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 ...
  • 1,046
12 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 ...
  • 15.1k
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 ...
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. ...
  • 7,478
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 ...
  • 29.2k
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 ...
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 ...
  • 29.2k
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 ...
  • 15.1k
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 ...
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 ...
  • 15.1k
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 ...
  • 15.1k
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 ...
  • 29.2k
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 ...
  • 19.3k
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 ...
3 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 ...
2 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.
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 ...
  • 29.2k
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 ...
  • 3,609
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 ...
  • 15.1k

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