arXiv subjects

Lan Cheng

Publications and source records attributed to Lan Cheng.

At least 19 recordsLinked to original sources

arXiv2026

Use of Cholesky decomposition in the CFOUR program package

An overview is given about our efforts to speed up high-level computations of molec- ular energies and properties using Cholesky decomposition (CD). We describe the cor- responding developments in the CFOUR quantum-chemical package with a focus on (a) the use of CD in complete-active space self-consistent-field (CASSCF) and coupled- cluster (CC) computations, (b) the implementation of analytic CC gradients using CD, (c) the efficient calculation of magnetic properties (NMR shieldings and magnetizabili- ties) using CD when carried out with explicit magnetic-field dependent basis functions, i.e., the so-called gauge-including atomic orbitals (GIAOs), (d) the efficient computa- tion of CASSCF response properties using CD, (e) the use of CD in relativistic two- and four-component computations, as well as (f) computations for molecules in finite magnetic fields using GIAOs together with CD. An outlook on future developments concerning the CFOUR package is given.

physics.chem-ph↗

arXiv2026

Limit on the nuclear Schiff moment of europium-153

The Schiff moment of a nucleus is a symmetry-violating nuclear moment that indicates new physics beyond the Standard Model. We place the limit, $|\mathscr{S}({}^{153}$Eu)$| < 1.7 \times 10^{-8}$ $e\,$fm$^3$ (95\% confidence), on the Schiff moment of the $^{153}$Eu nucleus, using nuclear spin resonances in two ensembles of oppositely-polarized $^{153}$Eu$^{3+}$ ions in a Y${}_2$SiO${}_5$ crystal. This measurement using octupolar nuclei in a mm-scale crystal constrains new physics at the TeV energy-scale.

physics.atom-ph↗

arXiv2026

Relativistic Complete Active Space Self-consistent-Field Method with a Hierarchy of Exact Two-Component Hamiltonians

The development of a novel exact two-component (X2C) scheme with the inclusion of the picture-change correction for the fluctuation potential, the X2Ccorr scheme, is reported, hereby establishing a hierarchy of X2C schemes with systematic improvement for the treatments of relativistic two-electron contributions. Using benchmark X2C complete active space self-consistent-field (CASSCF) calculations for zero-field splittings in chalcogen diatomics, the contributions of two-electron spin-orbit coupling, electron spin-spin coupling, and quantum electrodynamics are carefully analyzed. The capability of the new Cholesky decomposition-based implementation for relativistic two-component CASSCF method using super-configuration-interaction algorithms is further demonstrated with calculations for the low-lying electronic states of neodymium aqua-ions with up to the second coordination shells.

physics.chem-ph↗

arXiv2025

Hyperfine-Resolved Spectroscopy of Dysprosium Monoxide (DyO) for Precision Measurements of the Nuclear Schiff Moment

We perform laser spectroscopy of dysprosium monoxide (DyO) to determine the hyperfine structure of the ground X8 and excited [17.1]7 states in the $^{161}$Dy and $^{163}$Dy isotopologues. These dysprosium nuclei have non-zero nuclear spin and dynamical octupole deformation, which gives them high sensitivity to time-reversal-violating new physics via the nuclear Schiff moment (NSM). The DyO molecule was recently identified as being amenable to optical cycling -- the basis for many laser cooling and quantum control techniques -- which makes it a practical candidate for NSM searches. The measurements reported here are prerequisites to implementing optical cycling, designing precision measurement protocols, and benchmarking calculations of molecular sensitivity to symmetry-violating effects. The measured hyperfine parameters are interpreted using simple molecular orbital diagrams and show excellent agreement with relativistic quantum chemical calculations.

physics.atom-ph↗

arXiv2025

Production and spectroscopy of cold radioactive molecules

Molecules with heavy, radioactive nuclei promise extreme sensitivity to fundamental nuclear and particle physics. However, these nuclei are available in limited quantities, which challenges their use in precision measurements. Here we demonstrate the gas-phase synthesis, cryogenic cooling, and high-resolution laser spectroscopy of radium monohydroxide, monodeuteroxide, and monofluoride molecules ($^{226}$RaOH, $^{226}$RaOD, and $^{226}$RaF) in a tabletop apparatus by combining novel radioactive target production protocols, optically driven chemistry in a cryogenic buffer gas, and low-background spectroscopic detection methods. The molecules are cooled in the lab frame, creating conditions that are the same starting points as many current molecular precision measurement and quantum information experiments. This approach is readily applied to a wide range of species and establishes key capabilities for molecular quantum sensing of exotic nuclei.

