ORCA: DLPNO-CCSD(T) Calculation

DLPNO-CCSD(T)

DLPNO-CCSD(T) is another technique of Couple-Cluster generalization method which produces the reasonable CCSD(T) results but requires very cheap computation cost compared to the traditional CCSD method. DLPNO-CCSD(T) stands for the domain-based local pair natural orbital methods. It has been implemented in ORCA quantum chemistry software, which developed by Frank Neese.

With DLPNO-CCSD(T) in ORCA, we are able to perform the linear scaling CCSD(T) calculations at the cost of a simple DFT calculation and reaching the experimental results. ORCA's DLPNO capabilities are also available for multi-reference systems as well as for the calculation of properties.

Input file preparation: What Keyword We use in ORCA ?

The following is a DLPNO-CCSD(T)/cc-pVTZ calculation of water.

! DLPNO-CCSD(T) cc-pVTZ cc-pVTZ/C

* int 0 1

O 0 0 0 0.0 0.000 0.000

H 1 0 0 1.0 0.000 0.000

H 1 2 0 1.0 104.060 0.000

DLPNO-CCSD(T) will give the following output. They are at almost the end of output file.

-------------------------------------------

DLPNO BASED TRIPLES CORRECTION

-------------------------------------------

⁝

Triples Correction (T) ... -0.007718230

Final correlation energy ... -0.278154390

E(CCSD) ... -76.321436435

E(CCSD(T)) ... -76.329154666

⁝

or go to the end of output file and see following

⁝

------------------------- --------------------

FINAL SINGLE POINT ENERGY -76.326535164724

------------------------- --------------------

⁝

The current implementation of DLPNO-CCSD(T) in ORCA is a black-box, where there is many parameters that needed fine-tune for unique complex. Therefore, the developer leaves the setting of this method for us. Three parameters we have to concern are following:

The PNO occupation number, called TCutPNO
Thestrong pair approximation cut-off, called TCutPairs
The domain size parameter, called TCutMK

The default values for each parameter cut-off.

LoosePNO NormalPNO TightPNO Loose2PNO Normal2PNO

TCutPairs e-3 e-4 e-5 e-5 e-5

TCutMKN e-3 e-3 e-4 e-3 e-3

TCutPNO e-6 3.3 x e-7 e-7 e-6 3.3 x e-7

Example input file for manual setting for TightPNO mode with Tight SCF cutoff

! DLPNO-CCSD(T) cc-pVTZ TightPNO TightSCF

or manually set their values in mdci block

! DLPNO-CCSD(T) cc-pVTZ TightSCF

% mdci

# Not construct density matrix

Density None

TCutPairs 1e-5

TCutPNO 1e-7

TCutMKN 1e-4

end

...

Speed Up Calculation

To speed up the calculation, you can also use RI approximation (so-called density fitting) and auxiliary basis sets together

! DLPNO-CCSD(T) RIJCOSX def2-TZVP def2-TZVP/C TightSCF

where RIJCOSX is RI-approximation, the RI-J for Coulomb integrals and COSX numerical integration for HF exchange, and def2-TZVP/C is auxiliary basis set for def2-TZVP.

Increasing memory can speed up calculation too!

For large jobs, the calculation may require a large amount of memory. We can set the total amount of (physical) memory for the job using maxcore keyword.

% maxcore MEMORY

Note that the memory is in MB unit and it would be requested per CPU core. Therefore, if you run in parallel mode with N CPU cores, the total amount of memory will be N x Memory! Make sure that your machine has memory enough.

! DLPNO-CCSD(T) TightPNO TightSCF

# Number of CPU processors

% pal nprocs 4 end

# Memory per core (MB)

% maxcore 3000

...

With the above setting, the total memory is 4 x 3000 = 12,000 MB, which roughly is 12 GB

Energy Extrapolation to Complete Basis Set (CBS)

To achieve more accurate results as a standard reference for your own studied system, we can extrapolate the DLPNO-CCSD(T)/BasisSet to complete basis set (CBS) using the following equation.

Energy(DLPNO-CCSD(T)/CBS) = Energy(MP2/CBS) + Energy^(DLPNO-CCSD(T))_(core)

Learn more about DLPNO-CCSD(T) at its home website.

Rangsiman Ketkaew