27 votes
Accepted

Is it right to neglect very small imaginary frequencies?

tldr: This is something of an eternal debate. IMHO very small imaginary frequencies can be okay, but it depends on your system and needs. As you might see from the various comments above, there are ...
Geoff Hutchison's user avatar
22 votes

What is the largest system that has been studied by the coupled cluster method?

ORCA The local pair-natural orbital (DLPNO) based coupled cluster method have been managed to investigate large organic molecule and small proteins (linear C150H302 (452 atoms, >8800 basis ...
Paulie Bao's user avatar
  • 3,953
18 votes

Is it right to neglect very small imaginary frequencies?

Just want to add a small comment upon Geoff's excellent answer. One may be tempted to think that a $<\pu{10 cm-1}$ imaginary frequency introduces a negligible error to the energy or the property of ...
wzkchem5's user avatar
  • 9,620
14 votes

Why is my ORCA calculation of nitrogen iodide failing?

There was a bug with the PModel guess of iodine, which leads to a very poor initial guess for molecules containing iodine, and consequently divergent SCF calculation. This bug has been fixed in ORCA 5....
wzkchem5's user avatar
  • 9,620
14 votes

How to calculate DFT energy with density from another level of theory?

Incidentally, I initiated some discussions along these lines in these questions for HF-DFT (also called 'density-corrected' DFT) here, and here. I'm familiar with Quantum ESPRESSO, so I can detail a ...
livars98's user avatar
  • 2,496
13 votes
Accepted

Analytic gradient for DLPNO-CCSD

Note that the DLPNO method is only implemented in ORCA. There are indeed analogous and similarly efficient and accurate methods, the PNO-LCCSD method in Molpro [doi.org/10.1021/acs.jctc.7b00799] and ...
Peter Nagy's user avatar
12 votes
Accepted

Simulating autoionization of small molecules using DFT or AIMD

This is because autoionization is very thermodynamically unfavored near room temperature. I don't know much about water in a water-DMSO mixture, but let's take pure water as an example. The ...
wzkchem5's user avatar
  • 9,620
11 votes

Quantum chemistry in external electrostatic field?

Electrostaic embedding is usually on the QM-side as simple point charges which add to the total hamiltonian of the nuceli+electrons. It should be just a simple coulumb interaction. Any modern ...
vkj's user avatar
  • 111
11 votes

What is the largest system that has been studied by the coupled cluster method?

I’ll expand on this later but here is the abbreviated version. CTF The CTF code can do very large iterative CCSD and CCSDT using a dense spin-orbital formalism. CCSD up to 55 (50) water molecules with ...
Jeff Hammond's user avatar
11 votes
Accepted

Running an ORCA calculation from Python

I ran your script on my PC with Python and it ran perfectly fine. Here are all of the required PC specifications: System Specs ...
Vandan Revanur's user avatar
10 votes
Accepted

How to swap orbitals in ORCA?

I figured out the correct formatting. The section defining the QM needs to have the MORead option, the %moinp section, and the <...
kskinnerx16's user avatar
  • 1,546
10 votes

What is the largest system that has been studied by the coupled cluster method?

Localized natural orbitals methods allow tackling huge system sizes; some links were already given above. Of course, the accuracy remains sometimes a question in such methods: the methods rely on ...
Susi Lehtola's user avatar
  • 19.1k
10 votes
Accepted

Different types of transition dipole moment in ORCA output

You have stumbled across a problem that was described in 1983 in the following way (note that the "40 years" refers to 1943-1983): "Over the past 40 years, many authors have tried to ...
Nike Dattani - No Free Time's user avatar
9 votes

Is It possible to do microkinetic modelling for liquid phase reactions, in the presence of a catalyst?

