Using the Available Systems

The systems that are available for use are described below.

System Defined by a Hamiltonian Matrix Only

A very minimal System can be defined using only a Hamiltonian matrix. This can be accomplished with the MatrixHamiltonian class, where the only required argument is the path to a triangular Hamiltonian matrix output in the style of HANDE.

Note that additional information does need to be specified for a MatrixHamiltonian object to use the following methods:

• generate_determinant_bitarrays()

• generate_excitation_matrix()

• get_bitarray_integers()

• get_virtual_orbitals()

Some Methods may use these class methods under the hood and will throw an error if they are not supplied. See the API reference for MatrixHamiltonian to know which optional parameters each of the above methods requires.

Integral-Defined System

Systems may be defined using integral files supplied in the HANDE FCIDUMP file format. Such systems are supported by the Integral class.

The Hamiltonian for Integral systems must be calculated from the integrals specified in the class’s input_file. Since this calculation may be computationally intensive, it is not performed by default. Instead, the hamiltonian attribute is not defined until generate_hamiltonian() is called. The user may call this function or it will be called automatically by any Method that requires a Hamiltonian.