Unitary perturbation theory applied to multiconfigurational reference functions

摘要: Unitary parametrization of the wave operator in form suggested by Mayer is studied multireference framework. The investigated unitary perturbation theory (UPT) constructs a first correction terms functions having nonzero interaction with reference state via Hamiltonian. Parameters exponential are determined two dimensional eigenvalue equations. Because mapping, UPT unaffected quasi-degeneracy problem, making it an ideal tool for correcting starting functions. Lack size-consistency however shortcoming method. Applications as well related degeneracy-corrected PT (DCPT) presented on intruder prone examples like symmetric dissociation water molecule, BeH2 system and lowest lying states scandium dimer. Size consistency violation analysed evaluated example Tractability excited examined computing singlet–triplet splitting CH2 molecule. © 2012 Wiley Periodicals, Inc.