Usually if you have a heterogeneous catalyst, the best way to model it is to use a periodic slab (a thin plate that is periodic in two dimensions). A cluster model (i.e. approximating the catalyst as ...
wzkchem5's user avatar
  • 9,620
9 votes
Accepted

ORCA: Dipole moment of LiH from SP calculation

As suggested by Tyberius, I now expand my comment into an answer. The true charge density of a molecule is a sum of nuclear and electronic contributions. While the former is a sum of delta functions (...
wzkchem5's user avatar
  • 9,620
8 votes
Accepted

Numerical Frequency Parallelization in ORCA

This is exactly what ORCA actually does. For example if you have 48 cores, then ORCA does 48 displacements at once, each using only a single core. This is trivial to implement and is one of the major ...
wzkchem5's user avatar
  • 9,620
8 votes
Accepted

Quantum chemistry in external electrostatic field?

vkj's answer addressed the third part of the question. I'll answer the first part. Which quantum chemistry methods can be used in an external electrostatic field? Also which software supports that? I'...
Susi Lehtola's user avatar
  • 19.1k
8 votes
Accepted

How do you calculate the change in dipole moment from ground to first excited state using ORCA?

This second dipole moment is almost surely the excited state. You can see the nuclear contributions are identical and the rotational constants are also the same. This means they are both ...
Tristan Maxson's user avatar
8 votes

Analytic gradient for DLPNO-CCSD

As far as I know, analytic gradients for DLPNO-CCSD are not available in ORCA. Analytic first derivatives are available for both closed-shell and high-spin open-shell cases, which could be used for ...
Dipayan Datta's user avatar
8 votes
Accepted

What are Pair Natural Orbitals?

I think Nike has answered to all questions adequately enough. I am sharing my understanding as one of the developers of the PNO-based local coupled-cluster (CC) methods, codes for computing response ...
Dipayan Datta's user avatar
8 votes

What are Pair Natural Orbitals?

I'll answer each of your three questions separately, but the one you say is "most important" will go first 😊 And most importantly, why are they used for correlation calculation? They can ...
Nike Dattani - No Free Time's user avatar
8 votes
Accepted

What are the output files generated by an NEB calculation?

This question is bit too broad for a specific answer. The most common types of files that are generated by Orca are: *.gbw files are binary wavefunction files, ...
S R Maiti's user avatar
  • 6,851
8 votes
Accepted

Bromine basis set problem

You can download it from this site: Basis Set Exchange: https://www.basissetexchange.org/ Here is the requested basis set for Br in ORCA format: ...
Camps's user avatar
  • 23.3k
8 votes
Accepted

QuasiRRHO in Orca

I have access to ORCA's source code, and I can assure you that ORCA indeed uses $\omega_0 = 100 \rm{~cm^{-1}}$. However $\omega_0$ is hard-coded in ORCA and not controlled by the input parameter ...
wzkchem5's user avatar
  • 9,620
8 votes

Running an ORCA calculation from Python

As the other answers and comments have suggested, the problem is probably coming from how Python is launching ORCA. Because I cannot see any reason the job should run from command line, but not from ...
S R Maiti's user avatar
  • 6,851
7 votes

Adding external electric field in ORCA with specific strength

From the ORCA forum thread: Actually there is an undocumented feature for this purpose: ...
Camps's user avatar
  • 23.3k
7 votes

Constraining a set of atoms in the MM region using pDynamo (CHARMM/ORCA interface)

To tether a set of atoms (index or fragment) in pDynamo, the below lines are needed (energy model can vary): ...
kskinnerx16's user avatar
  • 1,546
7 votes
Accepted

How can I calculate the energy gap between the singlet and triplet state using ORCA?

First of all, if you are only interested in molecules with about the same size as the ones in the article you cited, then TDDFT is probably not the best method for this purpose. ORCA supports many ...
wzkchem5's user avatar
  • 9,620
7 votes

How to get the .47 file from ORCA for natural bonding orbital analysis

All files are always created in the run directory where ORCA runs. I specifically did not want to deal with environment variables and path names and all that. Hope that helps! best wishes, Frank Neese
FrankNeese's user avatar
7 votes
Accepted

Modify ORCA orbitals for CASSCF

You can use the "rotate" keyword in the SCF block, like %scf rotate {30,27,90} (38,41,90} {39,48,90} end end The "90"s mean "90 degrees&...
wzkchem5's user avatar
  • 9,620

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