# An alternative choice of the zeroth-order Hamiltonian in CASPT2 theory.

@article{Kollmar2020AnAC, title={An alternative choice of the zeroth-order Hamiltonian in CASPT2 theory.}, author={C. Kollmar and K. Sivalingam and F. Neese}, journal={The Journal of chemical physics}, year={2020}, volume={152 21}, pages={ 214110 } }

A zeroth-order Hamiltonian based on Koopmans matrices for complete active space second-order perturbation theory (CASPT2) is presented. This Hamiltonian involves three types of Fock matrices. The original CASPT2 Fock matrix is retained for excitation classes where the excitation does not change the number of electrons in the complete active space (CAS). For excitation classes involving a change in the number of electrons in the CAS, two alternative Fock matrices corresponding to either positive… CONTINUE READING

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