Coupled Cluster Scaling Question

In summary, this conversation does not provide a clear answer about how long a CCSDT calculation would take on the same system. It is difficult to estimate, and CCSDT is generally not a very useful method.
  • #1
Morberticus
85
0
Hi,

I have a relatively small system. It takes 10 minutes to perform a CCSDT calculation. I know scaling is notoriously bad, but does anyone have a rough idea of how long a CCSDTQ calculation would take on the same system? Are we talking days or months?

Thomas
 
Physics news on Phys.org
  • #2
First: Depends on the program. For these kinds of things you should use Kallay's MRCC. It will even give you a coarse estimation of the time per iteration. Like in CCSD(T) (which you probably want to do in Molpro or CFOUR), different programs can differ by one or two orders of magnitude in calculation time. (While producing exactly the same number to 12 digits precision)

Second: Depends on the system (symmetries, ratio of occupied to virtual orbitals). So it's hard to tell. In principle, CCSDT is N^8 and CCSDTQ is N^10, but that does not translate well into the factors for *the same system*. It is very hard to estimate.

Also note that CCSDT is generally not a very useful method, because in a large majority of cases it gives worse results than CCSD(T) (see e.g., Jan Martin's W4 paper). Generally, the next thing better than CCSD(T) is CCSDT(Q) (N^9) or CCSDTQ; so unless you are doing the CCSDT as part of a basis set extrapolation series, it is better to not do it at all.
 
  • #3
cgk said:
First: Depends on the program. For these kinds of things you should use Kallay's MRCC. It will even give you a coarse estimation of the time per iteration. Like in CCSD(T) (which you probably want to do in Molpro or CFOUR), different programs can differ by one or two orders of magnitude in calculation time. (While producing exactly the same number to 12 digits precision)

Second: Depends on the system (symmetries, ratio of occupied to virtual orbitals). So it's hard to tell. In principle, CCSDT is N^8 and CCSDTQ is N^10, but that does not translate well into the factors for *the same system*. It is very hard to estimate.

Also note that CCSDT is generally not a very useful method, because in a large majority of cases it gives worse results than CCSD(T) (see e.g., Jan Martin's W4 paper). Generally, the next thing better than CCSD(T) is CCSDT(Q) (N^9) or CCSDTQ; so unless you are doing the CCSDT as part of a basis set extrapolation series, it is better to not do it at all.

You have no idea how useful this is to me. Tearing my hair out over explaining CCSDT discrepancies when the answer was right in the W4 paper. Thanks!
 

What is the Coupled Cluster Scaling Question?

The Coupled Cluster Scaling Question is a fundamental question in computational chemistry that seeks to understand the relationship between the accuracy and computational cost of coupled cluster methods.

Why is the Coupled Cluster Scaling Question important?

The Coupled Cluster Scaling Question is important because it helps us understand the limitations and capabilities of coupled cluster methods, which are widely used in computational chemistry for simulating the electronic structure of molecules. By understanding the scaling of these methods, we can optimize their use in different applications and make more accurate predictions about molecular properties.

What factors affect the scaling of coupled cluster methods?

There are several factors that can affect the scaling of coupled cluster methods, including the size of the system (i.e. the number of atoms), the basis set used, and the level of theory (e.g. single, double, or triple excitations). Additionally, the type of correlation treatment and the algorithm used for solving the coupled cluster equations can also impact the scaling behavior.

What is the difference between linear and polynomial scaling?

Linear scaling means that the computational cost of a method increases linearly with the size of the system, while polynomial scaling means that the cost increases at a higher rate (e.g. quadratic or cubic). Linear scaling is desirable because it allows for larger systems to be studied without significant increases in computational cost.

Are there any solutions to the Coupled Cluster Scaling Question?

While there is no single solution to the Coupled Cluster Scaling Question, researchers are constantly working on developing new algorithms and methods to improve the scaling behavior of coupled cluster methods. Additionally, approximations and simplifications can be made to the equations to reduce the computational cost. However, it is important to balance accuracy and cost when choosing a method for a specific application.

Similar threads

  • Computing and Technology
2
Replies
35
Views
3K
  • Classical Physics
Replies
7
Views
1K
Replies
3
Views
960
  • Atomic and Condensed Matter
Replies
5
Views
2K
  • STEM Career Guidance
Replies
21
Views
2K
Replies
8
Views
691
Replies
5
Views
1K
  • Beyond the Standard Models
Replies
1
Views
2K
  • Sci-Fi Writing and World Building
Replies
4
Views
2K
  • Other Physics Topics
Replies
1
Views
992
Back
Top