Questions tagged [coupled-cluster]

For questions related to the Coupled Cluster electronic structure method.

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

In quantum chemistry, coupled-cluster methods, especially CCSD(T), with complete basis set extrapolation are often considered a "gold-standard" for closed-shell molecules. This means that we consider ...
Greg's user avatar
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20 votes
3 answers
594 views

What is the largest system that has been studied by the coupled cluster method?

This is parallel to the analogous question about the largest DFT calculation: What is the largest material that has been studied using density functional theory? I assume we allow high performance ...
Paulie Bao's user avatar
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15 votes
3 answers
694 views

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

HF or DFT methods are variational so I can confidently say that any basis set that gives the lowest energy is the best for that system. However, I have learnt that post-HF methods such as MP2, MP3, ...
S R Maiti's user avatar
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15 votes
1 answer
1k views

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

I’m doing CCSD(T) calculations (for both transition and ground state geometries) on a series of small molecules in Gaussian09. Several similar calculations terminated without any problems but one TS ...
QuantumX's user avatar
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13 votes
2 answers
892 views

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

I am doing a CCSD(T) geometry optimization on a series of molecules. One of the geometry optimizations took about 2 weeks even though an analogous molecule with the same number of basis functions took ...
QuantumX's user avatar
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13 votes
1 answer
467 views

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

I’m planning to run a calculation of single electron transfer where my system goes from singlet to triplet biradical (presumably). So I’m wondering how to decide if I need to use milticonfigurational ...
Roman's user avatar
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11 votes
2 answers
973 views

Analytic gradient for DLPNO-CCSD

Is there a formula for analytical gradients (for geometry optimization) for the DLPNO-CCSD method? I ask because I know that it is not implemented in ORCA. Maybe PNO-CCSD is implemented in MRCC or ...
Paulie Bao's user avatar
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11 votes
3 answers
627 views

CCSD and CISD size-consistency problem

I have performed a Gaussian16 calculation to explore the size-consistency problem for the CISD method. The system I am studying is the water dimer and the results are not what I expected at first. I ...
Paul Logan's user avatar
11 votes
1 answer
129 views

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

The segmented contracted basis sets are usually handled well by the QM programs than generally contracted basis sets, as the primitives are not repeated. The correlation consistent basis sets (cc-PVnZ)...
S R Maiti's user avatar
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9 votes
1 answer
276 views

Basis sets optimized for CCSDT(Q) calculations

CCSDT(Q) and CCSDTQ calculations are extraordinarily expensive, both in terms of FLOP count and RAM capacity requirements, eg. CCSDTQ requires a computational effort that scales as ~$N^{10}$, where N ...
TiborGY's user avatar
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9 votes
2 answers
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What are Pair Natural Orbitals?

Recently, I came across electron correlation methods that use local pair natural orbitals (LPNOs) or domain-based local pair natural orbitals (DLPNO), such as DLPNO-MP2, DLPNO-CCSD etc. The dynamic ...
S R Maiti's user avatar
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9 votes
1 answer
149 views

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

In other works and from the paper: Scaling Up Electronic Structure Calculations on Quantum Computers: The Frozen Natural Orbital Based Method of Increments. I have seen that the corrected CCSD ...
Wychh's user avatar
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9 votes
1 answer
69 views

Extended Hybrid Methods

Hybrid DFT methods, where the functional is supplemented with Hartree-Fock exchange, have become increasingly popular due to their low cost and decent accuracy. Double hybrids, which mix in an MP2 ...
Tyberius's user avatar
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8 votes
1 answer
777 views

What do size-extensivity and size-consistency mean?

I have heard both terms in various lectures and books on quantum chemistry, however, I have not found a proper explanation of them. As I understand now, size consistency of a method means for example, ...
S R Maiti's user avatar
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8 votes
1 answer
201 views

Interfacing MRCC with CFOUR for dynamic polarizability

I have asked this question on the MRCC forum but did not get an answer. I am trying to understand how to use CFOUR with the MRCC program for computing dynamic polarizability. To begin with, I am ...
ankit7540's user avatar
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8 votes
1 answer
469 views

List of programs for post-CCSD(T) calculations

Implementations of CCSD(T) are almost too numerous to list, but CCSDT and CCSDT(Q) are much more rare. I have compiled a list of such programs below, and I would be interested to hear if anyone is ...
TiborGY's user avatar
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7 votes
1 answer
344 views

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

I have seen an increase in the use of the coupled-cluster doubles (CCD) method recently. CCD uses the exponential ansatz of the coupled cluster equations, but only includes amplitudes to double ...
jheindel's user avatar
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7 votes
1 answer
555 views

When is coupled cluster preferred over DFT?

I know that coupled cluster is considered to provide better accuracy than DFT, but I am not sure for which materials science applications or types of molecules that this higher accuracy is considered ...
Todd Law's user avatar
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6 votes
1 answer
91 views

Is this notation for single and double excited Slater determinants correct?

