arXiv subjects

Tina N. Mihm

Publications and source records attributed to Tina N. Mihm.

5 recordsLinked to original sources

arXiv2025

Sulfur and sulfur-oxide compounds as potential optically active defects on SWCNTs

Semiconducting single-walled carbon nanotubes (SWCNT) functionalized with covalent defects are a promising class of optoelectronic materials with strong, tunable photoluminescence and demonstrated single photon emission (SPE). Here, we investigate sulfur-oxide containing compounds as a new class of optically active dopants on (6,5) SWCNT. Experimentally, it has been found that when the SWCNT is exposed to sodium dithionite, the resulting compound displays a red-shifted and bright photoluminescence peak that is characteristic of doping with covalent defects. We perform density functional theory calculations on the possible adsorbed compounds that may be the source of doping (S, SO, SO2 and SO3). We predict that the two smallest molecules strongly bind to the SWCNT with binding energies of ~ 1.5-1.8 eV and 0.56 eV for S and SO, respectively, and introduce in-gap electronic states into the bandstructure of the tube consistent with the measured red-shift of (0.1-0.3) eV, consistent with measurements. In contrast, the larger compounds are found to be either unbound or weakly physisorbed with no appreciable impact on the electronic structure of the tube, indicating that they are unlikely to occur. Overall, our study suggests that sulfur-based compounds are promising new dopants for (6,5) SWCNT with tunable electronic properties.

cond-mat.mtrl-sci↗

arXiv2023

How the exchange energy can affect the power laws used to extrapolate the coupled cluster correlation energy to the thermodynamic limit

Finite size error is commonly removed from coupled cluster theory calculations by $N^{-1}$ extrapolations over correlation energy calculations of different system sizes ($N$), where the $N^{-1}$ scaling comes from the total energy. However, previous studies in the quantum Monte Carlo community suggest an exchange-energy-like power law of $N^{-2/3}$ is also be present in the correlation energy when using the conventional Coulomb interaction. The rationale for this is that the total energy goes as $N^{-1}$ and the exchange energy as $N^{-2/3}$; so, the correlation energy should be a combination of the two power laws. Further, in coupled cluster theory, these power laws are related to the low $G$ scaling of the transition structure factor, $S(G)$, which is a property of the coupled cluster wavefunction calculated from the amplitudes. We show that data from coupled cluster doubles calculations on the uniform electron gas(UEG) fit a function with a low $G$ behavior of $S(G)$~$G$. The pre-factor for this linear term is derived from the exchange energy to be consistent with an $N^{-2/3}$ power law at large $N$. Incorporating the exchange structure factor into the transition structure factor results in a combined structure factor of $S(G)$~$G^2$, consistent with an $N^{-1}$ scaling of the exchange-correlation energy. We then look for the presence of an $N^{-2/3}$ power law in the energy. First, we develop a plane-wave cutoff scheme with less noise than the traditional basis set used for the UEG. Then, we collect data from a wide range of electron numbers and densities to systematically test 5 methods using $N^{-1}$ scaling, $N^{-2/3}$ scaling, or combinations of both scaling behaviors. We find that power laws that incorporate both $N^{-1}$ and $N^{-2/3}$ scaling perform better than either alone, especially when the pre-factor for $N^{-2/3}$ scaling can be found from exchange energy calculations.

physics.comp-ph↗

arXiv2022

Machine learning for a finite size correction in periodic coupled cluster theory calculations

We introduce a straightforward Gaussian process regression (GPR) model for the transition structure factor of metal periodic coupled cluster singles and doubles (CCSD) calculations. This is inspired by the method introduced by Liao and Grüneis for interpolating over the transition structure factor to obtain a finite size correction for CCSD [J. Chem. Phys. 145, 141102 (2016)], and by our own prior work using the transition structure factor to efficiently converge CCSD for metals to the thermodynamic limit [Nat. Comput. Sci. 1, 801 (2021)]. In our CCSD-FS-GPR method to correct for finite size errors, we fit the structure factor to a 1D function in the momentum transfer, $G$. We then integrate over this function by projecting it onto a k-point mesh to obtain comparisons with extrapolated results. Results are shown for lithium, sodium, and the uniform electron gas.

physics.comp-ph↗

arXiv2021

Effective Hamiltonians for the study of real metals using quantum chemical theories

Computationally efficient and accurate quantum mechanical approximations to solve the many-electron Schrödinger equation are at the heart of computational materials science. In that respect the coupled cluster hierarchy of methods plays a central role in molecular quantum chemistry because of its systematic improvability and computational efficiency. In this hierarchy, coupled cluster singles and doubles (CCSD) is one of the most important steps in moving towards chemical accuracy and, in recent years, its scope has successfully been expanded to the study of insulating surfaces and solids. Here, we show that CCSD theory can also be applied to real metals. In so doing, we overcome the limitation of needing extremely large supercells to capture long range electronic correlation effects. An effective Hamiltonian can be found using the transition structure factor--a map of electronic excitations from the Hartree--Fock wavefunction--which has fewer finite size effects than conventional periodic boundary conditions. This not only paves the way of applying coupled cluster methods to real metals but also reduces the computational cost by two orders of magnitude compared to previous methods. Our applications to phases of lithium and silicon show a resounding success in reaching the thermodynamic limit, taking the first step towards a truly universal quantum chemical treatment of solids.

cond-mat.mtrl-sci↗

arXiv2020

Accelerating convergence to the thermodynamic limit with twist angle selection applied to methods beyond many-body perturbation theory

We recently developed a scheme to use low-cost calculations to find a single twist angle where the couple cluster doubles energy of a single calculation matches the twist-averaged coupled cluster doubles energy in a finite unit cell. We used initiator full configuration interaction quantum Monte Carlo ($i$-FCIQMC) as an example of an exact method beyond coupled cluster doubles theory to show that this selected twist angle approach had comparable accuracy in methods beyond coupled cluster. Further, at least for small system sizes, we show that the same twist angle can also be found by comparing the energy directly (at the level of second-order Moller-Plesset theory) suggesting a route toward twist angle selection which requires minimal modification to existing codes which can perform twist averaging.

physics.chem-ph↗