Interface to the Orca package (https://orcaforum.cec.mpg.de/, free license for academic use). The interface was developed for version 2.9 and tested to work with 3.0.2. There are some changes in the input for Orca 4.0 and the interface was not tested thoroughly with this version.

Orca provides access to wide range of DFT and ab initio methods, both DFT and correled calculations can used the Resolution of identity approximation.

Orca implements small-basis HF/DFT calculations with the 3c corrections. The 3c stands for three corrrections for 1) dispersion, 2) basis set superposition error and 3) additional basis set incompleteness effects. Two versions are available, HF-3c based on Hartree-Fock calculation in minimal basis set[1] and PBEh-3c based on DFT in a double-zeta basis.[2] Using these methods activates the complete setup including the basis set for which the corrections were developed, no further setup is needed – see the example below.

- When the RI approximation is used and no auxiliary basis set is specified, cuby attempts to use an auxilary basis of the same name as the AO basis. If it does not exist, the calculation fails. This replaces the default procedure in orca where a small fitting basis is used silently - the calculation will run but the result may not be accurate enough for every purpose.

- R. Sure, S. Grimme, J. Comput. Chem., 2013, 34, 1672-1685
- S. Grimme, J. G. Brandenburg, C. Bannwarth, A. Hansen, The Journal of Chemical Physics, 2015, 143, 054107

- hf - supports calculation of energy, gradient, point_charges, point_charges_gradient
- dft - supports calculation of energy, gradient, point_charges, point_charges_gradient, native_opt, hessian
- mp2 - supports calculation of energy, gradient, native_opt
- mp3 - supports calculation of energy
- ccsd - supports calculation of energy
- ccsd(t) - supports calculation of energy
- qcisd - supports calculation of energy
- qcisd(t) - supports calculation of energy
- hf-3c - supports calculation of energy, gradient
- pbeh-3c - supports calculation of energy, gradient
- b97-3c - supports calculation of energy, gradient

- cosmo
- smd
- cpcm

- orca_bin_dir
- orca_dispersion_correction
- orca_dlpno
- orca_dlpno_accuracy
- orca_ecp
- orca_extra_input
- orca_guess
- orca_keywords
- orca_mpi_setup
- orca_opt_coords
- orca_optlimit_d
- orca_optlimit_e
- orca_optlimit_g
- orca_version

- auxiliary_basis_mp2
- auxiliary_basis_mp2_elements
- auxiliary_basis_scf
- auxiliary_basis_scf_elements
- basisset
- basisset_elements
- charge
- correlation_frozen_core
- delete_large_files
- density_fitting
- development
- dft_grid
- dft_grid_custom
- existing_calc_dir
- functional
- functional_custom
- functional_custom_type
- job_cleanup
- maxcycles
- mem
- mem_core
- method
- multiplicity
- parallel
- scf_convergence
- scf_cycles
- scf_integral_storage
- scs_os
- scs_ss
- solvent_epsilon
- solvent_model
- solvent_name
- spin_component_scaling
- spin_restricted
- start_from_previous

The following examples, along with all other files needed to run them, can be found in the directory cuby4/interfaces/orca/examples

```
#===============================================================================
# Orca example 1: setup, HF calculation
#===============================================================================
# Simple input for HF calculation, all other options use default values.
job: energy
interface: orca
method: hf
basisset: cc-pVDZ
geometry: A24:water # Water molecule from the internal database
charge: 0
#-------------------------------------------------------------------------------
# Interface configuration
#-------------------------------------------------------------------------------
# Path to Orca binaries
orca_bin_dir: /home/rezac/bin/orca/orca-3.0.2/arch/amd64/orca_3_0_2_linux_x86-64
# The following examples assume that the interface is set up in the config file.
```

```
#===============================================================================
# Orca example 2: DFT and RI-DFT calculation
#===============================================================================
# Multistep protocol is used to run two calculations from one input
job: multistep
steps: dft, ridft
calculation_dft:
job: energy
interface: orca
method: dft
functional: b-lyp
basisset: SV
density_fitting: none # disable RI (by default, it is enabled)
geometry: A24:water
charge: 0
calculation_ridft:
job: energy
interface: orca
method: dft
functional: b-lyp
basisset: SV
density_fitting: scf # enable RI (not necessary, enabled by default)
auxiliary_basis_scf: SV # Auxiliary basis for RI has to be specified
geometry: A24:water
charge: 0
```

```
#===============================================================================
# Orca example 3: MP2 and RI-MP2 calculation
#===============================================================================
# Multistep protocol is used to run two calculations from one input
job: multistep
steps: mp2, rimp2
# In contrast to the previous example, we gather the setup used in all steps
# into a common subsection:
calculation_common:
job: energy
interface: orca
method: mp2
basisset: cc-pVTZ
geometry: A24:water
charge: 0
calculation_mp2:
density_fitting: none # disable RI (by default, it is enabled)
calculation_rimp2:
density_fitting: correlation # enable RI (not necessary, enabled by default)
auxiliary_basis_mp2: cc-pVTZ # Auxiliary basis for RI has to be specified
```

```
#===============================================================================
# Orca example 4: CCSD(T) calculation
#===============================================================================
job: energy
interface: orca
method: ccsd(t)
basisset: cc-pVDZ
geometry: A24:water # Water molecule from the internal database
charge: 0
# Print components of the CCSD(T) energy
print: energy_decomposition
# Other options affecting this calculation
correlation_frozen_core: yes # Frozen-core approximation, on by default
scf_convergence: 8 # For CCSD(T), we set tighter SCF convergence, default is 7
```

```
#===============================================================================
# Orca example 5: DLPNO-CCSD(T) calculation
#===============================================================================
# Approximate CCSD(T) calculation applicable to large systems -
# Domain-Localized Pair Natural Orbitals CSCD(T)
job: energy
interface: orca
method: ccsd(t)
basisset: cc-pVDZ
geometry: A24:water # Water molecule from the internal database
charge: 0
# Print components of the CCSD(T) energy
print: energy_decomposition
# Setup needed in addition to plain CCSD(T)
orca_dlpno: yes # enable the approximation
auxiliary_basis_mp2: cc-pVDZ # Auxiliary basis for RI-MP2 is needed
```

```
#===============================================================================
# Orca example 6: HF-3c calculation
#===============================================================================
# The HF-3c method, a Hartree-foc calculation in minimal basis with corrections
# for basis set incompleteness and dispersion.
job: energy
interface: orca
# When the 3c methods are used, no setup besides the selection of the method
# is needed
method: hf-3c
# System specification: water molecule
geometry: A24:water
charge: 0
```