DynHeMat: A program for zero-point averaged dynamics of pure and doped helium nanodroplets

DynHeMat is a parallel program aimed at modeling the dynamics of pure and doped helium nanodroplets (HNDs) by means of zero-point averaged dynamics (ZPAD), a method where the quantum nature of helium atoms is taken into account through the use of a He-He pseudopotential which includes zero-point effects of helium clusters on an average manner. Three He-He pseudopotentials, defined for applications in different contexts, are implemented. Large HNDs can be formed by successive coalescences of smaller HNDs keeping in mind that, depending on the HND size and He-He pseudopotential in use, the liquid character of the HND is more or less pronounced. Files containing the positions and velocities of HNDs formed with the three aforementioned He-He pseudopotentials are collected in a local databank, called ZPAD_DB. ZPAD simulations can be carried out at constant energy or temperature, then enabling the user to investigate collision, coagulation or submersion processes in pure or doped HNDs. Impurities can be rare-gas atoms (Ne, Ar, Kr, Xe and Rn), alkali atoms (Li, Na, K, Rb, Cs), or homogeneous clusters composed of such atoms. The program provides information on trajectories, namely positions, velocities, energies, radial distribution functions, and the initial distribution of HND surface atoms. Extension to other impurities or He-He pseudopotentials is made possible by the current structure of the program and keyword system. PROGRAM SUMMARY Program title : DynHeMat CPC Library link to program files: Developer’s respository link: Licensing provisions : GNU General Public License 3 (GPL) Programming language : Fortran 90 Nature of problem : Helium nanodroplets (HNDs) are large quantum systems containing from a few thousands to billion atoms. The more quantum approaches, like quantum Monte Carlo, time-dependent density functional theory and path integral molecular dynamics, are often limited to the treatment of a few hundreds or thousands atoms or to small statistics in terms of projectile velocities or impact parameters, for instance. On the contrary, a classical approach would enable simulations on larger systems provided that the quantum nature of helium atoms is included on an average manner in the calculation in order to ensure that the expected heliophilic or heliophobic nature of impurities can be maintained. Solution method : The zero-point averaged dynamics (ZPAD) includes zero-point effects on an average manner in classical simulations through the use of an effective He-He potential, and possibly effective He-impurity potentials, which makes the HND liquid and drastically improves the agreement with quantum calculations compared to standard classical simulations. Initially used to tackle the fragmentation of rare-gas clusters embedded in HNDs ionized by electron impact, the ZPAD approach was later applied to study the coagulation of rare-gas atoms in liquid helium and the collision of alkali atoms with small HNDs. In the current version of the program, impurities can be rare gases (from Ne to Rn) or alkali atoms (from Li to Cs). The program is parallelized with MPI in such a way that several independent trajectories can be performed simultaneously, each of them being distributed over a set of CPU cores, which allows simulations on large HNDs ( N ≈ 10 5 ) to be carried out in a reasonable amount of time. Additional comments including Restrictions and Unusual features: The program was especially tested on × 86 and ARM architectures under operating systems linux RedHat 9 and MacOS Ventura, and the number of trajectories run in parallel should not exceed 999. Unlike TDDFT, ZPAD does not take into account superfluidity and quantum vortices cannot be investigated with this approach. Moreover, a significant amount of time may be needed to produce pure HNDs before running ZPAD simulations to investigate collision, coagulation or submersion processes. To compensate for this drawback, a databank which contains XYZ files, including positions and velocities, of He N (1000 ≤ N < 90000) is made available to DynHeMat users. References: 1. Modeling the fragmentation dynamics of ionic clusters inside helium nanodroplets: The case of He 100 Ne 4 + , D. Bonhommeau, P. T. Lake, Jr, C. Le Quiniou, M. Lewerenz and N. Halberstadt, J. Chem. Phys. 126 , 051104 (2007). DOI: 10.1063/1.2515225 2. Fragmentation of ionized doped helium nanodroplets: Theoretical evidence for a dopant ejection mechanism , D. Bonhommeau, M. Lewerenz and N. Halberstadt, J. Chem. Phys. 128 , 054302 (2008). DOI: 10.1063/1.2823101 3. Fragmentation dynamics of Ar 4 He 1000 upon electron impact ionization: competition between ion ejection and trapping , N. Halberstadt and D. A. Bonhommeau, J. Chem. Phys. 152 , 234305 (2020). DOI: 10.1063/5.0009363 4. Zero-point averaged dynamics of Ar n + He 1000 : Dopant-size influence on potential-energy surfaces, mass spectra and fragmentation patterns , D. A. Bonhommeau, Chem. Phys. 550 (2021) 111307. DOI: 10.1016/j.chemphys.2021.111307 5. Clustering, collision, and relaxation dynamics in pure and doped helium nanoclusters: Density- vs particle-based approaches , E. García-Alfonso, M. Barranco, D. A. Bonhommeau, N. Halberstadt, M. Pi, et F. Calvo, J. Chem. Phys. 157 , 014106 (2022). DOI: 10.1063/5.0091942 6. Collision of rare-gas atoms on helium nanodroplets: Theoretical evidence for an efficient coagulation of heavy rare-gas atoms , B. Fixot, E. Louaas and D. A. Bonhommeau, J. Chem. Phys. 161 , 044307 (2024). DOI: 10.1063/5.0220027 7. Collision of cesium atoms on helium nanodroplets: Unraveling mechanisms for surface capture at experimental velocities , D. A. Bonhommeau, J. Chem. Phys. 161 , 184302 (2024). DOI: 10.1063/5.0231641