# Tag Info

9

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 with Hans-Joachim Werner) and Nick Handy in 1989. This algorithm is still used by a lot of people today, and is considered one of the most efficient ways to do ...

7

MS2 is related to the spin. Specifically, it is the number of unpaired electrons. Your molecule is a singlet, which is why it says 0. ORBSYM is the list of symmetries for each orbital. In this case $\ce{H2}$ is in $D_{\infty h}$ but almost all electronic structure codes do not support non-Abelian groups, so instead we would almost always use $D_{2h}$. The ...

5

"I was able to guess most of results, except the ones with value 0.663472." I am able to confirm that your integrals are correct even for the 0.663472 case, because by calculating them myself in MOLPRO (a completely different program from what you used), I get the same indices of 2 2 1 1: &FCI NORB= 2,NELEC= 2,MS2= 0, ORBSYM=1,5,ISYM=0, 0....

5

1RDMs are just simple matrices, which can be stored in either dense or sparse form, possibly combined with triangular storage (the matrix is often symmetric). 1RDMs are passed around in a number of formats, like Gaussian formatted checkpoint and Molden, and can be visualized for density isosurfaces etc. 2RDMs are a bit more problematic, since they can become ...

4

I would like to post a partial answer that I have for now. For the indices ijkl in the FCIDUMP file, it corresponds to the integral (see https://theochem.github.io/horton/2.0.2/user_hamiltonian_io.html) $$\tag{1} \int dx_1dx_2\chi_i^*(x_1)\chi_k^*(x_2)\chi_j(x_1)\chi_l(x_2)$$ while the indices in the paper, it is  \tag{2} \int dx_1dx_2\chi_i^*(x_1)\chi_j^...

4

MRCC MRCC stores 1- and 2-electron integrals in an ASCII file called fort.55 which is formatted very similarly to FCIDUMP. The fort.55 file stores the integral values and integral indices in exactly the same way as in FCIDUMP, but has a different header. One has to delete the header of the FCIDUMP file and replace it with something like this (here my example ...

3

GAMESS(US) has this facility implemented, but this feature may not be available in the public release.

3

Psi4 Psi4 provides functions for reading and writing FCIDUMP files from wavefunction objects. Using Psi4 as a Python module, these look something like: import psi4 #Writing (default fname is "INTDUMP") E, wfn = psi4.energy('scf', return_wfn=True) psi4.fcidump(wfn) #Reading ##Returns dict of numerous properties including 1e ("hcore") and ...

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