openmaterials.ai / codes / ORCA

ORCA: quantum chemistry for molecules, from DFT to multireference methods

A molecular electronic structure program. In version 5.0 the SHARK integral engine makes analytic Hessians, TD-DFT and magnetic properties about 5 to 10 times faster.

What it is and how it works

Spectroscopy from first principles

ORCA grew out of the spectroscopy of open-shell transition metal complexes. It computes g tensors, hyperfine couplings, NMR shieldings, spin-spin couplings, harmonic frequencies and excited states (Neese 2022).

The SHARK integral engine

ORCA 5.0 computes two-electron integrals with a new algorithm, up to 5 to 6 times faster for g and higher angular momentum. Segmented and ANO bases run well (Neese 2022).

Semi-empirical to multireference methods

Semi-empirical, Hartree-Fock, 588 density functionals through LibXC 5.1.0, DLPNO-CCSD(T) and DLPNO-MP2 local correlation, CASSCF with NEVPT2 and CASPT2 (Neese 2022).

Self-adapting grids

Typical DefGrid2 errors are 0.01 kcal/mol in relative energies, 0.1 per cm in analytic harmonic frequencies and below 0.001 eV in excitation energies (Neese 2022).

SCF convergence

DIIS with SOSCF is the default. A trust-radius augmented Hessian solver starts only when a case stalls. Less numerical noise cuts SCF and geometry cycles by about 30%.

Multilevel and embedding

QM/MM with CHARMM, AMBER and OpenFF force fields, ONIOM with up to three layers, and Crystal-QMMM embedding set up from a .cif by orca_crystalprep (Neese 2022).

What people do with it

  • 14-15xparallel speedup on 16 cores, Vancomycin SCFNeese 2022
  • 1008 sPenicillin B3LYP/def2-TZVP Hessian, was 17,925 sNeese 2022
  • 0.1 cm-1typical DefGrid2 error, analytic frequenciesNeese 2022

Spin-phonon relaxation in [(tpaPh)Fe]-

ORCA CASSCF D tensors along every normal mode give an effective barrier of 19.7 per cm against 26 per cm measured, below the computed first excitation at 94 per cm. Lunghi 2017

Phonon-driven relaxation of a vanadyl qubit

ORCA CASSCF+NEVPT2 g tensors and PBE hyperfine A tensors for VO(acac)2. Above 5 T the Zeeman modulation by acoustic phonons dominates the relaxation. Below 1 T the hyperfine coupling does. Lunghi 2019

The killer phonon mode of DNTT

ORCA PBE0/def2-SVP site energies and transfer integrals along each mode single out the sliding mode, 23 per cm in DNTT and 24 per cm in C8-DNTT-C8, with 45 and 75 percent of fluctuations. Schweicher 2019

How it is used

  1. Write the input

    Method and basis set keywords, then the geometry. orca_crystalprep reads a .cif.

  2. Converge and optimize

    SCF with DIIS or TRAH, geometry optimization or NEB to a transition state.

  3. Frequencies and properties

    Analytic Hessian, then NMR, EPR, TD-DFT, polarizabilities or coupled-cluster energies.

  4. Read the results

    More than 250 properties are written to the ASCII property file (Neese 2022).

  5. Chain with compound

    Compound scripts chain steps, such as reoptimizing until no imaginary mode is left.