A Hessian free method to prevent zero-point energy leakage in classical trajectory simulation

The problem associated with the zero-point energy (ZPE) leak in classical trajectory calculations is well known. Since ZPE is a manifestation of the quantum uncertainty principle, there are no restrictions on energy during the classical propagation of nuclei. This phenomenon can lead to unphysical results, such as forming products without the ZPE in the internal vibrational degrees of freedom (DOFs). The ZPE leakage also permits reactions below the quantum threshold for the reaction. We have developed a new Hessian-free method, inspired by the Lowe-Andersen thermostat model, to prevent energy dipping below a threshold in the local-pair (LP) vibrational DOFs. The idea is to pump the leaked energy to the corresponding local vibrational mode, taken from the other vibrational DOFs. We have applied the new correction protocol on the ab initio ground-state molecular dynamics simulation of the water dimer (H20)2, which dissociates due to unphysical ZPE spilling from the high-frequency OH modes. The LP-ZPE method has been able to prevent the ZPE spilling of the OH stretching modes by pumping back the leaked energy into the corresponding modes while this energy is taken from the other modes of the dimer itself, keeping the system as a microcanonical ensemble.

Study of the Effect of Higher-order Dispersions on Photoionisation Induced by Ultrafast Laser Pulses Applying a Classical Theoretical Method

We investigated the effect of higher order dispersion on ultrafast photoionisation with Classical Trajectory Monte Carlo (CTMC) method for hydrogen and krypton atoms. In our calculations we used linearly polarised ultrashort 7 fs laser pulses, 6.5 × 1014 W/cm2 intensity, and a central wavelength of 800 nm. Our results show that electrons with the highest kinetic energies are obtained with transform limited (TL) pulses. The shaping of the pulses with negative second- third- or fourth- order dispersion results in higher ionisation yield and electron energies compared to pulses shaped with positive dispersion values. We have also investigated how the Carrier Envelope Phase (CEP) dependence of the ionisation is infuenced by dispersion. We calculated the left-right asymmetry as a function of energy and CEP for sodium atoms employing pulses of 4.5 fs, 800 nm central wavelength, and 4 × 1012 W/cm2 intensity. We found that the left-right asymmetry is more pronounced for pulses shaped with positive Group Delay Dispersion (GDD). It was also found that shaping a pulse with increasing amounts of GDD in absolute value blurs the CEP dependence, which is attributed to the increasing number of optical cycles.

Validating Experiment for the Reaction H2 + NH2- by Dynamical Computations on an Accurate Full-Dimensional Potential Energy Surface

Ion-neutral molecular reactions play key roles in the field of ion related chemistry. As a prototypical multi-channel ion-molecular reaction, the reaction H2 + NH2- → NH3 + H- has been studied for decades. In this work, we develop a globally accurate potential energy surface (PES) for the title system H2 + NH2- based on nearly hundreds of thousands points over a wide dynamically relevant region. The permutational invariants polynomials neural network (PIP-NN) method is used for fitting and the total root mean squared error (RMSE) is extremely small, only 0.026 kcal mol-1. Extensive dynamical and rate coefficient calculations are carried out on this new PIP-NN PES by the quasi-classical trajectory (QCT) method. The calculated rate coefficients for H2 / D2 + NH2- agree well with the experimental results that show a inverse temperature dependence from 50 to 300 K, consistent with the capture nature of this barrierless reaction. A significant kinetic isotope effect has been well reproduced by the QCT computations. In addition, we report a unique phenomenon of significant reactivity suppression by exciting the rotational mode of H2, particular at low collision energies. Further analysis shows that the excitation of rotational mode of H2 would prevent the formation of the reactant complex and thus suppress reactivity.

Addition Coupled Electron Transfer (ACET) and Addition Coupled Electron Coupled Proton Transfer (ACPCET)

The new types of elementary reaction in which a nucleophilic addition (A) to quinones is coupled with electron transfer (ET) and even further proton transfer (PT) are suggested herein by density functional theory calculation, which are called Addition Coupled Electron Transfer (ACET) or Addition Coupled Electron Coupled Proton Transfer (ACPCET). With a [2.2]paracyclophane-derived biquinone (1) as the substrate, the nature of nucleophilic addition onto its sp2 carbons exhibits a change from stepwise A-ET-PT to ACET-PT and further to ACPCET, in parallel with the decreased nucleophilicity of the attacking reagent. In addition, we further proposed six possible potential energy surfaces and the coupling modes between A, ET and PT, in which three have been found in this work. Quasi-classical trajectory shows that the ACET and PT event can also be dynamically concerted even for an ACET-PT mechanism.

Addition Coupled Electron Transfer (ACET) and Addition Coupled Electron Coupled Proton Transfer (ACPCET)

The new types of elementary reaction in which a nucleophilic addition (A) to quinones is coupled with electron transfer (ET) and even further proton transfer (PT) are suggested herein by density functional theory calculation, which are called Addition Coupled Electron Transfer (ACET) or Addition Coupled Electron Coupled Proton Transfer (ACPCET). With a [2.2]paracyclophane-derived biquinone (1) as the substrate, the nature of nucleophilic addition onto its sp2 carbons exhibits a change from stepwise A-ET-PT to ACET-PT and further to ACPCET, in parallel with the decreased nucleophilicity of the attacking reagent. In addition, we further proposed six possible potential energy surfaces and the coupling modes between A, ET and PT, in which three have been found in this work. Quasi-classical trajectory shows that the ACET and PT event can also be dynamically concerted even for an ACET-PT mechanism.

Target electron removal in C5+ + H collision

We present target ionization and charge exchange cross sections in a collision between C5+ ion and H atom. We treat the collision dynamics classically using a four-body classical trajectory Monte Carlo (CTMC) and a four-body quasi-classical Monte Carlo (QCTMC) model when the Heisenberg correction term is added to the standard CTMC model via model potential. The calculations were performed in the projectile energy range between 1.0 keV/amu and 10 MeV/amu. We found that the cross sections obtained by the QCTMC model are higher than that of the cross sections calculated by the standard CTMC model and these cross sections are closer to the previous experimental and theoretical data. Moreover, for the case of ionization, we show that the interaction between the projectile and the target electrons plays a dominant role in the enhancement of the cross sections at lower energies.