# Tag Info

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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 functions) , Crambin with 644 atoms, and more than 6400 basis functions ,C350H902 (>1000 atoms, > 20000 basis functions)). Riplinger, C., Sandhoefer, B., Hansen, ...

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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 the LNO-CCSD method in MRCC [doi.org/10.1021/acs.jctc.9b00511]. To my knowledge exact analytical gradients are not implemented for either of them. There is an ...

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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 basis set. So it's variational character in the basis set sense that matters here. Furthermore, variational character is not the most pertinent thing to ...

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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 a different approach (the Berny Algorithm) for methods which have analytical gradients, CCSD(T) is one of the few methods for which these still aren't ...

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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 cc-pVDZ http://solomonik.cs.illinois.edu/talks/molssi-monterey-may-2017.pdf The 8-water CCSDT problem in Table 3 took 15 minutes on 1024 nodes of BG/Q and ...

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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 what is a better result, you need a reference value to compare against. One choice for this reference is experimental results. At the end of the day, the goal of ...

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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 calculations, can take a bit more mental energy if you want to make sure your active space is appropriate, but on the other hand, the result could be much more ...

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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 computing other first-order properties. As the first exercise to implement analytic gradients within the DLPNO setup, the DLPNO-MP2 method was considered and the ...

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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 thresholds, which may not have always been converged! So you should keep in mind that these are not black-box tools like conventional coupled-cluster theory. The ...

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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 discard functions with tiny coefficients and reoptimize the segmented exponents and contraction coefficients to end up with a segmented basis set. However, as you ...

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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 properties in particular. In coupled-cluster theory, the correlated wavefunction is described in terms of "cluster amplitudes" (which are the ...

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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 significantly reduce the cost of a calculation on a big system, especially when there is a large number of "virtual" orbitals (unoccupied orbitals) in ...

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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 of 1000 Å, you should get the same thing as you would get by calculating the energy of a $\ce{Li}$ atom and multiplying it by two, and this would work for all &...

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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 state (or excited states) of the Schrödinger equation, the energy reproduced by any given method typically does behave variationally with respect to the basis set....

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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, starting from (usually) a Hartree-Fock reference wave function, you figure out which orbitals are the most strongly correlated, and which ones yield only small ...

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I have got your calculation successfully completed: The final electronic energy is -0.928161992153724 a.u. This computation required 318.38 seconds (walltime). My output (and input) files are here. They are the same as yours, except my ZMAT uses my basis set database, which I don't think will make a difference for you,...

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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 not always be obvious why. There is probably not one method that will work best generically. For quantum spin systems in $d\geq 2$ (lattice Hamiltonians with ...

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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 need to master Z-matrices. Now back to your question. If you know it should be symmetric, why not simply impose symmetry?

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Not a full answer to your particular problem, but a summary of the methods suggested for dealing with the headline issue of a diverging geometry optimization. Step size: if you are drifting away from what you believe to be a minimum, decreasing the size of each optimization step can help you avoid jumping out of a small well around the minimum. Guess: if ...

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As far as I am aware, double hybrids were first proposed by Truhlar and coworkers in J. Phys. Chem. A 2004, 108, 21, 4786–4791. If you look at the paper, in addition to MP2 they also proposed a double hybrid based on QCISD, i.e. the quadratic configuration interaction method which can be viewed as an approximate coupled-cluster method. There's no reason a ...

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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 usually do not yield even qualitatively correct results even at the SCF level of theory. To be reliable, a post-HF calculation should employ basis sets of at least ...

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The issue might be numerical as well. At theory level, CCSD could be rigorously prove to be size extensive. However, in practice, when one solves CC residue equation numerically, there is some convergence threshold that affect the accuracy of the result. Math is always simple and graceful but numerics can be complicated and nesty

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You should change your input to avoid defining a linear angle. This can be done by introducing a dummy atom. Here is an example with your input: 0 1 H Xe 1 R12 X 2 1.0 1 A1 H 2 R13 3 A1 1 D1 R12 = 1.92821600 R13 = 1.92934900 A1 = 90.0000000 D1 = 180.000000 By defining the hydrogen as perpendicular to the dummy ...

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It turns out, splitting the scratch files doesn't help. I have a 32-bit version of Gaussian, and it has a 16Gb limit for the combined size of the scratch files (I don't know how I could miss that). So splitting the read-write file can't help here. Looks like I'll have to find better computational resources.

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