physics.atom-ph↗

arXiv2025

High-Efficiency Quantum-State Detection of ThF$^+$ with Resonance-Enhanced Multiphoton Asymmetric Dissociation

Efficient quantum-state detection is crucial for many precision control experiments, such as the ongoing effort to probe the electron's electric dipole moment using trapped molecular $^{232}\mathrm{ThF}^+$ ions at JILA. While quantum state detection through state-selective photodissociation has been successfully implemented on this molecule, progress has been hindered by low dissociation efficiency. In this work, we perform spectroscopy on the molecule to identify excited states that facilitate more efficient photodissociation. For the most favorable transition, we achieve a dissociation efficiency of 57(14)% with quantum state selectivity. Additionally, we discuss several state detection protocols that leverage favorable excited states that will facilitate simultaneous readout of all EDM relevant states, allowing further improvement of overall statistics.

physics.atom-ph↗

arXiv2025

Nuclear Schiff moment of fluorine isotope $^{19}$F

Nuclear Schiff moments (NSMs) are sensitive probes for physics beyond the Standard Model of particle physics, signaling violations of time-reversal and parity-inversion symmetries in atomic nuclei. In this Letter, we report the first-ever calculation of a NSM in a nuclear ab initio framework, employing the no-core shell model to study the fluorine isotope $^{19}$F. We further perform quantum-chemistry calculations to evaluate the sensitivity of the hafnium monofluoride cation, HfF$^+$, to the NSM of $^{19}$F. Combined with recent high-precision measurements of the molecular electric dipole moment of HfF$^+$, our results enable the first experimental bound on the NSM of $^{19}$F. Although the resulting bounds on the pion-nucleon-nucleon ($\pi$NN) coupling constants are not yet the most stringent, this work establishes the foundation for constraining $\pi$NN interactions using nuclear ab initio methods.

nucl-th↗

arXiv2025

Nuclear Electric Quadrupole Moment-Induced Parity Doubling in Molecules for Symmetry-Violation Searches

Searches for the nuclear magnetic quadrupole moment (MQM) and nuclear Schiff moment (NSM) have high discovery potential for violations of time ($T$) and parity ($P$) reversal symmetries beyond the Standard Model. Molecules containing heavy nuclei are typically used to enhance the sensitivity to MQMs and NSMs due to their strong internal electric fields and potential octupole deformation. To extract these effects in the laboratory frame, a bias electric field is required to polarize the molecule by mixing states of opposite parity (parity doublets). Typical heavy nuclei that are sensitive to symmetry-violation also possess large nuclear electric quadrupole moments (EQMs) when its nuclear spin is $I\geq1$. We show that EQMs can significantly modify the energy splitting between parity doublet states and thus change the required polarizing electric field. As a result, the EQM-induced energy splitting must be taken into account in designing such experiments. We provide qualitative estimates of parity doubling from EQMs and supporting \textit{ab initio} calculations, along with implications for candidate molecules in symmetry-violation searches.

physics.atom-ph↗

arXiv2025

Relativistic Two-Electron Contributions within Exact Two-Component Theory

The development of relativistic exact two-component (X2C) theory is briefly reviewed, with an emphasis on cost-effective treatments of relativistic two-electron contributions by means of model potential (MP) techniques and closely related atomic mean-field (AMF) approaches. The correct MP or AMF contribution to the electronic energy is elucidated. The performance of one-center approximations to relativistic two-electron contributions is carefully assessed using benchmark calculations of molecular properties.

physics.chem-ph↗

arXiv2025

Unconventional Chemical Bonding of Lanthanide-OH Molecules

We present a theoretical study of the low lying adiabatic relativistic electronic states of lanthanide monohydroxide (Ln-OH) molecules near their linear equilibrium geometries. We focus on heavy, magnetic DyOH and ErOH relevant to fundamental symmetry tests. We use a restricted-active-space self-consistent field method combined with spin-orbit coupling as well as a relativistic coupled-cluster method. In addition, electric dipole and magnetic moments are computed with the self-consistent field method. Analysis of the results from both methods shows that the dominant molecular configuration of the ground state is one where an electron from the partially filled and submerged 4f orbital of the lanthanide atom moves to the hydroxyl group, leaving the closed outer-most 6s$^2$ lone electron pair of the lanthanide atom intact in sharp contrast to the bonding in alkaline-earth monohydroxides and YbOH, where an electron from the outer-most s shell moves to the hydroxyl group. For linear molecules the projection of the total electron angular momentum on the symmetry axis is a conserved quantity with quantum number $\Omega$ and we study the polynomial $\Omega$ dependence of the energies of the ground states as well as their electric and magnetic moments. We find that the lowest energy states have $|\Omega|=15/2$ and 1/2 for DyOH and ErOH, respectively. The zero field splittings among these $\Omega$ states is approximately $hc\times 1\,000$~cm$^{-1}$. We find that the permanent dipole moments for both triatomics are fairly small at 0.23 atomic units. The magnetic moments are closely related to that of the corresponding atomic Ln$^+$ ion in an excited electronic state. We also realize that the total electron angular momentum is to good approximation conserved and has a quantum number of 15/2 for both triatomic molecules.

