ORCA¶
ORCA is a modern, freely available quantum chemistry software package developed by the Neese Research Group for performing electronic structure calculations on molecules ranging from small organic systems to large transition-metal complexes. It provides an extensive collection of Hartree–Fock (HF), Density Functional Theory (DFT), post-Hartree–Fock, and multireference electronic structure methods, making it one of the most widely used quantum chemistry programs in both academic research and industrial applications.
ORCA is particularly recognized for its efficient implementations of multiconfigurational methods, high-level electron correlation techniques, spectroscopic property calculations, relativistic effects, and transition-metal chemistry.
In this documentation, the focus is on wavefunction-based multireference calculations commonly used in theoretical chemistry research. Starting from the Hartree–Fock reference wavefunction, we progressively introduce multiconfigurational methods and dynamic correlation techniques including CASSCF, CASPT2, and NEVPT2.
Official Resources¶
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ORCA Manual
Official documentation describing computational methods, keywords, input syntax, and examples.
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ORCA Website
Download ORCA, read release notes, licensing information, and software updates.
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ORCA Tutorials
Official collection of tutorials covering electronic structure methods and practical examples.
Recommended Learning Workflow¶
The tutorials are arranged in the recommended order for learning multireference electronic structure calculations in ORCA.
┌────────────────────┐
│ RHF │
└─────────┬──────────┘
▼
┌────────────────────┐
│ CASSCF │
└─────────┬──────────┘
▼
┌────────────────────┐
│ CASPT2 │
└─────────┬──────────┘
▼
┌────────────────────┐
│ NEVPT2 │
└────────────────────┘
Each tutorial introduces an increasingly sophisticated description of electron correlation.
- Restricted Hartree–Fock (RHF) constructs the single-determinant reference wavefunction used by many correlated methods.
- CASSCF introduces static electron correlation by optimizing molecular orbitals and configuration interaction coefficients within a chosen active space.
- CASPT2 incorporates dynamic electron correlation through second-order perturbation theory using the CASSCF wavefunction as the reference.
- NEVPT2 provides an alternative perturbative treatment of dynamic correlation that is size-consistent and free from intruder-state problems.
Tutorials¶
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Restricted Hartree–Fock (RHF)
Generate the self-consistent field reference wavefunction and molecular orbitals for subsequent correlated calculations.
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CASSCF
Perform Complete Active Space Self-Consistent Field calculations to describe multiconfigurational electronic structures.
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CASPT2
Recover dynamic electron correlation using second-order perturbation theory based on the CASSCF reference.
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NEVPT2
Compute dynamic correlation using N-electron Valence State Perturbation Theory with improved numerical robustness.
Future Tutorials¶
Additional ORCA capabilities will be included in future updates.
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Density Functional Theory (DFT)
Ground-state electronic structure calculations using modern exchange-correlation functionals.
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TD-DFT
Excited-state calculations, UV–Visible absorption spectra, and excited-state properties.
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EPR Calculations
Compute g tensors, hyperfine coupling constants, zero-field splitting, and related magnetic properties.
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Spectroscopic Properties
Vibrational frequencies, IR spectra, Raman spectra, and electronic spectroscopy.
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Population Analysis
Mulliken, Löwdin, Natural Population Analysis, and charge decomposition methods.
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Relativistic Methods
Scalar relativistic Hamiltonians, spin–orbit coupling, and relativistic corrections.