Present-day state-of-the-art ab initio many-body calculations on f-block containing cold molecules heavily focus on perturbative approaches for spin-orbit coupling and exclude a substantial part of the atomic transitions in the $f$- and $d$-shell. Here, we demonstrate the cruciality of a proper relativistic treatment of the $f$- and $d$-shell in Yb-containing diatomics and the inclusion of $f\rightarrow d$ transitions to obtain physically sound elastic scatterings and pre-dissociation lifetimes. We focus on state-of-the-art relativistic many-body calculations for the Yb atom's ground- and excited-state and the YbLi$^+$ potential energy surface. For that purpose, we exploit various quantum many-body methods, namely a spin-free and four-component implementation of the coupled cluster singles and doubles (CCSD) model and its equation of motion extensions, spin-free complete active space self-consistent field, and internally contracted multi-reference (MR) configuration interaction approaches, and spin-free MRCCSD with a perturbative and full triples correction. We oppose scalar relativistic calculations to four-component variants to support the reliability of our EOM-CCSD study and shed new light on the interplay between these systems' spin-orbit coupling and the proper treatment of relativistic effects. Most importantly, we observe a significant shift in the electronic spectra of the $f\rightarrow d$ excitation block. We also provide new reference potential energy surfaces for ground and excited states for which theoretically sound elastic scattering and pre-dissociation lifetimes are calculated.

physics.atom-ph↗

arXiv2024

Exploring electron affinities, LUMO energies, and band gaps with electron-pair theories

We introduce the electron attachment equation-of-motion pair coupled cluster doubles (EA-EOM-pCCD) ansatz, which allows us to inexpensively compute electron affinities, energies of unoccupied orbitals, and electron attachment spectra. We assess the accuracy of EA-EOM-pCCD for a representative data set of organic molecules for which experimental data is available, as well as the electron attachment process in uranyl dichloride. EA-EOM-pCCD provides more reliable energies for the LUMO than its ionization potential EOM counterpart for the HOMO. The advantage of EA-EOM-pCCD is demonstrated for rylene and rylene diimide units of different chain lengths, where the differences between theoretical and experimental EAs approach chemical accuracy.

physics.chem-ph↗

arXiv2024

Delving into the Catalytic Mechanism of Molybdenum Cofactors: A Novel Coupled Cluster Study

In this work, we use modern electronic structure methods to model the catalytic mechanism of different variants of the molybdenum cofactor (Moco). We investigate the dependence of various Moco model systems on structural relaxation and the importance of environmental effects for five critical points along the reaction coordinate with the DMSO and NO$_3^-$ substrates. Furthermore, we scrutinize the performance of various coupled-cluster approaches for modeling the relative energies along the investigated reaction paths, focusing on several pair coupled cluster doubles (pCCD) flavors and conventional coupled cluster approximations. Moreover, we elucidate the Mo--O bond formation using orbital-based quantum information measures, which highlight the flow of $σ_{\rm M-O}$ bond formation and $σ_{\rm N/S-O}$ bond breaking. Our study shows that pCCD-based models are a viable alternative to conventional methods and offer us unique insights into the bonding situation along a reaction coordinate. Finally, this work highlights the importance of environmental effects or changes in the core and, consequently, in the model itself to elucidate the change in activity of different Moco variants.

physics.chem-ph↗

arXiv2023

Accelerating Pythonic coupled cluster implementations: a comparison between CPUs and GPUs

We scrutinize how to accelerate the bottleneck operations of Pythonic coupled cluster implementations performed on a \texttt{NVIDIA} Tesla V100S PCIe 32GB (rev 1a) Graphics Processing Unit (GPU). The \texttt{NVIDIA} Compute Unified Device Architecture (CUDA) API is interacted with via \texttt{CuPy}, an open-source library for Python, designed as a \texttt{NumPy} drop-in replacement for GPUs. The implementation uses the Cholesky linear algebra domain and is done in {PyBEST}, the Pythonic Black-box Electronic Structure Tool -- a fully-fledged modern electronic structure software package. Due to the limitations of Video Memory (VRAM), the GPU calculations must be performed batch-wise. Timing results of some contractions containing large tensors are presented. The \texttt{CuPy} implementation leads to factor 10 speed-up compared to calculations on 36 CPUs. Furthermore, we benchmark several Pythonic routines for time and memory requirements to identify the optimal choice of the tensor contraction operations available. Finally, we compare an example CCSD and pCCD-LCCSD calculation performed solely on CPUs to their CPU--GPU hybrid implementation. Our results indicate a significant speed-up (up to a factor of 16 regarding the bottleneck operations) when offloading specific contractions to the GPU using \texttt{CuPy}.

physics.chem-ph↗

arXiv2023

Geminal-based strategies for modeling large building blocks of organic electronic materials

We elaborate on unconventional electronic structure methods based on geminals and their potential to advance the rapidly developing field of organic photovoltaics (OPV). Specifically, we focus on the computational advantages of geminal-based methods over standard approaches and identify the critical aspects of OPV development. Examples are reliable and efficient computations of orbital energies, electronic spectra, and van-der-Waals interactions. Geminal-based models can also be combined with quantum embedding techniques and a quantum information analysis of orbital interactions to gain a fundamental understanding of the electronic structures and properties of realistic OPV building blocks. Furthermore, other organic components present in, for instance, dye-sensitized solar cells (DSSC) represent another promising scope of application. Finally, we provide numerical examples predicting the properties of a small building block of OPV components and two carbazole-based dyes proposed as possible DSSC sensitizers.

