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
14 votes
Accepted

Basis sets optimized for CCSDT(Q) calculations

To the best of my knowledge, there have not been basis sets optimized for post-CCSD(T) calculations. Note that the cc-pVXZ have been optimized using CISD. One would expect these to be also close to ...
Frank Jensen's user avatar
  • 1,776
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
11 votes
Accepted

How should I compare different basis sets for post-Hartree-Fock methods?

First of all, MP2 (for example) is actually guaranteed to converge from above to the basis set limit even though MP2 in one specific basis set can give an energy lower than the FCI energy in that same ...
Nike Dattani - No Free Time's user avatar
11 votes

CCSD(T) Transition State and Ground State Calculations in Gaussian09: "Unable to Determine Lambda"?

It's not a descriptive error message, but this occurs when you reach the max number of iterations for the Eigenvector Following (EF) algorithm and still haven't converged a lambda. While Gaussian uses ...
Tyberius's user avatar
  • 15.5k
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
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

How should I compare different basis sets for post-Hartree-Fock methods?

In general, when computing any property with different models (e.g. level of theory, basis set, etc), if you don't have some kind of theoretical bound (like the variational principle) to determine ...
Tyberius's user avatar
  • 15.5k
8 votes
Accepted

What is a good practical way to determine whether or not I need to use a multiconfigurational method?

which way is more correct and methodologically appropriate? Nothing is more "correct" or less correct, but there's certainly advantages and disadvantages. MCSCF calculations such as CASSCF ...
Nike Dattani - No Free Time'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

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
7 votes
Accepted

Is it possible to make segmented contracted basis sets for correlated calculations?

Yes, it is perfectly possible. As I've discussed here, it is possible to convert generally contracted basis sets into (somewhat) segmented sets without any formal loss of accuracy; next, one would ...
Susi Lehtola's user avatar
  • 19.1k
7 votes

What do size-extensivity and size-consistency mean?

Size-consistency: Your description is correct. $E_{AB}(r=\infty) = E_A + E_B$. Size-extensivity: I explained this in my answer "Constraint #13: Size-Extensivity" to the question asking for ...
Nike Dattani - No Free Time's user avatar
6 votes

CCSD and CISD size-consistency problem

CISD is not size-consistent for more than two electrons, and CCSD is size-consistent and size-extensive for any number of electrons. With CCSD, if you calculate the $\ce{Li2}$ energy for a bond length ...
Nike Dattani - No Free Time's user avatar
6 votes
Accepted

How does coupled cluster doubles (CCD) compare to MP2?

Part of your question was: Does MP2 implicitly include any excitations that CCD does not (perhaps some single excitations)? From the expansion of the exponential form of the cluster operator, we ...
Antimon's user avatar
  • 546
6 votes
Accepted

The correlation energy from frozen natural orbtials (FNOs) using CCSD and MPBT(2)?

The idea in the FNO method is to do a MP2 calculation for the one-particle density matrix and diagonalize it to get the natural orbitals (NOs) and natural orbital occupation numbers (NOONs). So, ...
Susi Lehtola's user avatar
  • 19.1k
6 votes

Is there a "gold standard" method in materials modeling for obtaining ground state energy?

I think the answer is probably: yes, but not just one. Or no, if you want to be very strict. Depending on the type of system you are studying, different methods may work better or worse and it may ...
taciteloquence's user avatar
6 votes
Accepted

Error in UCCSD(T) Calculation in PySCF for S atom?

The stack trace shows that the program tries to divide a number by the number of alpha virtual orbitals (nvira), which is zero in the case of STO-3G triplet sulfur atom. This caused a ...
wzkchem5's user avatar
  • 9,620
6 votes
Accepted

Calculate the energy of a single electron in an atom

Normally $\textit{ab initio}$ methods should give approximately the orbital energies that the electrons will occupy after applying the exclusion principle. The accuracy of the total energy is related ...
M06-2x's user avatar
  • 906
6 votes
Accepted

Kill or let be: Is this calculation stuck or just very slow?

"I don't know if the calculation is just stuck and wasting compute resources, or if it is actually still working." When that happens, I log into the compute node and run the ...
Nike Dattani - No Free Time's user avatar
6 votes

OpenMOLCAS error while running CCT in &CCSDT module

I have run a calculation with that input file, and can see that an error occurred when trying to calculate the (T) part of the CCSD(T) calculation, as you can see that we have ...
Nike Dattani - No Free Time's user avatar
5 votes
Accepted

Interfacing MRCC with CFOUR for dynamic polarizability

I have got your calculation successfully completed: ...
Nike Dattani - No Free Time's user avatar
5 votes

A CCSD(T) geometry optimization in Gaussian09 is deviating too far from the minimum. How do I deal with it?

I'm always using CFOUR for CCSD(T) calculations, so bear with me. With CFOUR you can actually specify the irrep occupations, and works very well for the optimizations. But, there is one big snag. You ...
MSwart's user avatar
  • 1,064
5 votes

How should I compare different basis sets for post-Hartree-Fock methods?

Well, Nike already answered the point about the variationality: even though methods like MP2, CCSD, and CCSD(T) are non-variational in that they may over- or underestimate the energy of the ground ...
Susi Lehtola's user avatar
  • 19.1k
5 votes

When is coupled cluster preferred over DFT?

Coupled cluster is theoretically more accurate that DFT, as it's limiting behaviour is an exact solution to the Schrödinger equation. By limiting behaviour, I mean including all possible excitations (...
Hayden S's user avatar
  • 930
5 votes

Is it okay to use semi-empirical methods to optimize structures if I perform high-level energy calculations?

I generally try to use the most accurate methods that my resources will allow me to use, within a reasonable budget for the desired accuracy. If you're interested in 20 water molecules or 60 atoms (...
Nike Dattani - No Free Time's user avatar
4 votes

Are multiplet free atoms multireference?

Open shell atoms are multireference pretty much by definition. Take fluorine, for example, which has a $1s^2 2s^2 2p^5$ electron configuration and a $^2P$ ground state. We know that the electronic ...
Susi Lehtola's user avatar
  • 19.1k
4 votes
Accepted

CCSD and CISD size-consistency problem

Your CISD and CCSD energies are practically the same, since you have not defined a basis set. It appears Gaussian still defaults to STO-3G in this case, although it is a minimal basis set, which ...
Susi Lehtola's user avatar
  • 19.1k

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