arXiv subjects

Lee-Ping Wang

Publications and source records attributed to Lee-Ping Wang.

4 recordsLinked to original sources

arXiv2026

Structural Origin of Water Heat Capacity Anomaly from Classical and Quantum Simulations

Water isobaric heat capacity is anomalously large under ambient conditions and exhibits a sharp maximum upon supercooling. Using classical and path-integral molecular dynamics with accurate machine-learning interatomic potentials, we show that nuclear quantum effects primarily act by suppressing high-frequency vibrations, while the anomalous temperature dependence of the isobaric heat capacity originates from structural fluctuations, quantified by the second-solvent-shell intruder order parameter. A simple two-state mapping reveals an effective enthalpy scale of about 4 kJ/mol associated with the interconversion of low- and high-density-like local structures, providing a microscopic link between their population changes and the excess heat capacity from supercooled to ambient conditions.

cond-mat.stat-mech↗

arXiv2022

Multireference configuration interaction study of the predissociation of C$_{2}$ via its $F\,^1Π_u$ state

Photodissociation is one of the main destruction pathways for dicarbon (C$_{2}$) in astronomical environments such as diffuse interstellar clouds, yet the accuracy of modern astrochemical models is limited by a lack of accurate photodissociation cross sections in the vacuum ultraviolet range. C$_{2}$ features a strong predissociative $F\,^1Π_u - X\,^1Σ_g^+$ electronic transition near 130 nm originally measured in 1969; however, no experimental studies of this transition have been carried out since, and theoretical studies of the $F\,^1Π_u$ state are limited. In this work, potential energy curves of excited electronic states of C$_{2}$ are calculated with the aim of describing the predissociative nature of the $F\,^1Π_u$ state and providing new ab initio photodissociation cross sections for astrochemical applications. Accurate electronic calculations of 56 singlet, triplet, and quintet states are carried out at the DW-SA-CASSCF/MRCI+Q level of theory with a CAS(8,12) active space and the aug-cc-pV5Z basis set augmented with additional diffuse functions. Photodissociation cross sections arising from the vibronic ground state to the $F\,^1Π_u$ state are calculated by a coupled-channel model. The total integrated cross section through the $F\,^1Π_u$ $v=0$ and $v=1$ bands is 1.198$\times$10$^{-13} $cm$^2$cm$^{-1}$, giving rise to a photodissociation rate of 5.02$\times$10$^{-10}$ s$^{-1}$ under the standard interstellar radiation field, much larger than the rate in the Leiden photodissociation database. In addition, we report a new $2\,^1Σ_u^+$ state that should be detectable via a strong $2\,^1Σ_u^+-X\,^1Σ_g^+$ band around 116 nm.

astro-ph.GA↗

arXiv2019

Ab initio Study of Ground-State CS Photodissociation Via Highly Excited Electronic States

Photodissociation by ultraviolet radiation is the key destruction pathway for CS in photon-dominated regions, such as diffuse clouds. However, the large uncertainties of photodissociation cross sections and rates of CS, resulting from a lack of both laboratory experiments and theoretical calculations, limit the accuracy of calculated abundances of S-bearing molecules by modern astrochemical models. Here we show a detailed \textit{ab initio} study of CS photodissociation. Accurate potential energy curves of CS electronic states were obtained by choosing an active space CAS(8,10) in MRCI+Q/aug-cc-pV(5+d)Z calculation with additional diffuse functions, with a focus on the $B$ and $C\,^1Σ^+$ states. Cross sections for both direct photodissociation and predissociation from the vibronic ground state were calculated by applying the coupled-channel method. We found that the $C-X$ $(0-0)$ transition has extremely strong absorption due to a large transition dipole moment in the Franck-Condon region and the upper state is resonant with several triplet states via spin-orbit couplings, resulting in predissociation to the main atomic products C $(^3P)$ and S $(^1D)$. Our new calculations show the photodissociation rate under the standard interstellar radiation field is $2.9\ee{-9}$\,s$^{-1}$, with a 57\% contribution from $C-X$ $(0-0)$ transition. This value is larger than that adopted by the Leiden photodissociation and photoionization database by a factor of 3.0. Our accurate \textit{ab initio} calculations will allow more secure determination of S-bearing molecules in astrochemical models.

astro-ph.GA↗

arXiv2018

Polarizable Molecular Simulations Reveal How Silicon-containing Functional Groups Govern the Desalination Mechanism in Nanoporous Graphene

We report a molecular dynamics (MD) simulation study of reverse osmosis desalination using nanoporous monolayer graphene passivated by SiH$_2$ and Si(OH)$_2$ functional groups. A highly accurate and detailed polarizable molecular mechanics force field model was developed for simulating graphene nanopores of various sizes and geometries. The simulated water fluxes and ion rejection percentages are explained using detailed atomistic mechanisms derived from analysis of the simulation trajectories. Our main findings are: (1) The Si(OH)$_2$ pores possess superior ion rejection rates due to selective electrostatic repulsion of Cl$^-$ ions, but Na$^+$ ions are attracted to the pore and block water transfer. (2) By contrast, the SiH$_2$ pores operate via a steric mechanism that excludes ions based on the size and flexibility of their hydration layers. (3) In absence of ions, water flux is directly proportional to the solvent accessible area within the pore; however, simulated fluxes are lower than those inferred from recent experimental work. We also provide some hypotheses that could resolve the differences between simulation and experiment.

physics.chem-ph↗