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# 7.12.8 Direct RAS-nSF-IP/EA (librassf)

(February 4, 2022)

The librassf module enables the use of Fock-space CI RAS-$n$SF-IP/EA calculations. The CAS-$n$SF-IP/EA approach combines non-particle-number-conserving and non-spin-conserving operators to model systems with spin and spatial degeneracies. For example, the CAS-1SF-IP operator is defined as follows:

 $|\Psi\rangle=\sum_{ij\bar{a}}c_{ij}^{\bar{a}}\hat{a}_{ij}^{\bar{a}}|\Psi^{Ref}\rangle$ (7.117)

The librassf module can also perform traditional spin-flip and IP/EA calculations. External hole and particle excitations (RAS(h) and RAS(p), respectively) can also be added; this orbital relaxation is particularly important for IP/EA calculations due to the change in electron count.

Our implementation uses explicit expressions for unique changes in alpha and beta electron counts. These methods have been efficiently implemented using direct tensor contractions rather than alpha and beta strings. The code also allows the user to select the appropriate number of spin-flips, limiting the number of determinants considered in cases with large active spaces.

Currently, the following schemes are implemented: CAS-1SF, RAS(h)-1SF, RAS(p)-1SF, RAS(h,p)-1SF, CAS-2SF, CAS-IP, RAS(h)-IP, CAS-EA, RAS(p)-EA, CAS-1SF-IP, RAS(h)-1SF-IP, CAS-1SF-EA, and RAS(p)-1SF-EA.