This package contains a fast implementation for LAMMPS of quantum-based MGPT multi-ion potentials. The MGPT or model GPT method derives from first-principles DFT-based generalized pseudopotential theory (GPT) through a series of systematic approximations valid for mid-period transition metals with nearly half-filled d bands. The MGPT method was originally developed by John Moriarty at Lawrence Livermore National Lab (LLNL). In the general matrix representation of MGPT, which can also be applied to f-band actinide metals, the multi-ion potentials are evaluated on the fly during a simulation through d- or f-state matrix multiplication, and the forces that move the ions are determined analytically. The mgpt pair style in this package calculates forces and energies using an optimized matrix-MGPT algorithm due to Tomas Oppelstrup at LLNL. See the doc page for the pair_style mgpt command for full details on using this package in LAMMPS. In particular, the user should note that the MGPT potentials are explicitly volume dependent, requiring special attention in their application. Useful example scripts are given in the "examples/PACKAGES/mgpt" directory. These scripts show the necessary steps to perform constant-volume calculations and simulations. It is strongly recommended that the user work through and understand these examples before proceeding to more complex simulations. Specific MGPT potential data for the transition metals tantalum (Ta4 and Ta6.8x potentials), molybdenum (Mo5.2 potentials), and vanadium (V6.1 potentials) are contained in the LAMMPS "potentials" directory. It is expected that MGPT potentials for additional materials will be added over time. The persons who created the MGPT package are Tomas Oppelstrup (oppelstrup2@llnl.gov) and John Moriarty (moriarty2@llnl.gov) Contact them directly if you have any questions.