DFTB+
A quantum-atomistic engine using approximate density-functional tight-binding methods, available as a standalone program or embeddable library.
Where it fits
Lower-cost geometry and dynamics studies when reliable parameters cover the chemistry.
Limits and practical considerations
Accuracy depends on the Hamiltonian and parameter set; verify element and bonding coverage.
Licensing and access
GNU LGPL software; parameter sets may carry separate licences and citation requirements.