arXiv subjects

Elvira R. Sayfutyarova

Publications and source records attributed to Elvira R. Sayfutyarova.

5 recordsLinked to original sources

arXiv2026

Orbital choice in constructing model Hamiltonians

Model Hamiltonians (e.g., the Hubbard or Heisenberg models) provide a simple yet physically meaningful description of complex electronic phenomena through a small number of parameters, such as electron hopping integrals or magnetic exchange couplings. Their construction typically exploits the local nature of electron correlation effects and therefore relies on localized orbitals. Although many localization schemes are available, the resulting localized orbitals depend not only on the chosen localization functional but also on the orbital space to which the localization procedure is applied. These choices are often treated as technical details and are rarely discussed explicitly. Here, we demonstrate that different, chemically reasonable choices of localized orbitals can lead to substantially different model Hamiltonian parameters, even when they produce nearly identical electronic energies. Using Density Matrix Downfolding, we systematically investigate how the localization functional and, more importantly, the choice of model orbital space affect effective Hamiltonians derived from ab initio calculations for representative $π$-conjugated systems and transition metal complexes.

physics.chem-ph↗

arXiv2018

Electron paramagnetic resonance g-tensors from state interaction spin-orbit coupling density matrix renormalization group

We present a state interaction spin-orbit coupling method to calculate electron paramagnetic resonance (EPR) $g$-tensors from density matrix renormalization group wavefunctions. We apply the technique to compute $g$-tensors for the \ce{TiF3} and \ce{CuCl4^2-} complexes, a [2Fe-2S] model of the active center of ferredoxins, and a \ce{Mn4CaO5} model of the S2 state of the oxygen evolving complex. These calculations raise the prospects of determining $g$-tensors in multireference calculations with a large number of open shells.

physics.chem-ph↗

arXiv2017

Automated construction of molecular active spaces from atomic valence orbitals

We introduce the atomic valence active space (AVAS), a simple and well-defined automated technique for constructing active orbital spaces for use in multi-configuration and multireference (MR) electronic structure calculations. Concretely, the technique constructs active molecular orbitals capable of describing all relevant electronic configurations emerging from a targeted set of atomic valence orbitals (e.g., the metal d orbitals in a coordination complex). This is achieved via a linear transformation of the occupied and unoccupied orbital spaces from an easily obtainable single-reference wavefunction (such as from a Hartree-Fock or Kohn-Sham calculations) based on projectors to targeted atomic valence orbitals. We discuss the premises, theory, and implementation of the idea, and several of its variations are tested. To investigate the performance and accuracy, we calculate the excitation energies for various transition metal complexes in typical application scenarios. Additionally, we follow the homolytic bond breaking process of a Fenton reaction along its reaction coordinate. While the described AVAS technique is not an universal solution to the active space problem, its premises are fulfilled in many application scenarios of transition metal chemistry and bond dissociation processes. In these cases the technique makes MR calculations easier to execute, easier to reproduce by any user, and simplifies the determination of the appropriate size of the active space required for accurate results.

physics.chem-ph↗

arXiv2017

The Python-based Simulations of Chemistry Framework (PySCF)

PySCF is a general-purpose electronic structure platform designed from the ground up to emphasize code simplicity, both to aid new method development, as well as for flexibility in computational workflow. The package provides a wide range of tools to support simulations of finite size systems, extended systems with periodic boundary conditions, low dimensional periodic systems, and custom Hamiltonians, using mean-field and post-mean-field methods with standard Gaussian basis functions. To ensure easy of extensibility, PySCF uses the Python language to implement almost all its features, while computationally critical paths are implemented with heavily optimized C routines. Using this combined Python/C implementation, the package is as efficient as the best existing C or Fortran based quantum chemistry programs. In this paper we document the capabilities and design philosophy of the current version of the PySCF package.

physics.chem-ph↗

arXiv2013

Charge transfer in the cold Yb$^+$ + Rb collisions

Charge-transfer cold Yb$^+$ + Rb collision dynamics is investigated theoretically using high-level {\it ab initio} potential energy curves, dipole moment functions and nonadiabatic coupling matrix elements. Within the scalar-relativistic approximation, the radiative transitions from the entrance $A^1Σ^+$ to the ground $X^1Σ^+$ state are found to be the only efficient charge-transfer pathway. The spin-orbit coupling does not open other efficient pathways, but alters the potential energy curves and the transition dipole moment for the $A-X$ pair of states. The radiative, as well as the nonradiative, charge-transfer cross sections calculated within the $10^{-3}-10$ cm$^{-1}$ collision energy range exhibit all features of the Langevin ion-atom collision regime, including a rich structure associated with centrifugal barrier tunneling (orbiting) resonances. Theoretical rate coefficients for two Yb isotopes agree well with those measured by immersing Yb$^+$ ion in an ultracold Rb ensemble in a hybrid trap. Possible origins of discrepancy in the product distributions and relations to previously studied similar processes are discussed.

physics.atom-ph↗