physics.chem-ph↗

arXiv2023

Static Embedding with Pair Coupled Cluster Doubles Based Methods

Quantum embedding methods have recently developed significantly to model large molecular structures. This work proposes a novel wave function theory in density functional theory (WTF-in-DFT) embedding scheme based on pair-coupled cluster doubles (pCCD)-type methods. While pCCD can reliably describe strongly-correlated systems with mean-field-like computational cost, the large extent of dynamic correlation can be accounted for by (linearized) coupled-cluster corrections on top of the pCCD wave function. Here we focus on the linearized coupled-cluster singles and doubles (LCCSD) ansatz for electronic ground states and its extension to excited states within the equation of motion (EOM) formalism. We test our EOM-pCCD-LCCSD-in-DFT approach for the vertical excitation energies of the hydrogen-bonded water--ammonia complex and uranyl tetrahalides (UO$_2$X$_4^{2-}$, X=F, Cl, Br). Furthermore, we assess the quality of the embedding potential using an orbital entanglement and correlation analysis. The approximate models successfully capture changes in the excitation energies going from bare fragments to supramolecular structures and represent a promising computation model for excited states in large molecular systems.

physics.chem-ph↗

arXiv2021

Reexamination of the ground state Born-Oppenheimer Yb$_2$ potential

The precision of the photoassociation spectroscopy of Yb dimer in degenerate gases is enough to improve the constraints on the new short-range gravity-like forces if the theoretical knowledge of the Born-Oppenheimer interatomic potential and non-Born-Oppenheimer interactions is refined [M. Borkowski et al. Sci. Rep. A {\bf 9}, 14807 (2019)]. The ground-state interaction potential of ytterbium dimer is investigated at the eXact 2-component core-correlated CCSD(T) level of {\it ab initio} theory in the complete basis set limit with extensive augmentation by diffuse functions. For the small basis set the comparison is made with the four-component relativistic finite-nuclei CCSD(T) calculations to identify the contraction of the dimer bond length as the main unrecoverable consequence of the scalar-relativistic approximation. Empirical constraint on the number of bound vibrational energy levels of the $^{174}$Yb$_2$ dimer is accounted for by representing the global {\it ab initio}-based Born-Oppenheimer potential with the model semianalytical function containing the scale and shift parameters. The results support the previous evaluation of the Yb dimer potentials from the photoassociation spectroscopy data and provide an accurate and flexible reference for future refinement of the constraints on the short-range gravity-like forces by ultracold atomic spectroscopy.

physics.atom-ph↗

arXiv2020

Pythonic Black-box Electronic Structure Tool (PyBEST). An open-source Python platform for electronic structure calculations at the interface between chemistry and physics

Pythonic Black-box Electronic Structure Tool (PyBEST) represents a fully-fledged modern electronic structure software package developed at Nicolaus Copernicus University in Toruń. The package provides an efficient and reliable platform for electronic structure calculations at the interface between chemistry and physics using unique electronic structure methods, analysis tools, and visualization. Examples are the (orbital-optimized) pCCD-based models for ground- and excited-states electronic structure calculations as well as the quantum entanglement analysis framework based on the single-orbital entropy and orbital-pair mutual information. PyBEST is written primarily in the Python3 programming language with additional parts written in C++, which are interfaced using Pybind11, a lightweight header-only library. By construction, PyBEST is easy to use, to code, and to interface with other software packages. Moreover, its modularity allows us to conveniently host additional Python packages and software libraries in future releases to enhance its performance. The electronic structure methods available in PyBEST are tested for the half-filled 1-D model Hamiltonian. The capability of PyBEST to perform large-scale electronic structure calculations is demonstrated for the model vitamin B12 compound. The investigated molecule is composed of 190 electrons and 777 orbitals for which an orbital optimization within pCCD and an orbital entanglement and correlation analysis are performed for the first time.

physics.chem-ph↗

arXiv2020

Modeling the electronic structures of the ground and excited states of the ytterbium atom and the ytterbium dimer: A modern quantum chemistry perspective

We present a comprehensive theoretical study of the electronic structures of the Yb atom and the Yb$_2$ molecule, respectively, focusing on their ground and lowest-lying electronically excited states. Our study includes various state-of-the-art quantum chemistry methods such as CCSD, CCSD(T), CASPT2 (including spin--orbit coupling), and EOM-CCSD as well as some recently developed pCCD-based approaches and their extensions to target excited states. Specifically, we scan the lowest-lying potential energy surfaces of the \ce{Yb2} dimer and provide a reliable benchmark set of spectroscopic parameters including optimal bond lengths, vibrational frequencies, potential energy depths, and adiabatic excitation energies. Our in-depth analysis unravels the complex nature of the electronic spectrum of \ce{Yb2}, which is difficult to model accurately by any conventional quantum chemistry method. Finally, we scrutinize the bi-excited character of the first $^1Σ_g^+$ excited state and its evolution along the potential energy surface.

physics.chem-ph↗