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15 votes
Accepted

How to overcome the exponential wall encountered in full configurational interaction methods?

It is hard to claim that any FCI code overcomes the exponential wall, especially for strongly correlated systems. There are many algorithms, e.g. CDFCI, HCI, FCIQMC, ACI, etc., which significantly ...
Yingzhou Li's user avatar
  • 1,086
13 votes

FCI without Hartree-Fock

Yes, you can surely do full CI without Hartree-Fock. However, the reason why one typically starts from Hartree-Fock is that this guarantees a good conditioning of the FCI matrix, which becomes ...
Susi Lehtola's user avatar
  • 19.7k
8 votes

How to overcome the exponential wall encountered in full configurational interaction methods?

Not all determinants contribute significant correlation, especially configurations for excitations of multiple electrons to higher energy orbitals, which is the basis for CAS methods; i.e. only choose ...
Alan Rask's user avatar
7 votes

Transforming from CI basis to MO basis

This is impossible (except in the trivial case where there is only one electron), because MOs span the space of single electron wavefunctions, while SDs span the space of multielectron wavefunctions. ...
wzkchem5's user avatar
  • 9,670
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
7 votes

Are there any programs that can do non-orthogonal CI?

To start with, we do have: LUCIA, a Correlation Program, 2019, developed by Kähler and Olsen. Nonorthogonal internally contracted multi-configurational perturbation theory and Dynamic correlation for ...
Meilani Wibowo'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

PySCF: eigenvalues, non-integer occupation, CISD calculations, and frozen lists

One should not confuse the "smearing" used to facilitate self-consistent field convergence, especially for density functional calculations in periodic systems, and the role of fractional ...
Susi Lehtola's user avatar
  • 19.7k
5 votes
Accepted

Which properties would you study to evaluate spatial confinement of a molecule

The vibrational properties of the molecule should change under confinement. You could compute IR spectra or more simply vibrational spectra of the molecule to probe this. The benefit of this is that ...
Stephen's user avatar
  • 721
5 votes
Accepted

ORCA: CIS calculation for Helium atom

What you have obtained in this calculation is the energy of the first excited state according to CIS. In practice, the CI expansion coefficients are obtained as the eigenvectors of the Hamiltonian ...
Raghunathan Ramakrishnan's user avatar
4 votes
Accepted

Extended Hybrid Methods

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 ...
Susi Lehtola's user avatar
  • 19.7k
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
  • 19.7k
4 votes

Which molecules have multi-reference character?

The following small- to medium-sized molecular have multi-reference character, meaning that your CISD calculation will likely show the reference wavefunction to have a weight that is quite a fair bit ...
Nike Dattani - No Free Time's user avatar
4 votes

Has non-orthogonal CI (NOCI) been implemented in PySCF?

NOCI is not available in the main branch of PySCF That is why your question on the PySCF GitHub page which you asked on the same day as this question (next time please let us know if you are "...
Nike Dattani - No Free Time's user avatar
3 votes

Start a CI calculation from HF/DFT chkfile in PySCF

I got an answer to this question at PySCF GitHub page from James E.T. Smith. I will post here in case someone has the same issue. It can be done both ways: ...
3 votes
Accepted

Is PySCF able to print checkpoint files during a CISD calculation?

As far as I know, there isn't a native way to do this, e.g. an option that can be passed to CISD. The dump_chk option is never passed to ...
John's user avatar
  • 268
3 votes
Accepted

Accurate electron-hole localization for charge-transfer transitions

The Multiwfn software is capable to run hole-electron analysis to fully characterize electron excitations. From its manual: The hole-electron analysis module of Multiwfn is quite powerful, it is able ...
Camps's user avatar
  • 23.4k
3 votes

FCI without Hartree-Fock

Full CI often indicates the exact solution of the electronic Schroedinger equation that can also be determined without starting with the Hartree-Fock reference wavefunction. For example, the Hylleraas ...
Raghunathan Ramakrishnan's user avatar
3 votes

Which properties would you study to evaluate spatial confinement of a molecule

The KS spectrum should change: the more confined is the molecule, the larger should be the HOMO-LUMO gap.
Peter Koval's user avatar
2 votes

Finding the Configuration State Functions of Atoms

A configuration state function (CSF) is a symmetry-adapted linear combination of Slater determinants. Even for a small basis set, there can be trillions of CSFs. Those will surely not all be tabulated ...
Nike Dattani - No Free Time's user avatar
2 votes

CCSD and CISD size-consistency problem

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 ...
Paulie Bao's user avatar
  • 3,973
2 votes

What are the other matrix elements in singlet symmetry-adapted CISD?

An easier route to using symmetry-adapted CI with hand-derived matrix elements is to implement CI with determinant strings. That is, you construct bitstrings of which orbitals are occupied in the ...
Susi Lehtola's user avatar
  • 19.7k
1 vote

Why is my CISD restart failing in PySCF?

To prioritize other people's questions in search results for unanswered questions, I'm taking this out of the unanswered queue. If have also promised to put a bounty on this question if someone does ...

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