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ANT 2023 is a program for quasiclassical and semiclassical trajectories, both single-surface trajectories for which the Born-Oppenheimer approximation is valid and multi-surface calculations with electronic state changes, i.e., for electronically adiabatic and electronically nonadiabatic trajectories. There are several methods available for multisurface problems: surface hopping with or without time uncertainty and with or without decoherence, semiclassical Ehrenfest, self-consistent decay of mixing, and coherent switching with decay of mixing (CSDM). The potential surface for single-surface problems may be an analytic potential function supplied by the user, or one may use direct dynamics. To use the adiabatic representation (i.e., electronically adiabatic basis functions) for electronically nonadiabatic dynamics, the user may either provide the adiabatic surfaces and nonadiabatic couplings by direct dynamics, or the program may calculate them from diabatic surfaces and diabatic couplings, which are usually analytic. One can also use analytic fits to the surfaces and couplings to carry out calculations entirely in the diabatic representation. The curvature-driven approximation is available as an option for use with CSDM and trajectory surface hopping. The ANT 2023 program is especially recommended for calculations with analytic potential energy surfaces and couplings because of its high efficiency for such calculations. Program Title: ANT 2023 , revision B CPC Library link to program files: https://doi.org/10.17632/gr74wwckcp.1 Developer's repository link: https://doi.org/10.5281/zenodo.10011563 Licensing Provisions: Apache-2.0 Programming language : Fortran 90 Interfaces to other programs : For direct dynamics, the code has well tested interfaces to the Gaussian 09 [1], Gaussian 16 [2], MOPACmn [3], and Molpro [4] electronic structure codes. The interfaces are included, but these electronic structure programs must be obtained separately. Users may also provide their own interfaces to other electronic structure programs. [1] M. J. Frisch et al. Gaussian 09, Gaussian, Inc., Wallingford CT, 2016. [2] M. J. Frisch et al., Gaussian 16, Gaussian, Inc., Wallingford CT, 2016. [3] J. J. P. Stewart et al., MOPAC 5.022mn, University of Minnesota, Minneapolis, MN, 2015; Zenodo. https://doi.org/10.5281/zenodo.8153079 [4] H.-J. Werner et al., J. Chem. Phys. 152 (2020) 144107. Nature of problem : classical, quasiclassical, or semiclassical molecular dynamics for electronically adiabatic or electronically nonadiabatic collisions and unimolecular processes of atoms, molecules, and clusters Solution method : The program integrates the classical or semiclassical equations of motion for the motion of the nuclei. ANT 2023 can use anharmonic initial-state selection for initial states of diatomics or harmonic initial-state selection for general polyatomics. The equations of motion for electronically adiabatic processes are Hamilton's equations with potential energy functions corresponding to the Born-Oppenheimer approximation. The equations of motion for electronically nonadiabatic processes (non-Born-Oppenheimer processes) include trajectory surface hopping (fewest switches with or without time uncertainty and with or without decoherence) or several possible mean-field methods, including the semiclassical Ehrenfest method (also called the time-dependent Hartree method), self-consistent decay of mixing, and coherent switching with decay of mixing. Electronically nonadiabatic processes may be treated using either the adiabatic representation or a diabatic representation, and the potential surfaces and couplings may be obtained from analytic fits in a user-supplied subroutine or by direct dynamics, which involves requesting energies, gradients, and couplings as needed from a quantum chemistry electronic structure program. In the adiabatic representation, the coupling may be nonadiabatic coupling vectors (based on gradients of electronic wave functions), time-derivative couplings (based on overlaps of electronic wave functions), or curvature-driven couplings (based on the curvatures of the adiabatic potential energy surfaces). Additional comments : The ANT 2023 program authors are Y. Shu, L. Zhang, J. Zheng, Z. H. Li, A. W. Jasper, D. A. Bonhommeau, R. Valero, R. Meana-Pañeda, S. L. Mielke, Z. Varga, and D. G. Truhlar [5]; the authors of the article [6] describing ANT 2023 are Y. Shu, L. Zhang, and D. G. Truhlar. References : [1] M. J. Frisch et al. Gaussian 09, Gaussian, Inc., Wallingford CT, 2016. [2] M. J. Frisch et al., Gaussian 16, Gaussian, Inc., Wallingford CT, 2016. [3] J. J. P. Stewart et al., MOPAC 5.022mn, University of Minnesota, Minneapolis, MN, 2015; Zenodo. https://doi.org/10.5281/zenodo.8153079 [4] H.-J. Werner et al., J. Chem. Phys. 152 (2020) 144107. [5] Y. Shu, L. Zhang, J. Zheng, Z. H. Li, A. W. Jasper, D. A. Bonhommeau, R. Valero, R. Meana-Pañeda, S. L. Mielke, Z. Varga, & D. G. Truhlar. (2023). ANT 2023: A Program for Adiabatic and Nonadiabatic Trajectories (Version 2023). Zenodo. https://doi.org/10.5281/zenodo.10011563 [6] Y. Shu, L. Zhang, & D. G. Truhlar. Computer Physics Communications, to be published. [ABSTRACT FROM AUTHOR]