A High-Level Ab Initio Study of the N-2 + N-2 Reaction Channel

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Molecular Physics of Elementary Processes relevant to Hypersonics: atom-molecule, molecule-molecule and atom-surface processes.

In the present chapter some prototype gas and gas-surface processes occurring within the hypersonic flow layer surrounding spacecrafts at planetary entry are discussed. The discussion is based on microscopic dynamical calculations of the detailed cross sections and rate coefficients performed using classical mechanics treatments for atoms, molecules and surfaces. Such treatment allows the evaluation of the efficiency of thermal processes (both at equilibrium and nonequilibrium distributions) based on state-to-state and state specific calculations properly averaged over the population of the initial states. The dependence of the efficiency of the considered processes on the initial partitioning of energy among the various degrees of freedom is discussed

Efficiency of Collisional O2 + N2 Vibrational Energy Exchange

10 pags.; 6 figs.; 5 tabs. In press.By following the scheme of the Grid Empowered Molecular Simulator (GEMS), a new O2 + N2 intermolecular potential, built on ab initio calculations and experimental (scattering and second virial coefficient) data, has been coupled with an appropriate intramolecular one. On the resulting potential energy surface detailed rate coefficients for collision induced vibrational energy exchanges have been computed using a semiclassical method. A cross comparison of the computed rate coefficients with the outcomes of previous semiclassical calculations and kinetic experiments has provided a foundation for characterizing the main features of the vibrational energy transfer processes of the title system as well as a critical reading of the trajectory outcomes and kinetic data. On the implemented procedures massive trajectory runs for the proper interval of initial conditions have singled out structures of the vibrational distributions useful to formulate scaling relationships for complex molecular simulations.The authors acknowledge financial support from the Phys4- entry FP7/2007-2013 project (Contract 242311), ARPA Umbria, INSTM, the EGI-Inspire project (Contract 261323), MIUR PRIN 2008 (2008KJX4SN 003) and 2010/2011 (2010ERFKXL_002), the ESA-ESTEC contract 21790/08/ NL/HE, and the Spanish CTQ2012-37404 and FIS2013- 48275-C2-1-P projects. Computations have been supported by the use of Grid resources and services provided by the European Grid Infrastructure (EGI) and the Italian Grid Infrastructure (IGI) through the COMPCHEM Virtual Organization. Thanks are also due to the COST CMST European Cooperative Project CHEMGRID (Action D37) EGI Inspire.Peer reviewe

Full Dimensional Potential Energy Function and Calculation of State-Specific Properties of the CO+N2 Inelastic Processes Within an Open Molecular Science Cloud Perspective

A full dimensional Potential Energy Surface (PES) of the CO + N2 system has been generated by extending an approach already reported in the literature and applied to N2-N2 (Cappelletti et al., 2008), CO2-CO2 (Bartolomei et al., 2012), and CO2-N2 (Lombardi et al., 2016b) systems. The generation procedure leverages at the same time experimental measurements and high-level ab initio electronic structure calculations. The procedure adopts an analytic formulation of the PES accounting for the dependence of the electrostatic and non-electrostatic components of the intermolecular interaction on the deformation of the monomers. In particular, the CO and N2 molecular multipole moments and electronic polarizabilities, the basic physical properties controlling the behavior at intermediate and long-range distances of the interaction components, were made to depend on relevant internal coordinates. The formulated PES exhibits substantial advantages when used for structural and dynamical calculations. This makes it also well suited for reuse in Open Molecular Science Cloud services

Development of MQCT Method for Calculations of Collisional Energy Transfer for Astrochemistry and Planetary Atmospheres

A mixed quantum/classical methodology and an efficient computer code, named MQCT, were developed to model molecular energy transfer processes relevant to astrochemical environments and planetary atmospheres and applied to several real systems. In particular, the rotational energy transfer in N2 + Na collisions was studied with the focus on quantum phase, differential cross-sections, and scattering resonances, and excellent agreement with full quantum results was found. For H2O + H2, detailed calculations were carried out with the focus on allowed vs. forbidden transitions between the ortho/para states of both collision partners. Again, excellent agreement with full quantum calculations was achieved. Calculations of rotational energy transfer in a collision of two asymmetric-top rotors, a unique capability of this code, were tested using H2O + H2O system where the full-quantum calculations are unfeasible. To make MQCT calculations practical, an approximate, very efficient version of the method was developed, in which the classical-like equations of motion for the translational degrees of freedom (scattering) are decoupled from the quantum-like equations for time-evolution of the internal molecular states (rotational, vibrational). The code MQCT was made publicly available to serve as an efficient computational tool for other members of the community. It can perform scattering calculations on larger molecules and at higher collision energy than it is currently possible with full quantum methods and codes. To study the rotational quenching of isotopically substituted sulfur molecules, such as 32S32S, 32S34S, and 34S34S, a new accurate potential energy surface was developed for S2+Ar system. Rotational state-to-state transition cross sections were computed using MQCT, and the master equation modeling of energy transfer kinetics was carried out. It is found that isotopically substituted asymmetric molecules such as 32S34S promote energy transfer due to symmetry breaking and transitions with odd ∆j that become allowed. This process may be responsible for mass-independent isotopic fractionation of sulfur isotopes, typical to the Archean surface deposits

Towards hybrid molecular simulations

In many biology, chemistry and physics applications molecular simulations can be used to study material and process properties. The level of detail needed in such simulations depends on the application. In some cases quantum mechanical simulations are indispensable. However, traditional ab-initio methods, usually employing plane waves or a linear combination of atomic orbitals as a basis, are extremely expensive in terms of computational as well as memory requirements. The well-known fact that electronic wave functions vary much more rapidly near the atomic nuclei than in inter-atomic regions calls for a multi-resolution approach, allowing one to use low resolution and to add extra resolution only in those regions where necessary, so limiting the costs. This is provided by an alternative basis formed of wavelets. Using such a wavelet basis, a method has been developed for solving electronic structure problems that has been applied successfully to 2D quantum dots and 3D molecular systems. In other cases, it suffices to use effective potentials to describe the atomic interaction instead of the use of the electronic structure, enabling the simulation of larger systems. Molecular dynamics simulations with such effective potentials have been used for a systematic study of surface wettability influence on particle and heat flow in nanochannels, showing that the effects at the solid-gas interface are crucial for the behavior of the whole nanochannel. Again in other cases even coarse grained models can be used where the average behavior of several atoms is combined into a single particle. Such a model, refraining from as much detail as possible while maintaining realistic behavior, has been developed for lipids and with this model the dynamics of membranes and vesicle formation have been studied in detail. A disadvantage of molecular dynamics simulations with effective potentials is that no reactions are possible. Therefore a new method has been developed, where molecular dynamics is coupled with stochastic reactions. Using this method, both unilamellar and multilamellar vesicle formation, and vesicle growth, bursting, and healing are shown. Still larger systems can be simulated using other methods, like the direct simulation Monte Carlo method. However, as shown for nanochannels, these methods are not always accurate enough. But, exploiting again that the finest level of detail is often only needed in part of the domain, a hybrid method has been developed coupling molecular dynamics, where needed for accuracy, and direct simulation Monte Carlo, where possible in order to speed up the calculation. Further development of such hybrid simulations will further increase molecular simulation’s scientific role