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ORCA Reference

Free reference guide: ORCA Reference

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About ORCA Reference

The ORCA Reference is a searchable quick-reference guide for the ORCA quantum chemistry program, one of the most widely used ab initio and DFT packages in computational chemistry. It covers calculation methods ranging from Hartree-Fock and popular DFT functionals (B3LYP, PBE0) with Grimme D3BJ dispersion corrections, through correlated methods like RI-MP2, to the gold-standard DLPNO-CCSD(T) for near-coupled-cluster accuracy on large molecules, and multireference methods including CASSCF and NEVPT2 for bond dissociation and excited states.

The reference provides practical guidance on calculation types including geometry optimization (Opt, TightOpt), frequency analysis for thermodynamic data and transition state verification, transition state search (OptTS), time-dependent DFT for excited states and UV-Vis prediction, nudged elastic band (NEB-TS) for reaction path exploration, and continuum solvation models (CPCM, SMD) for solution-phase calculations. Basis set entries cover the Ahlrichs def2 family (SVP through QZVP), Dunning correlation-consistent sets (cc-pVnZ), and automatic auxiliary basis generation for RI approximations.

For day-to-day ORCA usage, the reference includes essential input file syntax for parallel processing (%pal nprocs), memory allocation (%maxcore), coordinate specification (* xyz and * xyzfile), per-element basis set assignment (%basis), SCF convergence troubleshooting (SlowConv, VerySlowConv, custom %scf settings), unrestricted calculations for open-shell systems (UKS/UHF), output control (%output), and post-processing tools (orca_plot for orbital visualization, orca_2mkl for Molden format conversion).

Key Features

  • DFT functional reference including B3LYP, PBE0, and Grimme D3BJ dispersion correction syntax
  • Correlated methods from RI-MP2 to DLPNO-CCSD(T) with TightPNO for benchmark accuracy
  • Multireference methods: CASSCF active space specification and NEVPT2 dynamic correlation
  • Geometry optimization, frequency analysis, OptTS transition state search, and NEB-TS reaction paths
  • TD-DFT for excited states and UV-Vis spectra with NRoots and MaxDim configuration
  • Complete basis set reference: def2-SVP/TZVP/QZVP, cc-pVnZ, aug- variants, and AutoAux for RI
  • Input syntax for parallel processing, memory, coordinates, solvation (CPCM/SMD), and output control
  • SCF convergence troubleshooting: SlowConv, VerySlowConv, custom tolerances, and open-shell (UKS/UHF) handling

Frequently Asked Questions

What calculation methods does the ORCA reference cover?

The reference covers Hartree-Fock (HF), DFT functionals (B3LYP, PBE0 with D3BJ dispersion), RI-MP2 with resolution-of-identity approximation, DLPNO-CCSD(T) for near-CCSD(T) accuracy on large molecules, and multireference methods CASSCF (with active space notation like (6,6)) and NEVPT2 for dynamic correlation on top of CASSCF.

How do I choose the right basis set in ORCA?

The reference covers the Ahlrichs def2 family: def2-SVP (double-zeta, fast preliminary calculations), def2-TZVP (triple-zeta, standard production), def2-TZVPP (triple-zeta double polarization), and def2-QZVP (quadruple-zeta benchmarks). It also covers Dunning cc-pVnZ sets with aug- variants for diffuse functions and auxiliary basis sets (/C, /J, /JK) for RI approximations.

What geometry optimization and transition state methods are covered?

The reference covers standard optimization (! Opt, TightOpt), frequency analysis (! Freq, NumFreq) for verifying minima vs. saddle points through imaginary frequency counts, transition state optimization (! OptTS with CalcHess for initial Hessian), and NEB-TS (Nudged Elastic Band) for finding reaction paths between reactant and product geometries.

How do I set up TD-DFT calculations in ORCA?

The reference shows the TD-DFT input syntax with the ! TDDFT keyword and %tddft block for setting NRoots (number of excited states) and MaxDim (Davidson expansion dimension). This is used for excited state calculations and UV-Vis spectral prediction.

How can I fix SCF convergence problems in ORCA?

The reference provides multiple strategies: SlowConv and VerySlowConv keywords for conservative convergence, custom %scf block settings (MaxIter, TolE, TolRMSP, DIISMaxEq, directresetfreq), and guidance on using UKS/UHF for open-shell systems like transition metal complexes and radicals that often have convergence difficulties.

How do I run solvation calculations?

The reference covers CPCM continuum solvation (! CPCM(Water)) with supported solvents including Water, DMSO, Acetonitrile, and THF, plus the more accurate SMD solvation model syntax (! SMD CPCM(Water)).

What input file settings are covered?

The reference covers parallel processor setup (%pal nprocs or ! PAL8 shorthand), memory allocation (%maxcore in MB per core), molecular coordinate input in both inline (* xyz charge mult) and file-based (* xyzfile) formats, per-element basis set specification (%basis with NewGTO and NewECP), and output verbosity control (%output).

Does the reference cover post-processing tools?

Yes, it covers orca_plot for generating cube files of molecular orbitals (HOMO/LUMO visualization) and electron density difference maps, plus orca_2mkl for converting ORCA orbital files (.gbw) to Molden format for visualization in external programs like Molden and Avogadro.