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20 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 ...
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12 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 ...
11 votes
Accepted

How to calculate homolytic bond dissociation energies?

Multi-reference or single-reference? While it is appreciated that near dissociation there will be a near-degeneracy of orbitals, requiring a multi-reference treatment, fortunately we don't have to ...
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10 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 ...
9 votes

How is "basis set projection" done?

What you're describing is very common, and is not limited to GAMESS and Q-Chem. First, here's how to do it in MOLPRO, MRCC, GAMESS, Q-Chem (in fact the only electronic structure software that I ...
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8 votes
Accepted

How is "basis set projection" done?

Supplementing Nike's answer above.. This is actually quite elementary math. Let's say I have an orbital in a basis $|\psi\rangle = \sum_i c_i |i\rangle$ and I want to find the expansion in some other ...
  • 15.1k
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 ...
8 votes

How to calculate homolytic bond dissociation energies?

BDEs are calculated by the below procedure: Calculate initial energy Perform homolytic bond cleavage and separate fragments Calculate energies of the fragments, add the energies together Calculate ...
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7 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 ...
  • 15.1k
6 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 ...
6 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 ...
  • 29.2k
6 votes

How to calculate homolytic bond dissociation energies?

One thing the other answers haven't mentioned is the zero point and thermal corrections to the BDE. As mentioned in the Wikipedia Morse potential article, a geometry optimization will take you, by ...
5 votes
Accepted

Interfacing MRCC with CFOUR for dynamic polarizability

I have got your calculation successfully completed: ...
  • 29.2k
5 votes

How is "basis set projection" done?

Note: I have corrected some omissions from my initial answer to avoid any confusion. For a rigorous explanation of the process, I would recommend looking at Susi's answer. The projection process for ...
  • 14.3k
2 votes

Which programs can calculate corrections to the Born-Oppenheimer approximation?

CFOUR + MRCC You asked about MOLPRO and Psi4. As recently as 2021, DBOC (diagonal Born-Oppenheimer correction) wasn't implemented in Psi4. The same is true for MOLPRO. No other types of Born-...
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