Is the following notation $\Phi^\mathrm{HF}_0 \equiv | \phi_1, \phi_2, \cdots, \phi_i, \phi_j, \cdots, \phi_N \rangle$ $\Phi_{ia} \equiv | \phi_1, \phi_2, \cdots, \phi_a, \phi_j, \cdots, \phi_N \...
M0M0's user avatar
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6 votes
1 answer
640 views

I get an error 'Conversion from Z-matrix to cartesian coordinates failed' in Gaussian09. Any suggestions?

I'm doing my calculations using G09W. My OS is Windows 7 (64-bit). This error occurs in the frequency calculation. What's even weirder is that it seems to happen right after everything is already ...
QuantumX's user avatar
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6 votes
1 answer
2k views

I get this error in Gaussian09 even though I have enough memory. "Error termination in NtrErr: NtrErr called from FIOCnC." Any suggestions?

Whenever read-write files get to about 20Gb, I get this error. I know that, in G09, one rw file is limited to 16Gb. So I have set the software to write several RWFs. But it still keeps producing this ...
QuantumX's user avatar
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5 votes
1 answer
266 views

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

I am trying to calculate the UCCSD energy and UCCSD(T) correction for the S atom using PySCF. Using sto-3g basis set, I found that the UCCSD calculation would be completed successfully, but the ...
Hemanth Haridas's user avatar
5 votes
1 answer
157 views

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

For non-covalent systems comprised of small-molecule monomers, folks are usually interested in understanding the interaction and binding energies of small (gas-phase) clusters ... things like $\ce{X(...
epalos's user avatar
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5 votes
1 answer
81 views

Are multiplet free atoms multireference?

In general, my understanding is that free radicals and non-equilibrium molecules sometimes need multireference electronic structure calculations, mainly because of different electronic states being ...
user avatar
4 votes
1 answer
383 views

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

My question is regarding a medium-to-large DLPNO-UCCSD(T)-F12 calculation. I have a ~250 electron system, an open-shell molecule inside a water droplet, and I'm trying to get the total energy, first ...
epalos's user avatar
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4 votes
1 answer
127 views

Question on CPU timings and/or ratio resp. DLPNO-CCSD(T) and wB97X-2-D for protein of ca. 1M Da

Suppose one has a protein of about 1 million Da, and performs a DLPNO-CCSD(T)/def2-TZVPPD/AutoAux/AutoAux calculation, and then performs a wB97X-2-D/def2-TZVPPD/AutoAux/AutoAux calculation, both via ...
Kanghun Kim's user avatar
4 votes
1 answer
222 views

What's the best code for getting 1-particle and 2-particle density matrices with CCSD(T) in Python?

I have been using PySCF to calculate the 1-particle and 2-particle density matrix from ccsd(T) wavefunction using these modules in-built in PySCF make_rdm1() and <...
Pro's user avatar
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4 votes
1 answer
102 views

Crash of DF-CCSD or DF-CCSD(T) geometry optimization (MRCC package)

I am interested in geometry optimization at the CCSD(T) or DF-CCSD(T) level, with MRCC. The calculation, however, crashes when executing prop: ...
EvGeniy's user avatar
  • 191
3 votes
2 answers
174 views

Calculate the energy of a single electron in an atom

When we employ Hartree-Fock or coupled cluster to calculate an atom's energy, can we directly obtain the valence electron energy? Is it as simple as looking at the orbital energy? I know that summing ...
Peter234's user avatar
  • 187
3 votes
1 answer
62 views

What is the gold standard ab initio pair-potential between two water molecules?

There are a myriad of pair potentials available for simulating the interactions between water molecules, ranging from simpler models like the SPC/E to more sophisticated ones based on high-level ...
Lodin Ellingsen's user avatar
3 votes
0 answers
93 views

Is the 4-index tensor in the coupled cluster code of PySCF stored in memory or on disk? [closed]

In the coupled cluster code of PySCF, is the four index tensor stored in memory or in disk?
Paulie Bao's user avatar
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3 votes
0 answers
481 views

STEOM-DLPNO-CCSD [closed]

I have to calculate the S1 - T1 energy gap using coupled cluster (STEOM-DLPNO-CCSD) in ORCA. The output file contains only singlet energies. IROOT=... (no notation singlet or triplet). I tried adding &...
Darya Tarakanovskaya's user avatar
2 votes
1 answer
201 views

Why does CCSD work, but not CCSD(T), after reading an FCIDUMP in PySCF?

CCSD(T) works after an RHF calculation: Input: ...
Nike Dattani's user avatar
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2 votes
1 answer
96 views

Calculating vertical excitation energies at the DLPNO-STEOM-CCSD Theory level

I want to calculate the vertical excitation energy using the DLPNO-STEOM-CCSD method, on molecules previously optimized using TD-DFT. the parameters being used are as follows: ...
Madeleine Fisher's user avatar
1 vote
0 answers
54 views

Question on possible cancellations in the computation of Dyson orbitals from FCC/FCI wavefunctions [closed]

Dyson orbitals are mathematically meaningful, in the sense that they, unlike the DFT orbitals for instance, are actual observables of the wavefunction, instead of being "the other way around"...
Kanghun Kim's user avatar