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1 1998

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Applicability of the almost-linear coupled-cluster method to nondynamically correlated states

Grabowski I., Jankowski K., Kowalski K.

Polish Journal of Chemistry

1998

Vol. 72, nr 7S

1411-1420

To study the reliability of the almost-linear coupled-cluster (AL-CC) approah when applied to the description of state disclosing nondynamical correlation (or quasidegeneracy) effects, calculation have been performed for several systems widely used in quantum-chemical model calculations for quasidegenerate states (H4 (MBS),H2O (DZ), BeH2 (DZ), BH (DZP)). The AL-CC approach represents a special application of the recently formulated splitamplitude strategy for aproximating the equations of the CC methods (Chem. Phys.Lett., 256,141 (1996)) which consist in splitting the individual cluster amplitudes into two components - one of fixed value and the other determined from a set of modified CC equations. The linearization of the latter set yields the equations of the AL-CC method. In the present study the cluster operator is presented in terms of one- and two-body operators (AL-CCSD). The energies calculated are free from the singular behavior of their counterparts obtained for some strongly quasidegenerate states when using the standard linearized CC methods (L-CCSD). Outside the very strong quasidegeneracy region the AL-CCSD energies are very close to the CCSD ones, which demonstrate the usefulness of the low cost AL-CC approach.