29
votes
Accepted
How accurate are the most accurate calculations?
Exhibit 1: Ground state hyperfine splitting of the H atom:
...
- 33.5k
15
votes
Accepted
Orbitals in full CI and DFT with true functional
FCI doesn't really have "occupied orbitals" in the mean-field sense, since all orbitals become fractionally occupied. What one usually does is look at natural orbitals, which one gets by ...
- 17.9k
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 ...
- 17.9k
11
votes
Accepted
How to calculate FCI matrix elements from an FCIDUMP file
Let's go back to the original paper where the fcidump format was first proposed: A determinant based full configuration interaction program by Peter Knowles (lead author of the MOLPRO program, along ...
- 33.5k
11
votes
What are the advantages of the Davidson diagonalization method over other sparse matrix diagonalization methods?
Following Nike Dattani's suggestion, I now add some comparisons of the Davidson method with methods that are more "modern" than it, to supplement the existing answers which compared the ...
- 9,076
9
votes
Accepted
What are the advantages of the Davidson diagonalization method over other sparse matrix diagonalization methods?
The Davidson-Method is the best algorithm when you want a comparatively small number of eigenvectors/eigenvalues from a sparse, diagonally dominant matrix.
MKL doesn’t have a sparse matrix ...
- 7,957
8
votes
What are the advantages of the Davidson diagonalization method over other sparse matrix diagonalization methods?
To supplement Cody's excellent answer, Davidson diagonalization originates from quantum chemistry, where solving configuration interaction problems requires diagonalizing the Hamiltonian in the space ...
- 17.9k
7
votes
What are the limitations of FCI?
Assuming that you have $N$ electrons and $K$ spatial orbitals, the total number of electron configurations that can be built is $$ N_{\rm confs} = {K \choose N_\alpha} {K \choose N_\beta} \approx {K ...
- 17.9k
6
votes
Accepted
Analytic Hessian at the level of full CI?
GAMESS also should be able to calculate analytic hessian for the full CI case. From the GAMESS manual:
$FORCE group
(optional, relevant for RUNTYP=HESSIAN,OPTIMIZE,SADPOINT)
This group controls the ...
- 6,516
6
votes
Analytic Hessian at the level of full CI?
Analytical Hessians have been derived for pretty much everything; they just aren't available in most programs.
CFOUR has analytical vibrational frequencies i.e. nuclear Hessians for HF and the ...
- 17.9k
6
votes
Accepted
What do the parameters at the top of an FCIDUMP file mean?
MS2 is related to the spin. Specifically, it is the number of unpaired electrons. Your molecule is a singlet, which is why it says ...
- 33.5k
5
votes
Open source FCI solvers for exact diagonalization of the many-electron Hamiltonian
MRCC
One of my favorite FCI programs is the one in MRCC, because it has a few advantages over the MOLPRO FCI program (which uses the program you described in your answer).
When the entire calculation ...
- 33.5k
5
votes
Open source FCI solvers for exact diagonalization of the many-electron Hamiltonian
LUCIA / LUCITA
Jeppe Olsen's LUCIA / LUCITA code has been included in e.g. Dalton, which is nowadays open source. The code has been successfully used in calculations including over one billion ...
- 17.9k
5
votes
Open source FCI solvers for exact diagonalization of the many-electron Hamiltonian
FCI
Peter Knowles' FCI program, published in Computer Physics Communications, 54, 75 (1989), is available at https://bitbucket.org/pjknowles/fci/src/master/
The code is Fortran 77 and can handle at ...
- 17.9k
5
votes
Accepted
Restarting an FCI calculation in PySCF
What you can do, is save the CI vector of your calculation and then use it as starting point in a subsequent calculation, like so:
...
- 346
4
votes
What are the limitations of FCI?
"it must be possible to conclusively argue where the exponential wall lies and how it comes to be."
The location of the "wall" depends on which "wall". With FCI you will ...
- 33.5k
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 ...
2
votes
Restarting an FCI calculation in PySCF
Preliminaries
The hint by Sverus Snape to use a numpy array was helpful, but how does one do that?
I found the answer at How to save and load numpy.array() data properly? and since there's also so ...
- 33.5k
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