OpenQuantum is an ab initio molecular orbital (MO) calculation program written in Rust. It performs Hartree-Fock and post-Hartree-Fock calculations within the Linear Combination of Atomic Orbitals (LCAO) framework. Supports RHF, UHF, and ROHF for closed- and open-shell molecules, MP2 and CCSD(T) correlation energies, geometry optimization (BFGS, Berny RFO, primitive/DLC/TRIC internal coordinates) with analytic gradients, IRC and NEB pathways, harmonic frequency analysis with analytical Hessians and thermochemistry, and effective core potentials (ECPs).

Note: The project is under active development and not yet ready for production use.

Full Features, examples, and instructions

https://lenhanpham.github.io/OpenQuantum

Features

• Molecular Input: XYZ Cartesian and Z-matrix geometry formats, Angstrom or Bohr units
• Basis Sets: STO-3G, 6-31G, 6-31G*, cc-pVDZ, cc-pVTZ, def2-SVP, def2-TZVP, and 500+ more (H–Xe and beyond)
• One-Electron Integrals: Overlap (S), kinetic energy (T), and nuclear attraction (V) via Obara-Saika recurrences
• Two-Electron Integrals: McMurchie-Davidson algorithm with 8-fold permutational symmetry; SP analytical fast paths for S/P shells (up to 10× speedup); Rys quadrature for D/F+ shells; direct SCF mode for large systems with symmetry-accelerated cache lookup; engine selection via INT section - applies uniformly to in-core ERI build, direct SCF, analytical gradients/Hessians, and semi-analytical (FD) Hessians
• SCF Methods: RHF, UHF, and ROHF with DIIS, level shifting, damping, Fermi broadening, multiple initial guesses (core Hamiltonian, Hückel, SAD), and Quadratic Convergence SCF (QC-SCF) with Newton-Raphson orbital optimization
• DFT Methods: A few popular functionals such as B3LYP and BP86 are supported
• Post-HF: MP2 and CCSD(T) correlation energies (not fully tested)
• Analytical Gradients: RHF and UHF analytic nuclear gradients with symmetry-accelerated ERI derivatives (skips symmetry-equivalent shell quartets); used by BFGS optimizer
• Analytical Hessians: Fully analytical RHF and UHF Hessians including CPHF response, occupied-occupied reorthonormalization, and analytic d²ERI integrals with symmetry acceleration (skips symmetry-equivalent quartets); semi-analytical (FD) path also propagates the selected ERI engine to all displaced SCF evaluations
• Geometry Optimization: With several optimization algorithms
• Frequency Analysis: Harmonic vibrational frequencies via semi-analytical (finite-difference of gradients) or fully analytical Hessian; IR intensities, thermochemistry
• Thermochemistry: Zero-point energy, thermal corrections (U, H, G), entropy (translational + rotational + vibrational) via RRHO model with symmetry number
• ECP Support: Effective core potentials (LANL2DZ, Stuttgart, etc.)
• Symmetry: Point group detection, character tables, irrep assignment; symmetry acceleration for all ERI-heavy computations — energy (in-core + direct SCF), analytical gradient, and analytical Hessian (matching GRAD2E SymShl — skips symmetry-equivalent shell quartets before expensive integral evaluation)
• CPHF Solver: Coupled-perturbed Hartree-Fock equations for response properties; RHF and coupled 2×2 UHF spins
• Analysis: Mulliken population analysis, orbital energies, spin contamination ⟨S²⟩, dipole moments, EFG
• Checkpointing: Save and restart SCF from binary checkpoint files; checkpoint and other temporary files are written to a configurable scratch directory
• Simplicit solvents: Support basis implicit solvent treatments

Installation

Download the binary file that fits your system

OpenQuantum binaries:

Platform	Target	Recommendation
Windows	x86_64-pc-windows-msvc.zip	Best compatibility
Ubuntu / Debian	x86_64-unknown-linux-gnu.zip	Good
Rocky / RHEL	x86_64-unknown-linux-gnu.zip	musl better for distribution
Rocky / RHEL	x86_64-unknown-linux-musl.zip	musl better for distribution
General Linux	x86_64-unknown-linux-musl.zip	Best (static, very portable)
macOS Intel	x86_64-apple-darwin.zip	Good
macOS Apple Silicon	aarch64-apple-darwin.zip	Good

License

NOT DECIDED YET

Contributing

Le Nhan Pham