physics.atom-ph↗

arXiv2025

Narrowline Laser Cooling and Spectroscopy of Molecules via Stark States

The electronic energy level structure of yttrium monoxide (YO) provides a long-lived, low-lying $^{2}\Delta$ state ideal for high-precision molecular spectroscopy, narrowline laser cooling at the single photon-recoil limit, and studying dipolar physics with unprecedented interaction strength. High-resolution laser spectroscopy of ultracold laser-cooled YO molecules is used to study the Stark effect in the A$^{\prime}\,^{2}\Delta_{3/2}\,J=3/2$ state. An immediate onset of the linear Stark effect is observed in the presence of weak applied electric fields due to the near degenerate $\Lambda$-doublet and the large electric dipole moment. By applying a small electric field the Stark insensitive state is spectroscopically isolated and the absolute transition frequency to the X$\,^2\Sigma^+$ electronic ground state is determined with a fractional frequency uncertainty of 9 $\times$ 10$^{-12}$. This electric field control is necessary to implement a quasi-closed photon cycling scheme that preserves parity. With this scheme the first narrowline laser cooling of a molecules is demonstrated, reducing the temperature of sub-Doppler cooled YO in two dimensions.

physics.atom-ph↗

arXiv2024

Optical Cycling and Sensitivity to the Electron's Electric Dipole Moment in Gold-Containing Molecules

We propose diatomic molecules built from gold and carbon-group atoms as promising candidates for optical cycling and precision measurements. We show that this class of molecules (AuX, X = C, Si, Ge, Sn, Pb) features laser-accessible electronic transitions with nearly diagonal Franck-Condon factors. The $^2\Pi_{1/2}$ ground states can be easily polarized in the laboratory frame and have near-zero magnetic moments, valuable features for quantum science and precision measurement applications. The sensitivities of AuX molecules to the electron electric dipole moment (EDM) are found to be favorable, with effective electric fields of 10-30 GV/cm. Together, these features imply that AuX molecules may enable significantly improved searches for time-reversal symmetry violation.

physics.atom-ph↗

arXiv2024

Relativistic Exact Two-Component Coupled-Cluster Study of Molecular Sensitivity Factors for Nuclear Schiff Moments

Relativistic exact two-component coupled-cluster calculations of molecular sensitivity factors for nuclear Schiff moments (NSMs) are reported. We focus on molecules containing heavy nuclei, especially octupole-deformed nuclei. Analytic relativistic coupled-cluster gradient techniques are used and serve as useful tools for identifying candidate molecules that sensitively probe for physics beyond the Standard Model in the hadronic sector. Notably, these tools enable straightforward ``black-box'' calculations. Two competing chemical mechanisms that contribute to the NSM are analyzed, illuminating the physics of ligand effects on NSM sensitivity factors.

physics.atom-ph↗

arXiv2024

Vibrational Branching Ratios for Laser-Cooling of Nonlinear Strontium-Containing Molecules

The vibrational branching ratios from the lowest excited electronic state for $\textrm{SrOCH}_3$, $\textrm{SrNH}_2$, and $\textrm{SrSH}$ are measured at the $< 0.1\%$ level. Spectra are obtained by driving the $\tilde{X} - \tilde{A}$ transitions and dispersing the fluorescence on a grating spectrometer. We also perform $\textit{ab initio}$ calculations for the energies of vibrational levels relevant for laser cooling, as well as branching ratios to support the interpretations of all molecular spectra. Symmetry group analysis is applied in conjunction with our data to study rotational closure in these molecules. These analyses indicate favorable prospects for laser cooling $\textrm{SrNH}_2$ and other similar alkaline-earth(-like) amides for future beyond the Standard Model physics searches using polyatomic molecules with long-lived parity doublets.

