Questions tagged [coupled-cluster]

For questions related to the Coupled Cluster electronic structure method.

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

In the coupled cluster code of PySCF, is the four index tensor stored in memory or in disk?
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5 votes
1 answer
65 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 ...
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7 votes
1 answer
157 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 ...
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8 votes
1 answer
134 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 ...
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6 votes
1 answer
188 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 ...
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4 votes
0 answers
137 views

STEOM-DLPNO-CCSD

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 &...
8 votes
1 answer
349 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, ...
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7 votes
1 answer
126 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 ...
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6 votes
1 answer
73 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 \...
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11 votes
3 answers
391 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 ...
13 votes
1 answer
221 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 ...
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15 votes
3 answers
435 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, ...
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11 votes
1 answer
111 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)...
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9 votes
2 answers
801 views

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 ...
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11 votes
2 answers
624 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 ...
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6 votes
1 answer
371 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 ...
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8 votes
1 answer
122 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 ...
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6 votes
1 answer
963 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 ...
  • 427
13 votes
2 answers
431 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 ...
  • 427
8 votes
1 answer
127 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 ...
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15 votes
1 answer
661 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 ...
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9 votes
1 answer
58 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 ...
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20 votes
3 answers
345 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 ...
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26 votes
1 answer
164 views

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 ...
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