# Tag Info

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### 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 ...
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### 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 ...
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### 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 ...
• 7,478

### 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....
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### 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 ...
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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 ...
• 206

### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 (...
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### 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 ...
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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 ...
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### 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 <...
• 1,496
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### 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'...
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### 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, ...
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### 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: ...
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### 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 ...
• 7,478
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### 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 ...
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### Adding external electric field in ORCA with specific strength

From the ORCA forum thread: Actually there is an undocumented feature for this purpose: ...
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### 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 ...
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### 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): ...
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### 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
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### 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 ...

### 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 ...
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### 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 ...
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### 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&...
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### Unrecognized or duplicated keyword error using ORCA

Grid4 has been removed in Orca 5, you must use defgridX instead, where X=1,2,3. In general the defaults should be much better and Grid4 is probably at least met by the default grid in Orca 5.
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