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 ...
- 9,395
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 ...
- 3,983
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 ...
- 10.2k
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....
- 10.2k
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 ...
- 2,506
14
votes
Accepted
ORCA 6 slower than ORCA 5?
Please note that in ORCA 6 we not only have performance improvements compared to ORCA 5, but also tightened many thresholds and increased the default grid size, so that the calculated results are more ...
- 10.2k
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 ...
- 216
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 ...
- 10.2k
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 ...
- 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 ...
- 310
11
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 ...
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
...
- 3,155
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 <...
- 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 ...
- 20.1k
10
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 ...
- 226
9
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 ...
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 ...
- 10.2k
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 (...
- 10.2k
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, ...
- 7,171
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'...
- 20.1k
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 ...
- 10.2k
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 ...
- 11.6k
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 ...
- 226
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:
...
- 24.2k
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 (EDIT 2024.5.11: from the next ORCA release, it ...
- 10.2k
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 ...
- 7,171
8
votes
Running multinode parallel Orca calculation without a work load manager (scheduler)
Not sure if ORCA can be run using a user-chosen MPI runner but there are a few steps that need to be done in order to run ORCA (or any other MPI program) on a distributed/interconnect cluster (...
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
- 96
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:
...
- 24.2k
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):
...
- 1,546
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