physics.atom-ph↗

arXiv2024

A new computational framework for spinor-based relativistic exact two-component calculations using contracted basis functions

A new computational framework for spinor-based relativistic exact two-component (X2C) calculations is developed using contracted basis sets with a spin-orbit contraction scheme. Generally contracted j-adapted basis sets using primitive functions in the correlation-consistent basis sets are constructed for the X2C Hamiltonian with atomic mean-field spin-orbit integrals (the X2CAMF scheme). The contraction coefficients are taken from atomic X2CAMF Hartree-Fock spinors, hereby following the simple concept of linear combination of atomic orbitals (LCAOs). Benchmark calculations of spin-orbit splittings, equilibrium bond lengths, and harmonic vibrational frequencies demonstrate the accuracy and efficacy of the j-adapted spin-orbit contraction scheme.

physics.chem-ph↗

arXiv2023

A Unified Pre-training and Adaptation Framework for Combinatorial Optimization on Graphs

Combinatorial optimization (CO) on graphs is a classic topic that has been extensively studied across many scientific and industrial fields. Recently, solving CO problems on graphs through learning methods has attracted great attention. Advanced deep learning methods, e.g., graph neural networks (GNNs), have been used to effectively assist the process of solving COs. However, current frameworks based on GNNs are mainly designed for certain CO problems, thereby failing to consider their transferable and generalizable abilities among different COs on graphs. Moreover, simply using original graphs to model COs only captures the direct correlations among objects, which does not consider the mathematical logicality and properties of COs. In this paper, we propose a unified pre-training and adaptation framework for COs on graphs with the help of the maximum satisfiability (Max-SAT) problem. We first use Max-SAT to bridge different COs on graphs since they can be converted to Max-SAT problems represented by standard formulas and clauses with logical information. Then, we further design a pre-training and domain adaptation framework to extract the transferable and generalizable features so that different COs can benefit from them. In the pre-training stage, Max-SAT instances are generated to initialize the parameters of the model. In the fine-tuning stage, instances from CO and Max-SAT problems are used for adaptation so that the transferable ability can be further improved. Numerical experiments on several datasets show that features extracted by our framework exhibit superior transferability and Max-SAT can boost the ability to solve COs on graphs.

cs.AI↗

arXiv2023

A Cholesky decomposition-based implementation of relativistic two-component coupled-cluster methods for medium-sized molecules

A Cholesky decomposition (CD)-based implementation of relativistic two-component coupled-cluster (CC) and equation-of-motion CC (EOM-CC) methods using an exact two-component Hamiltonian augmented with atomic-mean-field spin-orbit integrals (the X2CAMF scheme) is reported. The present CD-based implementation of X2CAMF-CC and EOM-CC methods employs atomic-orbital-based algorithms to avoid the construction of two-electron integrals and intermediates involving three and four virtual indices. Our CD-based implementation extends the applicability of X2CAMF-CC and EOM-CC methods to medium-sized molecules with the possibility to correlate around 1000 spinors. Benchmark calculations for uranium-containing small molecules have been performed to assess the dependence of the CC results on the Cholesky threshold. A Cholesky threshold of $10^{-4}$ is shown to be sufficient to maintain chemical accuracy. Example calculations to illustrate the capability of the CD-based relativistic CC methods are reported for the bond-dissociation energy of the uranium hexafluoride molecule, UF$_6$, with up to quadruple-zeta basis sets, and the lowest excitation energy in solvated uranyl ion [UO$_2^{2+}$(H$_2$O)$_{12}$].

physics.chem-ph↗

arXiv2023

Intensity-borrowing mechanisms pertinent to laser cooling of linear polyatomic molecules

A study of the intensity-borrowing mechanisms important to optical cycling transitions in laser-coolable polyatomic molecules arising from non-adiabatic coupling, contributions beyond the Franck-Condon approximation, and Fermi resonances is reported. It has been shown to be necessary to include non-adiabatic coupling to obtain computational accuracy that is sufficient to be useful for laser cooling of molecules. The predicted vibronic branching ratios using perturbation theory based on the non-adiabatic mechanisms have been demonstrated to agree well with those obtained from variational discrete variable representation calculations for representative molecules including CaOH, SrOH, and YbOH. The electron-correlation and basis-set effects on the calculated transition properties, including the vibronic coupling constants, the spin-orbit coupling matrix elements, and the transition dipole moments, and on the calculated branching ratios have been thoroughly studied. The vibronic branching ratios predicted using the present methodologies demonstrate that RaOH is a promising radioactive molecule candidate for laser cooling.

physics.chem-ph↗