FCIDump

class FCIDump

Bases: object

An electronic structure Hamiltonian in FCIDump format.

Definition

The FCIDump format was originally defined by Knowles and Handy, 1989 [1]. It is a widespread format for exporting electronic structure Hamiltonians in a plain-text file.

The present data structure only stores the information relevant for constructing the second quantized operator. However, this implementation goes beyond the original definition by supporting unrestricted spin data to be loaded. The table below outlines how integrals are associated with spin species based on the intervals in which the indices fall (assuming a header with NORB=n):

Integral Type	i	j	k	l
Constant	${0}$	${0}$	${0}$	${0}$
1-body alpha	${0}$	${0}$	$[1,n]$	$[1,n]$
1-body beta	${0}$	${0}$	$[n+1,2n]$	$[n+1,2n]$
2-body alpha-alpha	$[1,n]$	$[1,n]$	$[1,n]$	$[1,n]$
2-body alpha-beta	$[1,n]$	$[1,n]$	$[n+1,2n]$	$[n+1,2n]$
2-body beta-beta	$[n+1,2n]$	$[n+1,2n]$	$[n+1,2n]$	$[n+1,2n]$

The only required values are the 1-body alpha-spin integrals.

The two-body integrals are expected in chemist ordering, $(ij|kl)$ , matching the FCIDump convention.

Note

The implementation of this data structure is opaque to Python and only provides a few attributes and methods documented at the end of this page.

Conversion

Operator implementations which can be constructed from an instance of FCIDump provide a from_fcidump classmethod:


`FermionOperator.from_fcidump()`	Constructs a `FermionOperator` from an `FCIDump`.

[1]

1. Knowles and N. C. Handy, Computer Physics Communications 54 (1989) 75-83.

Attributes

ms2

Returns twice the spin quantum number, $2S$ .

This number, $2S$ , is extracted from the MS2=2S field in the header of the FCIDump file.

nelec

Returns the number of electrons.

This number is extracted from the NELEC field in the header of the FCIDump file.

norb

Returns the number of orbitals.

This number, $n$ , is extracted from the NORB=n field in the header of the FCIDump file.

Methods

from_file

classmethod from_file(file_path)

Parses an FCIDump file.

Assuming you have an FCIDump file called molecule.fcidump, you use this method like so:

from qiskit_fermions.operators.library import FCIDump

fcidump = FCIDump.from_file("molecule.fcidump")

Parameters

file_path – the path to the FCIDump file.

Returns

The constructed data structure.