# Psi4FockCI: A General Fock-Space CI For Spin-Flip And IP/EA¶

Code author: Shannon E. Houck

Section author: Shannon E. Houck

This plugin is an implementation of the RAS-nSF-IP/EA approach detailed in the paper by Houck et. al. [Houck:2019:2278]. This approach handles systems with both spin and spatial degeneracies by combining the spin-flip (SF) [Krylov:2001:522] and ionization potential/electron affinity (IP/EA) [Nooijen:1995:3629] approaches.

The Psi4FockCI plugin allows one to perform spin-flip (SF), ionization potential (IP), and electron affinity (EA) calculations, as well as combined RAS-SF-IP/EA calculations, through the DETCI (CI: Configuration Interaction) module.

## Installation¶

• Download the plugin from the GitHub repository:

>>> git clone https://github.com/shannonhouck/psi4fockci.git

• Once downloaded, the plugin can be installed as follows:

>>> cd {top-level-psi4fockci-directory}
>>> python -m pip install .


## Sample Input¶

To run a CAS-nSF-IP/EA calculation, start with a molecule with the correct charge and multiplicity for the reference state (i.e. some state well-represented by a single reference). Then, run an energy calculation, passing in the charge and multiplicity of the desired state; the number of spin-flips and IP/EA will be automatically determined based on this input. If additional excitations outside of the RAS II space are desired, one can set the level of external excitations using the conf_space keyword.

A sample input file for a RAS(h)-2SF-IP calculation is shown below:

molecule {
0 7
N 0.0 0.0 0.0
N 0.0 0.0 1.3
symmetry c1
}

set {
basis cc-pVDZ
}

energy('psi4fockci', new_charge=1, new_multiplicity=1, conf_space="h")


Note that for calculations involving IP/EA, inclusion of hole (for IP) and particle (for EA) excitations is strongly recommended. Additional keywords can be found in the documentation.

This input file can be run with Psi4:

>>> psi4 input.dat